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Nettle: a low-level cryptographic library

Table of Contents

Nettle

This document describes the Nettle low-level cryptographic library. You can use the library directly from your C programs, or write or use an object-oriented wrapper for your favorite language or application.

This manual is for the Nettle library (version 3.4), a low-level cryptographic library.

Originally written 2001 by Niels Möller, updated 2017.

This manual is placed in the public domain. You may freely copy it, in whole or in part, with or without modification. Attribution is appreciated, but not required.

 — The Detailed Node Listing —

Reference

Hash functions

Cipher modes

Keyed Hash Functions

Public-key algorithms

Elliptic curves

1 Introduction

Nettle is a cryptographic library that is designed to fit easily in more or less any context: In crypto toolkits for object-oriented languages (C++, Python, Pike, ...), in applications like LSH or GNUPG, or even in kernel space. In most contexts, you need more than the basic cryptographic algorithms, you also need some way to keep track of available algorithms, their properties and variants. You often have some algorithm selection process, often dictated by a protocol you want to implement.

And as the requirements of applications differ in subtle and not so subtle ways, an API that fits one application well can be a pain to use in a different context. And that is why there are so many different cryptographic libraries around.

Nettle tries to avoid this problem by doing one thing, the low-level crypto stuff, and providing a simple but general interface to it. In particular, Nettle doesn’t do algorithm selection. It doesn’t do memory allocation. It doesn’t do any I/O.

The idea is that one can build several application and context specific interfaces on top of Nettle, and share the code, test cases, benchmarks, documentation, etc. Examples are the Nettle module for the Pike language, and LSH, which both use an object-oriented abstraction on top of the library.

This manual explains how to use the Nettle library. It also tries to provide some background on the cryptography, and advice on how to best put it to use.

2 Copyright

Nettle is dual licenced under the GNU General Public License version 2 or later, and the GNU Lesser General Public License version 3 or later. When using Nettle, you must comply fully with all conditions of at least one of these licenses. A few of the individual files are licensed under more permissive terms, or in the public domain. To find the current status of particular files, you have to read the copyright notices at the top of the files.

This manual is in the public domain. You may freely copy it in whole or in part, e.g., into documentation of programs that build on Nettle. Attribution, as well as contribution of improvements to the text, is of course appreciated, but it is not required.

A list of the supported algorithms, their origins, and exceptions to the above licensing:

AES

The implementation of the AES cipher (also known as rijndael) is written by Rafael Sevilla. Assembler for x86 by Rafael Sevilla and Niels Möller, Sparc assembler by Niels Möller.

ARCFOUR

The implementation of the ARCFOUR (also known as RC4) cipher is written by Niels Möller.

ARCTWO

The implementation of the ARCTWO (also known as RC2) cipher is written by Nikos Mavroyanopoulos and modified by Werner Koch and Simon Josefsson.

BLOWFISH

The implementation of the BLOWFISH cipher is written by Werner Koch, copyright owned by the Free Software Foundation. Also hacked by Simon Josefsson and Niels Möller.

CAMELLIA

The C implementation is by Nippon Telegraph and Telephone Corporation (NTT), heavily modified by Niels Möller. Assembler for x86 and x86_64 by Niels Möller.

CAST128

The implementation of the CAST128 cipher is written by Steve Reid. Released into the public domain.

CHACHA

Implemented by Joachim Strömbergson, based on the implementation of SALSA20 (see below). Assembly for x86_64 by Niels Möller.

DES

The implementation of the DES cipher is written by Dana L. How, and released under the LGPL, version 2 or later.

GOSTHASH94

The C implementation of the GOST94 message digest is written by Aleksey Kravchenko and was ported from the rhash library by Nikos Mavrogiannopoulos. It is released under the MIT license.

MD2

The implementation of MD2 is written by Andrew Kuchling, and hacked some by Andreas Sigfridsson and Niels Möller. Python Cryptography Toolkit license (essentially public domain).

MD4

This is almost the same code as for MD5 below, with modifications by Marcus Comstedt. Released into the public domain.

MD5

The implementation of the MD5 message digest is written by Colin Plumb. It has been hacked some more by Andrew Kuchling and Niels Möller. Released into the public domain.

PBKDF2

The C implementation of PBKDF2 is based on earlier work for Shishi and GnuTLS by Simon Josefsson.

RIPEMD160

The implementation of RIPEMD160 message digest is based on the code in libgcrypt, copyright owned by the Free Software Foundation. Ported to Nettle by Andres Mejia.

SALSA20

The C implementation of SALSA20 is based on D. J. Bernstein’s reference implementation (in the public domain), adapted to Nettle by Simon Josefsson, and heavily modified by Niels Möller. Assembly for x86_64 and ARM by Niels Möller.

SERPENT

The implementation of the SERPENT cipher is based on the code in libgcrypt, copyright owned by the Free Software Foundation. Adapted to Nettle by Simon Josefsson and heavily modified by Niels Möller. Assembly for x86_64 by Niels Möller.

POLY1305

Based on the implementation by Andrew M. (floodyberry), modified by Nikos Mavrogiannopoulos and Niels Möller. Assembly for x86_64 by Niels Möller.

SHA1

The C implementation of the SHA1 message digest is written by Peter Gutmann, and hacked some more by Andrew Kuchling and Niels Möller. Released into the public domain. Assembler for x86, x86_64 and ARM by Niels Möller, released under the LGPL.

SHA2

Written by Niels Möller, using Peter Gutmann’s SHA1 code as a model.

SHA3

Written by Niels Möller.

TWOFISH

The implementation of the TWOFISH cipher is written by Ruud de Rooij.

UMAC

Written by Niels Möller.

RSA

Written by Niels Möller. Uses the GMP library for bignum operations.

DSA

Written by Niels Möller. Uses the GMP library for bignum operations.

ECDSA

Written by Niels Möller. Uses the GMP library for bignum operations. Development of Nettle’s ECC support was funded by the .SE Internet Fund.

3 Conventions

For each supported algorithm, there is an include file that defines a context struct, a few constants, and declares functions for operating on the context. The context struct encapsulates all information needed by the algorithm, and it can be copied or moved in memory with no unexpected effects.

For consistency, functions for different algorithms are very similar, but there are some differences, for instance reflecting if the key setup or encryption function differ for encryption and decryption, and whether or not key setup can fail. There are also differences between algorithms that don’t show in function prototypes, but which the application must nevertheless be aware of. There is no big difference between the functions for stream ciphers and for block ciphers, although they should be used quite differently by the application.

If your application uses more than one algorithm of the same type, you should probably create an interface that is tailor-made for your needs, and then write a few lines of glue code on top of Nettle.

By convention, for an algorithm named foo, the struct tag for the context struct is foo_ctx, constants and functions uses prefixes like FOO_BLOCK_SIZE (a constant) and foo_set_key (a function).

In all functions, strings are represented with an explicit length, of type size_t, and a pointer of type uint8_t * or const uint8_t *. For functions that transform one string to another, the argument order is length, destination pointer and source pointer. Source and destination areas are usually of the same length. When they differ, e.g., for ccm_encrypt_message, the length argument specifies the size of the destination area. Source and destination pointers may be equal, so that you can process strings in place, but source and destination areas must not overlap in any other way.

Many of the functions lack return value and can never fail. Those functions which can fail, return one on success and zero on failure.

4 Example

A simple example program that reads a file from standard input and writes its SHA1 check-sum on standard output should give the flavor of Nettle.

#include <stdio.h>
#include <stdlib.h>

#include <nettle/sha1.h>

#define BUF_SIZE 1000

static void
display_hex(unsigned length, uint8_t *data)
{
  unsigned i;

  for (i = 0; i<length; i++)
    printf("%02x ", data[i]);

  printf("\n");
}

int
main(int argc, char **argv)
{
  struct sha1_ctx ctx;
  uint8_t buffer[BUF_SIZE];
  uint8_t digest[SHA1_DIGEST_SIZE];
  
  sha1_init(&ctx);
  for (;;)
  {
    int done = fread(buffer, 1, sizeof(buffer), stdin);
    sha1_update(&ctx, done, buffer);
    if (done < sizeof(buffer))
      break;
  }
  if (ferror(stdin))
    return EXIT_FAILURE;

  sha1_digest(&ctx, SHA1_DIGEST_SIZE, digest);

  display_hex(SHA1_DIGEST_SIZE, digest);
  return EXIT_SUCCESS;  
}

On a typical Unix system, this program can be compiled and linked with the command line

gcc sha-example.c -o sha-example -lnettle

5 Linking

Nettle actually consists of two libraries, libnettle and libhogweed. The libhogweed library contains those functions of Nettle that uses bignum operations, and depends on the GMP library. With this division, linking works the same for both static and dynamic libraries.

If an application uses only the symmetric crypto algorithms of Nettle (i.e., block ciphers, hash functions, and the like), it’s sufficient to link with -lnettle. If an application also uses public-key algorithms, the recommended linker flags are -lhogweed -lnettle -lgmp. If the involved libraries are installed as dynamic libraries, it may be sufficient to link with just -lhogweed, and the loader will resolve the dependencies automatically.

6 Reference

This chapter describes all the Nettle functions, grouped by family.

6.1 Hash functions

A cryptographic hash function is a function that takes variable size strings, and maps them to strings of fixed, short, length. There are naturally lots of collisions, as there are more possible 1MB files than 20 byte strings. But the function is constructed such that is hard to find the collisions. More precisely, a cryptographic hash function H should have the following properties:

One-way

Given a hash value H(x) it is hard to find a string x that hashes to that value.

Collision-resistant

It is hard to find two different strings, x and y, such that H(x) = H(y).

Hash functions are useful as building blocks for digital signatures, message authentication codes, pseudo random generators, association of unique ids to documents, and many other things.

The most commonly used hash functions are MD5 and SHA1. Unfortunately, both these fail the collision-resistance requirement; cryptologists have found ways to construct colliding inputs. The recommended hash functions for new applications are SHA2 (with main variants SHA256 and SHA512). At the time of this writing (Autumn 2015), SHA3 has recently been standardized, and the new SHA3 and other top SHA3 candidates may also be reasonable alternatives.

6.1.1 Recommended hash functions

The following hash functions have no known weaknesses, and are suitable for new applications. The SHA2 family of hash functions were specified by NIST, intended as a replacement for SHA1.

6.1.1.1 SHA256

SHA256 is a member of the SHA2 family. It outputs hash values of 256 bits, or 32 octets. Nettle defines SHA256 in <nettle/sha2.h>.

Context struct: struct sha256_ctx

Constant: SHA256_DIGEST_SIZE

The size of a SHA256 digest, i.e. 32.

Constant: SHA256_BLOCK_SIZE

The internal block size of SHA256. Useful for some special constructions, in particular HMAC-SHA256.

Function: void sha256_init (struct sha256_ctx *ctx)

Initialize the SHA256 state.

Function: void sha256_update (struct sha256_ctx *ctx, size_t length, const uint8_t *data)

Hash some more data.

Function: void sha256_digest (struct sha256_ctx *ctx, size_t length, uint8_t *digest)

Performs final processing and extracts the message digest, writing it to digest. length may be smaller than SHA256_DIGEST_SIZE, in which case only the first length octets of the digest are written.

This function also resets the context in the same way as sha256_init.

Earlier versions of nettle defined SHA256 in the header file <nettle/sha.h>, which is now deprecated, but kept for compatibility.

6.1.1.2 SHA224

SHA224 is a variant of SHA256, with a different initial state, and with the output truncated to 224 bits, or 28 octets. Nettle defines SHA224 in <nettle/sha2.h> (and in <nettle/sha.h>, for backwards compatibility).

Context struct: struct sha224_ctx

Constant: SHA224_DIGEST_SIZE

The size of a SHA224 digest, i.e. 28.

Constant: SHA224_BLOCK_SIZE

The internal block size of SHA224. Useful for some special constructions, in particular HMAC-SHA224.

Function: void sha224_init (struct sha224_ctx *ctx)

Initialize the SHA224 state.

Function: void sha224_update (struct sha224_ctx *ctx, size_t length, const uint8_t *data)

Hash some more data.

Function: void sha224_digest (struct sha224_ctx *ctx, size_t length, uint8_t *digest)

Performs final processing and extracts the message digest, writing it to digest. length may be smaller than SHA224_DIGEST_SIZE, in which case only the first length octets of the digest are written.

This function also resets the context in the same way as sha224_init.

6.1.1.3 SHA512

SHA512 is a larger sibling to SHA256, with a very similar structure but with both the output and the internal variables of twice the size. The internal variables are 64 bits rather than 32, making it significantly slower on 32-bit computers. It outputs hash values of 512 bits, or 64 octets. Nettle defines SHA512 in <nettle/sha2.h> (and in <nettle/sha.h>, for backwards compatibility).

Context struct: struct sha512_ctx

Constant: SHA512_DIGEST_SIZE

The size of a SHA512 digest, i.e. 64.

Constant: SHA512_BLOCK_SIZE

The internal block size of SHA512, 128. Useful for some special constructions, in particular HMAC-SHA512.

Function: void sha512_init (struct sha512_ctx *ctx)

Initialize the SHA512 state.

Function: void sha512_update (struct sha512_ctx *ctx, size_t length, const uint8_t *data)

Hash some more data.

Function: void sha512_digest (struct sha512_ctx *ctx, size_t length, uint8_t *digest)

Performs final processing and extracts the message digest, writing it to digest. length may be smaller than SHA512_DIGEST_SIZE, in which case only the first length octets of the digest are written.

This function also resets the context in the same way as sha512_init.

6.1.1.4 SHA384 and other variants of SHA512

Several variants of SHA512 have been defined, with a different initial state, and with the output truncated to shorter length than 512 bits. Naming is a bit confused, these algorithms are called SHA512-224, SHA512-256 and SHA384, for output sizes of 224, 256 and 384 bits, respectively. Nettle defines these in <nettle/sha2.h> (and in <nettle/sha.h>, for backwards compatibility).

Context struct: struct sha512_224_ctx

Context struct: struct sha512_256_ctx

Context struct: struct sha384_ctx

These context structs are all the same as sha512_ctx. They are defined as simple preprocessor aliases, which may cause some problems if used as identifiers for other purposes. So avoid doing that.

Constant: SHA512_224_DIGEST_SIZE

Constant: SHA512_256_DIGEST_SIZE

Constant: SHA384_DIGEST_SIZE

The digest size for each variant, i.e., 28, 32, and 48, respectively.

Constant: SHA512_224_BLOCK_SIZE

Constant: SHA512_256_BLOCK_SIZE

Constant: SHA384_BLOCK_SIZE

The internal block size, same as SHA512_BLOCK_SIZE, i.e., 128. Useful for some special constructions, in particular HMAC-SHA384.

Function: void sha512_224_init (struct sha512_224_ctx *ctx)

Function: void sha512_256_init (struct sha512_256_ctx *ctx)

Function: void sha384_init (struct sha384_ctx *ctx)

Initialize the context struct.

Function: void sha512_224_update (struct sha512_224_ctx *ctx, size_t length, const uint8_t *data)

Function: void sha512_256_update (struct sha512_256_ctx *ctx, size_t length, const uint8_t *data)

Function: void sha384_update (struct sha384_ctx *ctx, size_t length, const uint8_t *data)

Hash some more data. These are all aliases for sha512_update, which does the same thing.

Function: void sha512_224_digest (struct sha512_224_ctx *ctx, size_t length, uint8_t *digest)

Function: void sha512_256_digest (struct sha512_256_ctx *ctx, size_t length, uint8_t *digest)

Function: void sha384_digest (struct sha384_ctx *ctx, size_t length, uint8_t *digest)

Performs final processing and extracts the message digest, writing it to digest. length may be smaller than the specified digest size, in which case only the first length octets of the digest are written.

These function also reset the context in the same way as the corresponding init function.

6.1.1.5 SHA3-224

The SHA3 hash functions were specified by NIST in response to weaknesses in SHA1, and doubts about SHA2 hash functions which structurally are very similar to SHA1. SHA3 is a result of a competition, where the winner, also known as Keccak, was designed by Guido Bertoni, Joan Daemen, Michaël Peeters and Gilles Van Assche. It is structurally very different from all widely used earlier hash functions. Like SHA2, there are several variants, with output sizes of 224, 256, 384 and 512 bits (28, 32, 48 and 64 octets, respectively). In August 2015, it was formally standardized by NIST, as FIPS 202, http://dx.doi.org/10.6028/NIST.FIPS.202.

Note that the SHA3 implementation in earlier versions of Nettle was based on the specification at the time Keccak was announced as the winner of the competition, which is incompatible with the final standard and hence with current versions of Nettle. The nette/sha3.h defines a preprocessor symbol NETTLE_SHA3_FIPS202 to indicate conformance with the standard.

Constant: NETTLE_SHA3_FIPS202

Defined to 1 in Nettle versions supporting FIPS 202. Undefined in earlier versions.

Nettle defines SHA3-224 in <nettle/sha3.h>.

Context struct: struct sha3_224_ctx

Constant: SHA3_224_DIGEST_SIZE

The size of a SHA3_224 digest, i.e., 28.

Constant: SHA3_224_BLOCK_SIZE

The internal block size of SHA3_224.

Function: void sha3_224_init (struct sha3_224_ctx *ctx)

Initialize the SHA3-224 state.

Function: void sha3_224_update (struct sha3_224_ctx *ctx, size_t length, const uint8_t *data)

Hash some more data.

Function: void sha3_224_digest (struct sha3_224_ctx *ctx, size_t length, uint8_t *digest)

Performs final processing and extracts the message digest, writing it to digest. length may be smaller than SHA3_224_DIGEST_SIZE, in which case only the first length octets of the digest are written.

This function also resets the context.

6.1.1.6 SHA3-256

This is SHA3 with 256-bit output size, and possibly the most useful of the SHA3 hash functions.

Nettle defines SHA3-256 in <nettle/sha3.h>.

Context struct: struct sha3_256_ctx

Constant: SHA3_256_DIGEST_SIZE

The size of a SHA3_256 digest, i.e., 32.

Constant: SHA3_256_BLOCK_SIZE

The internal block size of SHA3_256.

Function: void sha3_256_init (struct sha3_256_ctx *ctx)

Initialize the SHA3-256 state.

Function: void sha3_256_update (struct sha3_256_ctx *ctx, size_t length, const uint8_t *data)

Hash some more data.

Function: void sha3_256_digest (struct sha3_256_ctx *ctx, size_t length, uint8_t *digest)

Performs final processing and extracts the message digest, writing it to digest. length may be smaller than SHA3_256_DIGEST_SIZE, in which case only the first length octets of the digest are written.

This function also resets the context.

6.1.1.7 SHA3-384

This is SHA3 with 384-bit output size.

Nettle defines SHA3-384 in <nettle/sha3.h>.

Context struct: struct sha3_384_ctx

Constant: SHA3_384_DIGEST_SIZE

The size of a SHA3_384 digest, i.e., 48.

Constant: SHA3_384_BLOCK_SIZE

The internal block size of SHA3_384.

Function: void sha3_384_init (struct sha3_384_ctx *ctx)

Initialize the SHA3-384 state.

Function: void sha3_384_update (struct sha3_384_ctx *ctx, size_t length, const uint8_t *data)

Hash some more data.

Function: void sha3_384_digest (struct sha3_384_ctx *ctx, size_t length, uint8_t *digest)

Performs final processing and extracts the message digest, writing it to digest. length may be smaller than SHA3_384_DIGEST_SIZE, in which case only the first length octets of the digest are written.

This function also resets the context.

6.1.1.8 SHA3-512

This is SHA3 with 512-bit output size.

Nettle defines SHA3-512 in <nettle/sha3.h>.

Context struct: struct sha3_512_ctx

Constant: SHA3_512_DIGEST_SIZE

The size of a SHA3_512 digest, i.e. 64.

Constant: SHA3_512_BLOCK_SIZE

The internal block size of SHA3_512.

Function: void sha3_512_init (struct sha3_512_ctx *ctx)

Initialize the SHA3-512 state.

Function: void sha3_512_update (struct sha3_512_ctx *ctx, size_t length, const uint8_t *data)

Hash some more data.

Function: void sha3_512_digest (struct sha3_512_ctx *ctx, size_t length, uint8_t *digest)

Performs final processing and extracts the message digest, writing it to digest. length may be smaller than SHA3_512_DIGEST_SIZE, in which case only the first length octets of the digest are written.

This function also resets the context.

6.1.2 Legacy hash functions

The hash functions in this section all have some known weaknesses, and should be avoided for new applications. These hash functions are mainly useful for compatibility with old applications and protocols. Some are still considered safe as building blocks for particular constructions, e.g., there seems to be no known attacks against HMAC-SHA1 or even HMAC-MD5. In some important cases, use of a “legacy” hash function does not in itself make the application insecure; if a known weakness is relevant depends on how the hash function is used, and on the threat model.

6.1.2.1 MD5

MD5 is a message digest function constructed by Ronald Rivest, and described in RFC 1321. It outputs message digests of 128 bits, or 16 octets. Nettle defines MD5 in <nettle/md5.h>.

Context struct: struct md5_ctx

Constant: MD5_DIGEST_SIZE

The size of an MD5 digest, i.e. 16.

Constant: MD5_BLOCK_SIZE

The internal block size of MD5. Useful for some special constructions, in particular HMAC-MD5.

Function: void md5_init (struct md5_ctx *ctx)

Initialize the MD5 state.

Function: void md5_update (struct md5_ctx *ctx, size_t length, const uint8_t *data)

Hash some more data.

Function: void md5_digest (struct md5_ctx *ctx, size_t length, uint8_t *digest)

Performs final processing and extracts the message digest, writing it to digest. length may be smaller than MD5_DIGEST_SIZE, in which case only the first length octets of the digest are written.

This function also resets the context in the same way as md5_init.

The normal way to use MD5 is to call the functions in order: First md5_init, then md5_update zero or more times, and finally md5_digest. After md5_digest, the context is reset to its initial state, so you can start over calling md5_update to hash new data.

To start over, you can call md5_init at any time.

6.1.2.2 MD2

MD2 is another hash function of Ronald Rivest’s, described in RFC 1319. It outputs message digests of 128 bits, or 16 octets. Nettle defines MD2 in <nettle/md2.h>.

Context struct: struct md2_ctx

Constant: MD2_DIGEST_SIZE

The size of an MD2 digest, i.e. 16.

Constant: MD2_BLOCK_SIZE

The internal block size of MD2.

Function: void md2_init (struct md2_ctx *ctx)

Initialize the MD2 state.

Function: void md2_update (struct md2_ctx *ctx, size_t length, const uint8_t *data)

Hash some more data.

Function: void md2_digest (struct md2_ctx *ctx, size_t length, uint8_t *digest)

Performs final processing and extracts the message digest, writing it to digest. length may be smaller than MD2_DIGEST_SIZE, in which case only the first length octets of the digest are written.

This function also resets the context in the same way as md2_init.

6.1.2.3 MD4

MD4 is a predecessor of MD5, described in RFC 1320. Like MD5, it is constructed by Ronald Rivest. It outputs message digests of 128 bits, or 16 octets. Nettle defines MD4 in <nettle/md4.h>. Use of MD4 is not recommended, but it is sometimes needed for compatibility with existing applications and protocols.

Context struct: struct md4_ctx

Constant: MD4_DIGEST_SIZE

The size of an MD4 digest, i.e. 16.

Constant: MD4_BLOCK_SIZE

The internal block size of MD4.

Function: void md4_init (struct md4_ctx *ctx)

Initialize the MD4 state.

Function: void md4_update (struct md4_ctx *ctx, size_t length, const uint8_t *data)

Hash some more data.

Function: void md4_digest (struct md4_ctx *ctx, size_t length, uint8_t *digest)

Performs final processing and extracts the message digest, writing it to digest. length may be smaller than MD4_DIGEST_SIZE, in which case only the first length octets of the digest are written.

This function also resets the context in the same way as md4_init.

6.1.2.4 RIPEMD160

RIPEMD160 is a hash function designed by Hans Dobbertin, Antoon Bosselaers, and Bart Preneel, as a strengthened version of RIPEMD (which, like MD4 and MD5, fails the collision-resistance requirement). It produces message digests of 160 bits, or 20 octets. Nettle defined RIPEMD160 in nettle/ripemd160.h.

Context struct: struct ripemd160_ctx

Constant: RIPEMD160_DIGEST_SIZE

The size of a RIPEMD160 digest, i.e. 20.

Constant: RIPEMD160_BLOCK_SIZE

The internal block size of RIPEMD160.

Function: void ripemd160_init (struct ripemd160_ctx *ctx)

Initialize the RIPEMD160 state.

Function: void ripemd160_update (struct ripemd160_ctx *ctx, size_t length, const uint8_t *data)

Hash some more data.

Function: void ripemd160_digest (struct ripemd160_ctx *ctx, size_t length, uint8_t *digest)

Performs final processing and extracts the message digest, writing it to digest. length may be smaller than RIPEMD160_DIGEST_SIZE, in which case only the first length octets of the digest are written.

This function also resets the context in the same way as ripemd160_init.

6.1.2.5 SHA1

SHA1 is a hash function specified by NIST (The U.S. National Institute for Standards and Technology). It outputs hash values of 160 bits, or 20 octets. Nettle defines SHA1 in <nettle/sha1.h> (and in <nettle/sha.h>, for backwards compatibility).

Context struct: struct sha1_ctx

Constant: SHA1_DIGEST_SIZE

The size of a SHA1 digest, i.e. 20.

Constant: SHA1_BLOCK_SIZE

The internal block size of SHA1. Useful for some special constructions, in particular HMAC-SHA1.

Function: void sha1_init (struct sha1_ctx *ctx)

Initialize the SHA1 state.

Function: void sha1_update (struct sha1_ctx *ctx, size_t length, const uint8_t *data)

Hash some more data.

Function: void sha1_digest (struct sha1_ctx *ctx, size_t length, uint8_t *digest)

Performs final processing and extracts the message digest, writing it to digest. length may be smaller than SHA1_DIGEST_SIZE, in which case only the first length octets of the digest are written.

This function also resets the context in the same way as sha1_init.

6.1.2.6 GOSTHASH94

The GOST94 or GOST R 34.11-94 hash algorithm is a Soviet-era algorithm used in Russian government standards (see RFC 4357). It outputs message digests of 256 bits, or 32 octets. Nettle defines GOSTHASH94 in <nettle/gosthash94.h>.

Context struct: struct gosthash94_ctx

Constant: GOSTHASH94_DIGEST_SIZE

The size of a GOSTHASH94 digest, i.e. 32.

Constant: GOSTHASH94_BLOCK_SIZE

The internal block size of GOSTHASH94, i.e., 32.

Function: void gosthash94_init (struct gosthash94_ctx *ctx)

Initialize the GOSTHASH94 state.

Function: void gosthash94_update (struct gosthash94_ctx *ctx, size_t length, const uint8_t *data)

Hash some more data.

Function: void gosthash94_digest (struct gosthash94_ctx *ctx, size_t length, uint8_t *digest)

Performs final processing and extracts the message digest, writing it to digest. length may be smaller than GOSTHASH94_DIGEST_SIZE, in which case only the first length octets of the digest are written.

This function also resets the context in the same way as gosthash94_init.

6.1.3 The struct nettle_hash abstraction

Nettle includes a struct including information about the supported hash functions. It is defined in <nettle/nettle-meta.h>, and is used by Nettle’s implementation of HMAC (see Keyed hash functions).

Meta struct: struct nettle_hash name context_size digest_size block_size init update digest

The last three attributes are function pointers, of types nettle_hash_init_func *, nettle_hash_update_func *, and nettle_hash_digest_func *. The first argument to these functions is void * pointer to a context struct, which is of size context_size.

Constant Struct: struct nettle_hash nettle_md2

Constant Struct: struct nettle_hash nettle_md4

Constant Struct: struct nettle_hash nettle_md5

Constant Struct: struct nettle_hash nettle_ripemd160

Constant Struct: struct nettle_hash nettle_sha1

Constant Struct: struct nettle_hash nettle_sha224

Constant Struct: struct nettle_hash nettle_sha256

Constant Struct: struct nettle_hash nettle_sha384

Constant Struct: struct nettle_hash nettle_sha512

Constant Struct: struct nettle_hash nettle_sha3_256

Constant Struct: struct nettle_hash nettle_gosthash94

These are all the hash functions that Nettle implements.

Nettle also exports a list of all these hashes.

Function: const struct nettle_hash **nettle_get_hashes(void)

Returns a NULL-terminated list of pointers to supported hash functions. This list can be used to dynamically enumerate or search the supported algorithms.

Macro: nettle_hashes

A macro expanding to a call to nettle_get_hashes, so that one could write, e.g., nettle_hashes[0]->name for the name of the first hash function on the list. In earlier versions, this was not a macro but the actual array of pointers. However, referring directly to the array makes the array size leak into the ABI in some cases.

6.2 Cipher functions

A cipher is a function that takes a message or plaintext and a secret key and transforms it to a ciphertext. Given only the ciphertext, but not the key, it should be hard to find the plaintext. Given matching pairs of plaintext and ciphertext, it should be hard to find the key.

There are two main classes of ciphers: Block ciphers and stream ciphers.

A block cipher can process data only in fixed size chunks, called blocks. Typical block sizes are 8 or 16 octets. To encrypt arbitrary messages, you usually have to pad it to an integral number of blocks, split it into blocks, and then process each block. The simplest way is to process one block at a time, independent of each other. That mode of operation is called ECB, Electronic Code Book mode. However, using ECB is usually a bad idea. For a start, plaintext blocks that are equal are transformed to ciphertext blocks that are equal; that leaks information about the plaintext. Usually you should apply the cipher is some “feedback mode”, CBC (Cipher Block Chaining) and CTR (Counter mode) being two of of the most popular. See See Cipher modes, for information on how to apply CBC and CTR with Nettle.

A stream cipher can be used for messages of arbitrary length. A typical stream cipher is a keyed pseudo-random generator. To encrypt a plaintext message of n octets, you key the generator, generate n octets of pseudo-random data, and XOR it with the plaintext. To decrypt, regenerate the same stream using the key, XOR it to the ciphertext, and the plaintext is recovered.

Caution: The first rule for this kind of cipher is the same as for a One Time Pad: never ever use the same key twice.

A common misconception is that encryption, by itself, implies authentication. Say that you and a friend share a secret key, and you receive an encrypted message. You apply the key, and get a plaintext message that makes sense to you. Can you then be sure that it really was your friend that wrote the message you’re reading? The answer is no. For example, if you were using a block cipher in ECB mode, an attacker may pick up the message on its way, and reorder, delete or repeat some of the blocks. Even if the attacker can’t decrypt the message, he can change it so that you are not reading the same message as your friend wrote. If you are using a block cipher in CBC mode rather than ECB, or are using a stream cipher, the possibilities for this sort of attack are different, but the attacker can still make predictable changes to the message.

It is recommended to always use an authentication mechanism in addition to encrypting the messages. Popular choices are Message Authentication Codes like HMAC-SHA1 (see Keyed hash functions), or digital signatures like RSA.

Some ciphers have so called “weak keys”, keys that results in undesirable structure after the key setup processing, and should be avoided. In Nettle, most key setup functions have no return value, but for ciphers with weak keys, the return value indicates whether or not the given key is weak. For good keys, key setup returns 1, and for weak keys, it returns 0. When possible, avoid algorithms that have weak keys. There are several good ciphers that don’t have any weak keys.

To encrypt a message, you first initialize a cipher context for encryption or decryption with a particular key. You then use the context to process plaintext or ciphertext messages. The initialization is known as key setup. With Nettle, it is recommended to use each context struct for only one direction, even if some of the ciphers use a single key setup function that can be used for both encryption and decryption.

6.2.1 AES

AES is a block cipher, specified by NIST as a replacement for the older DES standard. The standard is the result of a competition between cipher designers. The winning design, also known as RIJNDAEL, was constructed by Joan Daemen and Vincent Rijnmen.

Like all the AES candidates, the winning design uses a block size of 128 bits, or 16 octets, and three possible key-size, 128, 192 and 256 bits (16, 24 and 32 octets) being the allowed key sizes. It does not have any weak keys. Nettle defines AES in <nettle/aes.h>, and there is one context struct for each key size. (Earlier versions of Nettle used a single context struct, struct aes_ctx, for all key sizes. This interface kept for backwards compatibility).

Context struct: struct aes128_ctx

Context struct: struct aes192_ctx

Context struct: struct aes256_ctx

Context struct: struct aes_ctx

Alternative struct, for the old AES interface.

Constant: AES_BLOCK_SIZE

The AES block-size, 16.

Constant: AES128_KEY_SIZE

Constant: AES192_KEY_SIZE

Constant: AES256_KEY_SIZE

Constant: AES_MIN_KEY_SIZE

Constant: AES_MAX_KEY_SIZE

Constant: AES_KEY_SIZE

Default AES key size, 32.

Function: void aes128_set_encrypt_key (struct aes128_ctx *ctx, const uint8_t *key)

Function: void aes128_set_decrypt_key (struct aes128_ctx *ctx, const uint8_t *key)

Function: void aes192_set_encrypt_key (struct aes192_ctx *ctx, const uint8_t *key)

Function: void aes192_set_decrypt_key (struct aes192_ctx *ctx, const uint8_t *key)

Function: void aes256_set_encrypt_key (struct aes256_ctx *ctx, const uint8_t *key)

Function: void aes256_set_decrypt_key (struct aes256_ctx *ctx, const uint8_t *key)

Function: void aes_set_encrypt_key (struct aes_ctx *ctx, size_t length, const uint8_t *key)

Function: void aes_set_decrypt_key (struct aes_ctx *ctx, size_t length, const uint8_t *key)

Initialize the cipher, for encryption or decryption, respectively.

Function: void aes128_invert_key (struct aes128_ctx *dst, const struct aes128_ctx *src)

Function: void aes192_invert_key (struct aes192_ctx *dst, const struct aes192_ctx *src)

Function: void aes256_invert_key (struct aes256_ctx *dst, const struct aes256_ctx *src)

Function: void aes_invert_key (struct aes_ctx *dst, const struct aes_ctx *src)

Given a context src initialized for encryption, initializes the context struct dst for decryption, using the same key. If the same context struct is passed for both src and dst, it is converted in place. These functions are mainly useful for applications which needs to both encrypt and decrypt using the same key, because calling, e.g., aes128_set_encrypt_key and aes128_invert_key, is more efficient than calling aes128_set_encrypt_key and aes128_set_decrypt_key.

Function: void aes128_encrypt (struct aes128_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Function: void aes192_encrypt (struct aes192_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Function: void aes256_encrypt (struct aes256_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Function: void aes_encrypt (struct aes_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Encryption function. length must be an integral multiple of the block size. If it is more than one block, the data is processed in ECB mode. src and dst may be equal, but they must not overlap in any other way.

Function: void aes128_decrypt (struct aes128_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Function: void aes192_decrypt (struct aes192_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Function: void aes256_decrypt (struct aes256_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Function: void aes_decrypt (struct aes_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Analogous to the encryption functions above.

6.2.2 ARCFOUR

ARCFOUR is a stream cipher, also known under the trade marked name RC4, and it is one of the fastest ciphers around. A problem is that the key setup of ARCFOUR is quite weak, you should never use keys with structure, keys that are ordinary passwords, or sequences of keys like “secret:1”, “secret:2”, .... If you have keys that don’t look like random bit strings, and you want to use ARCFOUR, always hash the key before feeding it to ARCFOUR. Furthermore, the initial bytes of the generated key stream leak information about the key; for this reason, it is recommended to discard the first 512 bytes of the key stream.

/* A more robust key setup function for ARCFOUR */
void
arcfour_set_key_hashed(struct arcfour_ctx *ctx,
                       size_t length, const uint8_t *key)
{
  struct sha256_ctx hash;
  uint8_t digest[SHA256_DIGEST_SIZE];
  uint8_t buffer[0x200];

  sha256_init(&hash);
  sha256_update(&hash, length, key);
  sha256_digest(&hash, SHA256_DIGEST_SIZE, digest);

  arcfour_set_key(ctx, SHA256_DIGEST_SIZE, digest);
  arcfour_crypt(ctx, sizeof(buffer), buffer, buffer);
}

Nettle defines ARCFOUR in <nettle/arcfour.h>.

Context struct: struct arcfour_ctx

Constant: ARCFOUR_MIN_KEY_SIZE

Minimum key size, 1.

Constant: ARCFOUR_MAX_KEY_SIZE

Maximum key size, 256.

Constant: ARCFOUR_KEY_SIZE

Default ARCFOUR key size, 16.

Function: void arcfour_set_key (struct arcfour_ctx *ctx, size_t length, const uint8_t *key)

Initialize the cipher. The same function is used for both encryption and decryption.

Function: void arcfour_crypt (struct arcfour_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Encrypt some data. The same function is used for both encryption and decryption. Unlike the block ciphers, this function modifies the context, so you can split the data into arbitrary chunks and encrypt them one after another. The result is the same as if you had called arcfour_crypt only once with all the data.

6.2.3 ARCTWO

ARCTWO (also known as the trade marked name RC2) is a block cipher specified in RFC 2268. Nettle also include a variation of the ARCTWO set key operation that lack one step, to be compatible with the reverse engineered RC2 cipher description, as described in a Usenet post to sci.crypt by Peter Gutmann.

ARCTWO uses a block size of 64 bits, and variable key-size ranging from 1 to 128 octets. Besides the key, ARCTWO also has a second parameter to key setup, the number of effective key bits, ekb. This parameter can be used to artificially reduce the key size. In practice, ekb is usually set equal to the input key size. Nettle defines ARCTWO in <nettle/arctwo.h>.

We do not recommend the use of ARCTWO; the Nettle implementation is provided primarily for interoperability with existing applications and standards.

Context struct: struct arctwo_ctx

Constant: ARCTWO_BLOCK_SIZE

The ARCTWO block-size, 8.

Constant: ARCTWO_MIN_KEY_SIZE

Constant: ARCTWO_MAX_KEY_SIZE

Constant: ARCTWO_KEY_SIZE

Default ARCTWO key size, 8.

Function: void arctwo_set_key_ekb (struct arctwo_ctx *ctx, size_t length, const uint8_t *key, unsigned ekb)

Function: void arctwo_set_key (struct arctwo_ctx *ctx, size_t length, const uint8_t *key)

Function: void arctwo_set_key_gutmann (struct arctwo_ctx *ctx, size_t length, const uint8_t *key)

Initialize the cipher. The same function is used for both encryption and decryption. The first function is the most general one, which lets you provide both the variable size key, and the desired effective key size (in bits). The maximum value for ekb is 1024, and for convenience, ekb = 0 has the same effect as ekb = 1024.

arctwo_set_key(ctx, length, key) is equivalent to arctwo_set_key_ekb(ctx, length, key, 8*length), and arctwo_set_key_gutmann(ctx, length, key) is equivalent to arctwo_set_key_ekb(ctx, length, key, 1024)

Function: void arctwo_encrypt (struct arctwo_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Encryption function. length must be an integral multiple of the block size. If it is more than one block, the data is processed in ECB mode. src and dst may be equal, but they must not overlap in any other way.

Function: void arctwo_decrypt (struct arctwo_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Analogous to arctwo_encrypt

6.2.4 BLOWFISH

BLOWFISH is a block cipher designed by Bruce Schneier. It uses a block size of 64 bits (8 octets), and a variable key size, up to 448 bits. It has some weak keys. Nettle defines BLOWFISH in <nettle/blowfish.h>.

Context struct: struct blowfish_ctx

Constant: BLOWFISH_BLOCK_SIZE

The BLOWFISH block-size, 8.

Constant: BLOWFISH_MIN_KEY_SIZE

Minimum BLOWFISH key size, 8.

Constant: BLOWFISH_MAX_KEY_SIZE

Maximum BLOWFISH key size, 56.

Constant: BLOWFISH_KEY_SIZE

Default BLOWFISH key size, 16.

Function: int blowfish_set_key (struct blowfish_ctx *ctx, size_t length, const uint8_t *key)

Initialize the cipher. The same function is used for both encryption and decryption. Checks for weak keys, returning 1 for good keys and 0 for weak keys. Applications that don’t care about weak keys can ignore the return value.

blowfish_encrypt or blowfish_decrypt with a weak key will crash with an assert violation.

Function: void blowfish_encrypt (struct blowfish_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Encryption function. length must be an integral multiple of the block size. If it is more than one block, the data is processed in ECB mode. src and dst may be equal, but they must not overlap in any other way.

Function: void blowfish_decrypt (struct blowfish_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Analogous to blowfish_encrypt

6.2.5 Camellia

Camellia is a block cipher developed by Mitsubishi and Nippon Telegraph and Telephone Corporation, described in RFC3713. It is recommended by some Japanese and European authorities as an alternative to AES, and it is one of the selected algorithms in the New European Schemes for Signatures, Integrity and Encryption (NESSIE) project. The algorithm is patented. The implementation in Nettle is derived from the implementation released by NTT under the GNU LGPL (v2.1 or later), and relies on the implicit patent license of the LGPL. There is also a statement of royalty-free licensing for Camellia at http://www.ntt.co.jp/news/news01e/0104/010417.html, but this statement has some limitations which seem problematic for free software.

Camellia uses a the same block size and key sizes as AES: The block size is 128 bits (16 octets), and the supported key sizes are 128, 192, and 256 bits. The variants with 192 and 256 bit keys are identical, except for the key setup. Nettle defines Camellia in <nettle/camellia.h>, and there is one context struct for each key size. (Earlier versions of Nettle used a single context struct, struct camellia_ctx, for all key sizes. This interface kept for backwards compatibility).

Context struct: struct camellia128_ctx

Context struct: struct camellia192_ctx

Context struct: struct camellia256_ctx

Contexts structs. Actually, camellia192_ctx is an alias for camellia256_ctx.

Context struct: struct camellia_ctx

Alternative struct, for the old Camellia interface.

Constant: CAMELLIA_BLOCK_SIZE

The CAMELLIA block-size, 16.

Constant: CAMELLIA128_KEY_SIZE

Constant: CAMELLIA192_KEY_SIZE

Constant: CAMELLIA256_KEY_SIZE

Constant: CAMELLIA_MIN_KEY_SIZE

Constant: CAMELLIA_MAX_KEY_SIZE

Constant: CAMELLIA_KEY_SIZE

Default CAMELLIA key size, 32.

Function: void camellia128_set_encrypt_key (struct camellia128_ctx *ctx, const uint8_t *key)

Function: void camellia128_set_decrypt_key (struct camellia128_ctx *ctx, const uint8_t *key)

Function: void camellia192_set_encrypt_key (struct camellia192_ctx *ctx, const uint8_t *key)

Function: void camellia192_set_decrypt_key (struct camellia192_ctx *ctx, const uint8_t *key)

Function: void camellia256_set_encrypt_key (struct camellia256_ctx *ctx, const uint8_t *key)

Function: void camellia256_set_decrypt_key (struct camellia256_ctx *ctx, const uint8_t *key)

Function: void camellia_set_encrypt_key (struct camellia_ctx *ctx, size_t length, const uint8_t *key)

Function: void camellia_set_decrypt_key (struct camellia_ctx *ctx, size_t length, const uint8_t *key)

Initialize the cipher, for encryption or decryption, respectively.

Function: void camellia128_invert_key (struct camellia128_ctx *dst, const struct camellia128_ctx *src)

Function: void camellia192_invert_key (struct camellia192_ctx *dst, const struct camellia192_ctx *src)

Function: void camellia256_invert_key (struct camellia256_ctx *dst, const struct camellia256_ctx *src)

Function: void camellia_invert_key (struct camellia_ctx *dst, const struct camellia_ctx *src)

Given a context src initialized for encryption, initializes the context struct dst for decryption, using the same key. If the same context struct is passed for both src and dst, it is converted in place. These functions are mainly useful for applications which needs to both encrypt and decrypt using the same key.

Function: void camellia128_crypt (struct camellia128_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Function: void camellia192_crypt (struct camellia192_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Function: void camellia256_crypt (struct camellia256_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Function: void camellia_crypt (struct camellia_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

The same function is used for both encryption and decryption. length must be an integral multiple of the block size. If it is more than one block, the data is processed in ECB mode. src and dst may be equal, but they must not overlap in any other way.

6.2.6 CAST128

CAST-128 is a block cipher, specified in RFC 2144. It uses a 64 bit (8 octets) block size, and a variable key size of up to 128 bits. Nettle defines cast128 in <nettle/cast128.h>.

Context struct: struct cast128_ctx

Constant: CAST128_BLOCK_SIZE

The CAST128 block-size, 8.

Constant: CAST128_MIN_KEY_SIZE

Minimum CAST128 key size, 5.

Constant: CAST128_MAX_KEY_SIZE

Maximum CAST128 key size, 16.

Constant: CAST128_KEY_SIZE

Default CAST128 key size, 16.

Function: void cast128_set_key (struct cast128_ctx *ctx, size_t length, const uint8_t *key)

Initialize the cipher. The same function is used for both encryption and decryption.

Function: void cast128_encrypt (struct cast128_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Encryption function. length must be an integral multiple of the block size. If it is more than one block, the data is processed in ECB mode. src and dst may be equal, but they must not overlap in any other way.

Function: void cast128_decrypt (struct cast128_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Analogous to cast128_encrypt

6.2.7 ChaCha

ChaCha is a variant of the stream cipher Salsa20, also designed by D. J. Bernstein. For more information on Salsa20, see below. Nettle defines ChaCha in <nettle/chacha.h>.

Context struct: struct chacha_ctx

Constant: CHACHA_KEY_SIZE

ChaCha key size, 32.

Constant: CHACHA_BLOCK_SIZE

ChaCha block size, 64.

Constant: CHACHA_NONCE_SIZE

Size of the nonce, 8.

Function: void chacha_set_key (struct chacha_ctx *ctx, const uint8_t *key)

Initialize the cipher. The same function is used for both encryption and decryption. Before using the cipher, you must also call chacha_set_nonce, see below.

Function: void chacha_set_nonce (struct chacha_ctx *ctx, const uint8_t *nonce)

Sets the nonce. It is always of size CHACHA_NONCE_SIZE, 8 octets. This function also initializes the block counter, setting it to zero.

Function: void chacha_crypt (struct chacha_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Encrypts or decrypts the data of a message, using ChaCha. When a message is encrypted using a sequence of calls to chacha_crypt, all but the last call must use a length that is a multiple of CHACHA_BLOCK_SIZE.

6.2.8 DES

DES is the old Data Encryption Standard, specified by NIST. It uses a block size of 64 bits (8 octets), and a key size of 56 bits. However, the key bits are distributed over 8 octets, where the least significant bit of each octet may be used for parity. A common way to use DES is to generate 8 random octets in some way, then set the least significant bit of each octet to get odd parity, and initialize DES with the resulting key.

The key size of DES is so small that keys can be found by brute force, using specialized hardware or lots of ordinary work stations in parallel. One shouldn’t be using plain DES at all today, if one uses DES at all one should be using “triple DES”, see DES3 below.

DES also has some weak keys. Nettle defines DES in <nettle/des.h>.

Context struct: struct des_ctx

Constant: DES_BLOCK_SIZE

The DES block-size, 8.

Constant: DES_KEY_SIZE

DES key size, 8.

Function: int des_set_key (struct des_ctx *ctx, const uint8_t *key)

Initialize the cipher. The same function is used for both encryption and decryption. Parity bits are ignored. Checks for weak keys, returning 1 for good keys and 0 for weak keys. Applications that don’t care about weak keys can ignore the return value.

Function: void des_encrypt (struct des_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Encryption function. length must be an integral multiple of the block size. If it is more than one block, the data is processed in ECB mode. src and dst may be equal, but they must not overlap in any other way.

Function: void des_decrypt (struct des_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Analogous to des_encrypt

Function: int des_check_parity (size_t length, const uint8_t *key);

Checks that the given key has correct, odd, parity. Returns 1 for correct parity, and 0 for bad parity.

Function: void des_fix_parity (size_t length, uint8_t *dst, const uint8_t *src)

Adjusts the parity bits to match DES’s requirements. You need this function if you have created a random-looking string by a key agreement protocol, and want to use it as a DES key. dst and src may be equal.

6.2.9 DES3

The inadequate key size of DES has already been mentioned. One way to increase the key size is to pipe together several DES boxes with independent keys. It turns out that using two DES ciphers is not as secure as one might think, even if the key size of the combination is a respectable 112 bits.

The standard way to increase DES’s key size is to use three DES boxes. The mode of operation is a little peculiar: the middle DES box is wired in the reverse direction. To encrypt a block with DES3, you encrypt it using the first 56 bits of the key, then decrypt it using the middle 56 bits of the key, and finally encrypt it again using the last 56 bits of the key. This is known as “ede” triple-DES, for “encrypt-decrypt-encrypt”.

The “ede” construction provides some backward compatibility, as you get plain single DES simply by feeding the same key to all three boxes. That should help keeping down the gate count, and the price, of hardware circuits implementing both plain DES and DES3.

DES3 has a key size of 168 bits, but just like plain DES, useless parity bits are inserted, so that keys are represented as 24 octets (192 bits). As a 112 bit key is large enough to make brute force attacks impractical, some applications uses a “two-key” variant of triple-DES. In this mode, the same key bits are used for the first and the last DES box in the pipe, while the middle box is keyed independently. The two-key variant is believed to be secure, i.e. there are no known attacks significantly better than brute force.

Naturally, it’s simple to implement triple-DES on top of Nettle’s DES functions. Nettle includes an implementation of three-key “ede” triple-DES, it is defined in the same place as plain DES, <nettle/des.h>.

Context struct: struct des3_ctx

Constant: DES3_BLOCK_SIZE

The DES3 block-size is the same as DES_BLOCK_SIZE, 8.

Constant: DES3_KEY_SIZE

DES key size, 24.

Function: int des3_set_key (struct des3_ctx *ctx, const uint8_t *key)

Initialize the cipher. The same function is used for both encryption and decryption. Parity bits are ignored. Checks for weak keys, returning 1 if all three keys are good keys, and 0 if one or more key is weak. Applications that don’t care about weak keys can ignore the return value.

For random-looking strings, you can use des_fix_parity to adjust the parity bits before calling des3_set_key.

Function: void des3_encrypt (struct des3_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Encryption function. length must be an integral multiple of the block size. If it is more than one block, the data is processed in ECB mode. src and dst may be equal, but they must not overlap in any other way.

Function: void des3_decrypt (struct des3_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Analogous to des_encrypt

6.2.10 Salsa20

Salsa20 is a fairly recent stream cipher designed by D. J. Bernstein. It is built on the observation that a cryptographic hash function can be used for encryption: Form the hash input from the secret key and a counter, xor the hash output and the first block of the plaintext, then increment the counter to process the next block (similar to CTR mode, see see CTR). Bernstein defined an encryption algorithm, Snuffle, in this way to ridicule United States export restrictions which treated hash functions as nice and harmless, but ciphers as dangerous munitions.

Salsa20 uses the same idea, but with a new specialized hash function to mix key, block counter, and a couple of constants. It’s also designed for speed; on x86_64, it is currently the fastest cipher offered by nettle. It uses a block size of 512 bits (64 octets) and there are two specified key sizes, 128 and 256 bits (16 and 32 octets).

Caution: The hash function used in Salsa20 is not directly applicable for use as a general hash function. It’s not collision resistant if arbitrary inputs are allowed, and furthermore, the input and output is of fixed size.

When using Salsa20 to process a message, one specifies both a key and a nonce, the latter playing a similar rôle to the initialization vector (IV) used with CBC or CTR mode. One can use the same key for several messages, provided one uses a unique random iv for each message. The iv is 64 bits (8 octets). The block counter is initialized to zero for each message, and is also 64 bits (8 octets). Nettle defines Salsa20 in <nettle/salsa20.h>.

Context struct: struct salsa20_ctx

Constant: SALSA20_128_KEY_SIZE

Constant: SALSA20_256_KEY_SIZE

The two supported key sizes, 16 and 32 octets.

Constant: SALSA20_KEY_SIZE

Recommended key size, 32.

Constant: SALSA20_BLOCK_SIZE

Salsa20 block size, 64.

Constant: SALSA20_NONCE_SIZE

Size of the nonce, 8.

Function: void salsa20_128_set_key (struct salsa20_ctx *ctx, const uint8_t *key)

Function: void salsa20_256_set_key (struct salsa20_ctx *ctx, const uint8_t *key)

Function: void salsa20_set_key (struct salsa20_ctx *ctx, size_t length, const uint8_t *key)

Initialize the cipher. The same function is used for both encryption and decryption. salsa20_128_set_key and salsa20_128_set_key use a fix key size each, 16 and 32 octets, respectively. The function salsa20_set_key is provided for backwards compatibility, and the length argument must be either 16 or 32. Before using the cipher, you must also call salsa20_set_nonce, see below.

Function: void salsa20_set_nonce (struct salsa20_ctx *ctx, const uint8_t *nonce)

Sets the nonce. It is always of size SALSA20_NONCE_SIZE, 8 octets. This function also initializes the block counter, setting it to zero.

Function: void salsa20_crypt (struct salsa20_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Encrypts or decrypts the data of a message, using salsa20. When a message is encrypted using a sequence of calls to salsa20_crypt, all but the last call must use a length that is a multiple of SALSA20_BLOCK_SIZE.

The full salsa20 cipher uses 20 rounds of mixing. Variants of Salsa20 with fewer rounds are possible, and the 12-round variant is specified by eSTREAM, see http://www.ecrypt.eu.org/stream/finallist.html. Nettle calls this variant salsa20r12. It uses the same context struct and key setup as the full salsa20 cipher, but a separate function for encryption and decryption.

Function: void salsa20r12_crypt (struct salsa20_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Encrypts or decrypts the data of a message, using salsa20 reduced to 12 rounds.

6.2.11 SERPENT

SERPENT is one of the AES finalists, designed by Ross Anderson, Eli Biham and Lars Knudsen. Thus, the interface and properties are similar to AES’. One peculiarity is that it is quite pointless to use it with anything but the maximum key size, smaller keys are just padded to larger ones. Nettle defines SERPENT in <nettle/serpent.h>.

Context struct: struct serpent_ctx

Constant: SERPENT_BLOCK_SIZE

The SERPENT block-size, 16.

Constant: SERPENT_MIN_KEY_SIZE

Minimum SERPENT key size, 16.

Constant: SERPENT_MAX_KEY_SIZE

Maximum SERPENT key size, 32.

Constant: SERPENT_KEY_SIZE

Default SERPENT key size, 32.

Function: void serpent_set_key (struct serpent_ctx *ctx, size_t length, const uint8_t *key)

Initialize the cipher. The same function is used for both encryption and decryption.

Function: void serpent_encrypt (struct serpent_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Encryption function. length must be an integral multiple of the block size. If it is more than one block, the data is processed in ECB mode. src and dst may be equal, but they must not overlap in any other way.

Function: void serpent_decrypt (struct serpent_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Analogous to serpent_encrypt

6.2.12 TWOFISH

Another AES finalist, this one designed by Bruce Schneier and others. Nettle defines it in <nettle/twofish.h>.

Context struct: struct twofish_ctx

Constant: TWOFISH_BLOCK_SIZE

The TWOFISH block-size, 16.

Constant: TWOFISH_MIN_KEY_SIZE

Minimum TWOFISH key size, 16.

Constant: TWOFISH_MAX_KEY_SIZE

Maximum TWOFISH key size, 32.

Constant: TWOFISH_KEY_SIZE

Default TWOFISH key size, 32.

Function: void twofish_set_key (struct twofish_ctx *ctx, size_t length, const uint8_t *key)

Initialize the cipher. The same function is used for both encryption and decryption.

Function: void twofish_encrypt (struct twofish_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Encryption function. length must be an integral multiple of the block size. If it is more than one block, the data is processed in ECB mode. src and dst may be equal, but they must not overlap in any other way.

Function: void twofish_decrypt (struct twofish_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Analogous to twofish_encrypt

6.2.13 The struct nettle_cipher abstraction

Nettle includes a struct including information about some of the more regular cipher functions. It can be useful for applications that need a simple way to handle various algorithms. Nettle defines these structs in <nettle/nettle-meta.h>.

Meta struct: struct nettle_cipher name context_size block_size key_size set_encrypt_key set_decrypt_key encrypt decrypt

The last four attributes are function pointers, of types nettle_set_key_func * and nettle_cipher_func *. The first argument to these functions is a const void * pointer to a context struct, which is of size context_size.

Constant Struct: struct nettle_cipher nettle_aes128

Constant Struct: struct nettle_cipher nettle_aes192

Constant Struct: struct nettle_cipher nettle_aes256

Constant Struct: struct nettle_cipher nettle_arctwo40

Constant Struct: struct nettle_cipher nettle_arctwo64

Constant Struct: struct nettle_cipher nettle_arctwo128

Constant Struct: struct nettle_cipher nettle_arctwo_gutmann128

Constant Struct: struct nettle_cipher nettle_arcfour128

Constant Struct: struct nettle_cipher nettle_camellia128

Constant Struct: struct nettle_cipher nettle_camellia192

Constant Struct: struct nettle_cipher nettle_camellia256

Constant Struct: struct nettle_cipher nettle_cast128

Constant Struct: struct nettle_cipher nettle_serpent128

Constant Struct: struct nettle_cipher nettle_serpent192

Constant Struct: struct nettle_cipher nettle_serpent256

Constant Struct: struct nettle_cipher nettle_twofish128

Constant Struct: struct nettle_cipher nettle_twofish192

Constant Struct: struct nettle_cipher nettle_twofish256

Nettle includes such structs for all the regular ciphers, i.e. ones without weak keys or other oddities.

Nettle also exports a list of all these ciphers without weak keys or other oddities.

Function: const struct nettle_cipher **nettle_get_ciphers(void)

Returns a NULL-terminated list of pointers to supported block ciphers. This list can be used to dynamically enumerate or search the supported algorithms.

Macro: nettle_ciphers

A macro expanding to a call to nettle_get_ciphers. In earlier versions, this was not a macro but the actual array of pointers.

6.3 Cipher modes

Cipher modes of operation specifies the procedure to use when encrypting a message that is larger than the cipher’s block size. As explained in See Cipher functions, splitting the message into blocks and processing them independently with the block cipher (Electronic Code Book mode, ECB), leaks information.

Besides ECB, Nettle provides several other modes of operation: Cipher Block Chaining (CBC), Counter mode (CTR), Cipher Feedback (CFB) and a couple of AEAD modes (see Authenticated encryption). CBC is widely used, but there are a few subtle issues of information leakage, see, e.g., SSH CBC vulnerability. Today, CTR is usually preferred over CBC.

Modes like CBC, CTR and CFB provide no message authentication, and should always be used together with a MAC (see Keyed hash functions) or signature to authenticate the message.

6.3.1 Cipher Block Chaining

When using CBC mode, plaintext blocks are not encrypted independently of each other, like in Electronic Cook Book mode. Instead, when encrypting a block in CBC mode, the previous ciphertext block is XORed with the plaintext before it is fed to the block cipher. When encrypting the first block, a random block called an IV, or Initialization Vector, is used as the “previous ciphertext block”. The IV should be chosen randomly, but it need not be kept secret, and can even be transmitted in the clear together with the encrypted data.

In symbols, if E_k is the encryption function of a block cipher, and IV is the initialization vector, then n plaintext blocks M_1,… M_n are transformed into n ciphertext blocks C_1,… C_n as follows:

C_1 = E_k(IV  XOR M_1)
C_2 = E_k(C_1 XOR M_2)

…

C_n = E_k(C_(n-1) XOR M_n)

Nettle’s includes two functions for applying a block cipher in Cipher Block Chaining (CBC) mode, one for encryption and one for decryption. These functions uses void * to pass cipher contexts around.

Function: void cbc_encrypt (const void *ctx, nettle_cipher_func *f, size_t block_size, uint8_t *iv, size_t length, uint8_t *dst, const uint8_t *src)

Function: void cbc_decrypt (const void *ctx, nettle_cipher_func *f, size_t block_size, uint8_t *iv, size_t length, uint8_t *dst, const uint8_t *src)

Applies the encryption or decryption function f in CBC mode. The final ciphertext block processed is copied into iv before returning, so that a large message can be processed by a sequence of calls to cbc_encrypt. The function f is of type

void f (void *ctx, size_t length, uint8_t dst, const uint8_t *src),

and the cbc_encrypt and cbc_decrypt functions pass their argument ctx on to f.

There are also some macros to help use these functions correctly.

Macro: CBC_CTX (context_type, block_size)

Expands to

{
   context_type ctx;
   uint8_t iv[block_size];
}

It can be used to define a CBC context struct, either directly,

struct CBC_CTX(struct aes_ctx, AES_BLOCK_SIZE) ctx;

or to give it a struct tag,

struct aes_cbc_ctx CBC_CTX (struct aes_ctx, AES_BLOCK_SIZE);

Macro: CBC_SET_IV (ctx, iv)

First argument is a pointer to a context struct as defined by CBC_CTX, and the second is a pointer to an Initialization Vector (IV) that is copied into that context.

Macro: CBC_ENCRYPT (ctx, f, length, dst, src)

Macro: CBC_DECRYPT (ctx, f, length, dst, src)

A simpler way to invoke cbc_encrypt and cbc_decrypt. The first argument is a pointer to a context struct as defined by CBC_CTX, and the second argument is an encryption or decryption function following Nettle’s conventions. The last three arguments define the source and destination area for the operation.

These macros use some tricks to make the compiler display a warning if the types of f and ctx don’t match, e.g. if you try to use an struct aes_ctx context with the des_encrypt function.

6.3.2 Counter mode

Counter mode (CTR) uses the block cipher as a keyed pseudo-random generator. The output of the generator is XORed with the data to be encrypted. It can be understood as a way to transform a block cipher to a stream cipher.

The message is divided into n blocks M_1,… M_n, where M_n is of size m which may be smaller than the block size. Except for the last block, all the message blocks must be of size equal to the cipher’s block size.

If E_k is the encryption function of a block cipher, IC is the initial counter, then the n plaintext blocks are transformed into n ciphertext blocks C_1,… C_n as follows:

C_1 = E_k(IC) XOR M_1
C_2 = E_k(IC + 1) XOR M_2

…

C_(n-1) = E_k(IC + n - 2) XOR M_(n-1)
C_n = E_k(IC + n - 1) [1..m] XOR M_n

The IC is the initial value for the counter, it plays a similar rôle as the IV for CBC. When adding, IC + x, IC is interpreted as an integer, in network byte order. For the last block, E_k(IC + n - 1) [1..m] means that the cipher output is truncated to m bytes.

Function: void ctr_crypt (const void *ctx, nettle_cipher_func *f, size_t block_size, uint8_t *ctr, size_t length, uint8_t *dst, const uint8_t *src)

Applies the encryption function f in CTR mode. Note that for CTR mode, encryption and decryption is the same operation, and hence f should always be the encryption function for the underlying block cipher.

When a message is encrypted using a sequence of calls to ctr_crypt, all but the last call must use a length that is a multiple of the block size.

Like for CBC, there are also a couple of helper macros.

Macro: CTR_CTX (context_type, block_size)

Expands to

{
   context_type ctx;
   uint8_t ctr[block_size];
}

Macro: CTR_SET_COUNTER (ctx, iv)

First argument is a pointer to a context struct as defined by CTR_CTX, and the second is a pointer to an initial counter that is copied into that context.

Macro: CTR_CRYPT (ctx, f, length, dst, src)

A simpler way to invoke ctr_crypt. The first argument is a pointer to a context struct as defined by CTR_CTX, and the second argument is an encryption function following Nettle’s conventions. The last three arguments define the source and destination area for the operation.

6.3.3 Cipher Feedback mode

Cipher Feedback mode (CFB) being a close relative to both CBC mode and CTR mode borrows some characteristics from stream ciphers.

The message is divided into n blocks M_1,… M_n, where M_n is of size m which may be smaller than the block size. Except for the last block, all the message blocks must be of size equal to the cipher’s block size.

If E_k is the encryption function of a block cipher, IV is the initialization vector, then the n plaintext blocks are transformed into n ciphertext blocks C_1,… C_n as follows:

C_1 = E_k(IV) XOR M_1
C_2 = E_k(C_1) XOR M_2

…

C_(n-1) = E_k(C_(n - 2)) XOR M_(n-1)
C_n = E_k(C_(n - 1)) [1..m] XOR M_n

Nettle’s includes two functions for applying a block cipher in Cipher Feedback (CFB) mode, one for encryption and one for decryption. These functions uses void * to pass cipher contexts around.

Function: void cfb_encrypt (const void *ctx, nettle_cipher_func *f, size_t block_size, uint8_t *iv, size_t length, uint8_t *dst, const uint8_t *src)

Function: void cfb_decrypt (const void *ctx, nettle_cipher_func *f, size_t block_size, uint8_t *iv, size_t length, uint8_t *dst, const uint8_t *src)

Applies the encryption or decryption function f in CFB mode. The final ciphertext block processed is copied into iv before returning, so that a large message can be processed by a sequence of calls to cfb_encrypt. Note that for CFB mode internally uses encryption only function and hence f should always be the encryption function for the underlying block cipher.

When a message is encrypted using a sequence of calls to cfb_encrypt, all but the last call must use a length that is a multiple of the block size.

Like for CBC, there are also a couple of helper macros.

Macro: CFB_CTX (context_type, block_size)

Expands to

{
   context_type ctx;
   uint8_t iv[block_size];
}

Macro: CFB_SET_IV(ctx, iv)

First argument is a pointer to a context struct as defined by CFB_CTX, and the second is a pointer to an initialization vector that is copied into that context.

Macro: CFB_ENCRYPT (ctx, f, length, dst, src)

A simpler way to invoke cfb_encrypt. The first argument is a pointer to a context struct as defined by CFB_CTX, and the second argument is an encryption function following Nettle’s conventions. The last three arguments define the source and destination area for the operation.

Macro: CFB_DECRYPT (ctx, f, length, dst, src)

A simpler way to invoke cfb_decrypt. The first argument is a pointer to a context struct as defined by CFB_CTX, and the second argument is an encryption function following Nettle’s conventions. The last three arguments define the source and destination area for the operation.

6.4 Authenticated encryption with associated data

Since there are some subtle design choices to be made when combining a block cipher mode with out authentication with a MAC. In recent years, several constructions that combine encryption and authentication have been defined. These constructions typically also have an additional input, the “associated data”, which is authenticated but not included with the message. A simple example is an implicit message number which is available at both sender and receiver, and which needs authentication in order to detect deletions or replay of messages. This family of building blocks are therefore called AEAD, Authenticated encryption with associated data.

The aim is to provide building blocks that it is easier for designers of protocols and applications to use correctly. There is also some potential for improved performance, if encryption and authentication can be done in a single step, although that potential is not realized for the constructions currently supported by Nettle.

For encryption, the inputs are:

• The key, which can be used for many messages.
• A nonce, which must be unique for each message using the same key.
• Additional associated data to be authenticated, but not included in the message.
• The cleartext message to be encrypted.

The outputs are:

• The ciphertext, of the same size as the cleartext.
• A digest or “authentication tag”.

Decryption works the same, but with cleartext and ciphertext interchanged. All currently supported AEAD algorithms always use the encryption function of the underlying block cipher, for both encryption and decryption.

Usually, the authentication tag should be appended at the end of the ciphertext, producing an encrypted message which is slightly longer than the cleartext. However, Nettle’s low level AEAD functions produce the authentication tag as a separate output for both encryption and decryption.

Both associated data and the message data (cleartext or ciphertext) can be processed incrementally. In general, all associated data must be processed before the message data, and all calls but the last one must use a length that is a multiple of the block size, although some AEAD may implement more liberal conventions. The CCM mode is a bit special in that it requires the message lengths up front, other AEAD constructions don’t have this restriction.

The supported AEAD constructions are Galois/Counter mode (GCM), EAX, ChaCha-Poly1305, and Counter with CBC-MAC (CCM). There are some weaknesses in GCM authentication, see http://csrc.nist.gov/groups/ST/toolkit/BCM/documents/comments/CWC-GCM/Ferguson2.pdf. CCM and EAX use the same building blocks, but the EAX design is cleaner and avoids a couple of inconveniences of CCM. Therefore, EAX seems like a good conservative choice. The more recent ChaCha-Poly1305 may also be an attractive but more adventurous alternative, in particular if performance is important.

6.4.1 EAX

The EAX mode is an AEAD mode whichcombines CTR mode encryption, See CTR, with a message authentication based on CBC, See CBC. The implementation in Nettle is restricted to ciphers with a block size of 128 bits (16 octets). EAX was defined as a reaction to the CCM mode, See CCM, which uses the same primitives but has some undesirable and inelegant properties.

EAX supports arbitrary nonce size; it’s even possible to use an empty nonce in case only a single message is encrypted for each key.

Nettle’s support for EAX consists of a low-level general interface, some convenience macros, and specific functions for EAX using AES-128 as the underlying cipher. These interfaces are defined in <nettle/eax.h>

6.4.1.1 General EAX interface

Context struct: struct eax_key

EAX state which depends only on the key, but not on the nonce or the message.

Context struct: struct eax_ctx

Holds state corresponding to a particular message.

Constant: EAX_BLOCK_SIZE

EAX’s block size, 16.

Constant: EAX_DIGEST_SIZE

Size of the EAX digest, also 16.

Function: void eax_set_key (struct eax_key *key, const void *cipher, nettle_cipher_func *f)

Initializes key. cipher gives a context struct for the underlying cipher, which must have been previously initialized for encryption, and f is the encryption function.

Function: void eax_set_nonce (struct eax_ctx *eax, const struct eax_key *key, const void *cipher, nettle_cipher_func *f, size_t nonce_length, const uint8_t *nonce)

Initializes ctx for processing a new message, using the given nonce.

Function: void eax_update (struct eax_ctx *eax, const struct eax_key *key, const void *cipher, nettle_cipher_func *f, size_t data_length, const uint8_t *data)

Process associated data for authentication. All but the last call for each message must use a length that is a multiple of the block size. Unlike many other AEAD constructions, for EAX it’s not necessary to complete the processing of all associated data before encrypting or decrypting the message data.

Function: void eax_encrypt (struct eax_ctx *eax, const struct eax_key *key, const void *cipher, nettle_cipher_func *f, size_t length, uint8_t *dst, const uint8_t *src)

Function: void eax_decrypt (struct eax_ctx *eax, const struct eax_key *key, const void *cipher, nettle_cipher_func *f, size_t length, uint8_t *dst, const uint8_t *src)

Encrypts or decrypts the data of a message. cipher is the context struct for the underlying cipher and f is the encryption function. All but the last call for each message must use a length that is a multiple of the block size.

Function: void eax_digest (struct eax_ctx *eax, const struct eax_key *key, const void *cipher, nettle_cipher_func *f, size_t length, uint8_t *digest);

Extracts the message digest (also known “authentication tag”). This is the final operation when processing a message. If length is smaller than EAX_DIGEST_SIZE, only the first length octets of the digest are written.

6.4.1.2 EAX helper macros

The following macros are defined.

Macro: EAX_CTX (context_type)

This defines an all-in-one context struct, including the context of the underlying cipher and all EAX state. It expands to

{
   struct eax_key key;
   struct eax_ctx eax;
   context_type cipher;
}

For all these macros, ctx, is a context struct as defined by EAX_CTX, and encrypt is the encryption function of the underlying cipher.

Macro: EAX_SET_KEY (ctx, set_key, encrypt, key)

set_key is the function for setting the encryption key for the underlying cipher, and key is the key.

Macro: EAX_SET_NONCE (ctx, encrypt, length, nonce)

Sets the nonce to be used for the message.

Macro: EAX_UPDATE (ctx, encrypt, length, data)

Process associated data for authentication.

Macro: EAX_ENCRYPT (ctx, encrypt, length, dst, src)

Macro: EAX_DECRYPT (ctx, encrypt, length, dst, src)

Process message data for encryption or decryption.

Macro: EAX_DIGEST (ctx, encrypt, length, digest)

Extract te authentication tag for the message.

6.4.1.3 EAX-AES128 interface

The following functions implement EAX using AES-128 as the underlying cipher.

Context struct: struct eax_aes128_ctx

The context struct, defined using EAX_CTX.

Function: void eax_aes128_set_key (struct eax_aes128_ctx *ctx, const uint8_t *key)

Initializes ctx using the given key.

Function: void eax_aes128_set_nonce (struct eax_aes128_ctx *ctx, size_t length, const uint8_t *iv)

Initializes the per-message state, using the given nonce.

Function: void eax_aes128_update (struct eax_aes128_ctx *ctx, size_t length, const uint8_t *data)

Process associated data for authentication. All but the last call for each message must use a length that is a multiple of the block size.

Function: void eax_aes128_encrypt (struct eax_aes128_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Function: void eax_aes128_decrypt (struct eax_aes128_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Encrypts or decrypts the data of a message. All but the last call for each message must use a length that is a multiple of the block size.

Function: void eax_aes128_digest (struct eax_aes128_ctx *ctx, size_t length, uint8_t *digest);

Extracts the message digest (also known “authentication tag”). This is the final operation when processing a message. If length is smaller than EAX_DIGEST_SIZE, only the first length octets of the digest are written.

6.4.2 Galois counter mode

Galois counter mode is an AEAD constructions combining counter mode with message authentication based on universal hashing. The main objective of the design is to provide high performance for hardware implementations, where other popular MAC algorithms (see Keyed hash functions) become a bottleneck for high-speed hardware implementations. It was proposed by David A. McGrew and John Viega in 2005, and recommended by NIST in 2007, NIST Special Publication 800-38D. It is constructed on top of a block cipher which must have a block size of 128 bits.

The authentication in GCM has some known weaknesses, see http://csrc.nist.gov/groups/ST/toolkit/BCM/documents/comments/CWC-GCM/Ferguson2.pdf. In particular, don’t use GCM with short authentication tags.

Nettle’s support for GCM consists of a low-level general interface, some convenience macros, and specific functions for GCM using AES or Camellia as the underlying cipher. These interfaces are defined in <nettle/gcm.h>

6.4.2.1 General GCM interface

Context struct: struct gcm_key

Message independent hash sub-key, and related tables.

Context struct: struct gcm_ctx

Holds state corresponding to a particular message.

Constant: GCM_BLOCK_SIZE

GCM’s block size, 16.

Constant: GCM_DIGEST_SIZE

Size of the GCM digest, also 16.

Constant: GCM_IV_SIZE

Recommended size of the IV, 12. Arbitrary sizes are allowed.

Function: void gcm_set_key (struct gcm_key *key, const void *cipher, nettle_cipher_func *f)

Initializes key. cipher gives a context struct for the underlying cipher, which must have been previously initialized for encryption, and f is the encryption function.

Function: void gcm_set_iv (struct gcm_ctx *ctx, const struct gcm_key *key, size_t length, const uint8_t *iv)

Initializes ctx using the given IV. The key argument is actually needed only if length differs from GCM_IV_SIZE.

Function: void gcm_update (struct gcm_ctx *ctx, const struct gcm_key *key, size_t length, const uint8_t *data)

Provides associated data to be authenticated. If used, must be called before gcm_encrypt or gcm_decrypt. All but the last call for each message must use a length that is a multiple of the block size.

Function: void gcm_encrypt (struct gcm_ctx *ctx, const struct gcm_key *key, const void *cipher, nettle_cipher_func *f, size_t length, uint8_t *dst, const uint8_t *src)

Function: void gcm_decrypt (struct gcm_ctx *ctx, const struct gcm_key *key, const void *cipher, nettle_cipher_func *f, size_t length, uint8_t *dst, const uint8_t *src)

Encrypts or decrypts the data of a message. cipher is the context struct for the underlying cipher and f is the encryption function. All but the last call for each message must use a length that is a multiple of the block size.

Function: void gcm_digest (struct gcm_ctx *ctx, const struct gcm_key *key, const void *cipher, nettle_cipher_func *f, size_t length, uint8_t *digest)

Extracts the message digest (also known “authentication tag”). This is the final operation when processing a message. It’s strongly recommended that length is GCM_DIGEST_SIZE, but if you provide a smaller value, only the first length octets of the digest are written.

To encrypt a message using GCM, first initialize a context for the underlying block cipher with a key to use for encryption. Then call the above functions in the following order: gcm_set_key, gcm_set_iv, gcm_update, gcm_encrypt, gcm_digest. The decryption procedure is analogous, just calling gcm_decrypt instead of gcm_encrypt (note that GCM decryption still uses the encryption function of the underlying block cipher). To process a new message, using the same key, call gcm_set_iv with a new iv.

6.4.2.2 GCM helper macros

The following macros are defined.

Macro: GCM_CTX (context_type)

This defines an all-in-one context struct, including the context of the underlying cipher, the hash sub-key, and the per-message state. It expands to

{
   struct gcm_key key; 
   struct gcm_ctx gcm;
   context_type cipher;
}

Example use:

struct gcm_aes128_ctx GCM_CTX(struct aes128_ctx);

The following macros operate on context structs of this form.

Macro: GCM_SET_KEY (ctx, set_key, encrypt, key)

First argument, ctx, is a context struct as defined by GCM_CTX. set_key and encrypt are functions for setting the encryption key and for encrypting data using the underlying cipher.

Macro: GCM_SET_IV (ctx, length, data)

First argument is a context struct as defined by GCM_CTX. length and data give the initialization vector (IV).

Macro: GCM_UPDATE (ctx, length, data)

Simpler way to call gcm_update. First argument is a context struct as defined by GCM_CTX

Macro: GCM_ENCRYPT (ctx, encrypt, length, dst, src)

Macro: GCM_DECRYPT (ctx, encrypt, length, dst, src)

Macro: GCM_DIGEST (ctx, encrypt, length, digest)

Simpler way to call gcm_encrypt, gcm_decrypt or gcm_digest. First argument is a context struct as defined by GCM_CTX. Second argument, encrypt, is the encryption function of the underlying cipher.

6.4.2.3 GCM-AES interface

The following functions implement the common case of GCM using AES as the underlying cipher. The variants with a specific AES flavor are recommended, while the fucntinos using struct gcm_aes_ctx are kept for compatibility with older versiosn of Nettle.

Context struct: struct gcm_aes128_ctx

Context struct: struct gcm_aes192_ctx

Context struct: struct gcm_aes256_ctx

Context structs, defined using GCM_CTX.

Context struct: struct gcm_aes_ctx

Alternative context struct, usign the old AES interface.

Function: void gcm_aes128_set_key (struct gcm_aes128_ctx *ctx, const uint8_t *key)

Function: void gcm_aes192_set_key (struct gcm_aes192_ctx *ctx, const uint8_t *key)

Function: void gcm_aes256_set_key (struct gcm_aes256_ctx *ctx, const uint8_t *key)

Initializes ctx using the given key.

Function: void gcm_aes_set_key (struct gcm_aes_ctx *ctx, size_t length, const uint8_t *key)

Corresponding function, using the old AES interface. All valid AES key sizes can be used.

Function: void gcm_aes128_set_iv (struct gcm_aes128_ctx *ctx, size_t length, const uint8_t *iv)

Function: void gcm_aes192_set_iv (struct gcm_aes192_ctx *ctx, size_t length, const uint8_t *iv)

Function: void gcm_aes256_set_iv (struct gcm_aes256_ctx *ctx, size_t length, const uint8_t *iv)

Function: void gcm_aes_set_iv (struct gcm_aes_ctx *ctx, size_t length, const uint8_t *iv)

Initializes the per-message state, using the given IV.

Function: void gcm_aes128_update (struct gcm_aes128_ctx *ctx, size_t length, const uint8_t *data)

Function: void gcm_aes192_update (struct gcm_aes192_ctx *ctx, size_t length, const uint8_t *data)

Function: void gcm_aes256_update (struct gcm_aes256_ctx *ctx, size_t length, const uint8_t *data)

Function: void gcm_aes_update (struct gcm_aes_ctx *ctx, size_t length, const uint8_t *data)

Provides associated data to be authenticated. If used, must be called before gcm_aes_encrypt or gcm_aes_decrypt. All but the last call for each message must use a length that is a multiple of the block size.

Function: void gcm_aes128_encrypt (struct gcm_aes128_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Function: void gcm_aes192_encrypt (struct gcm_aes192_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Function: void gcm_aes256_encrypt (struct gcm_aes256_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Function: void gcm_aes_encrypt (struct gcm_aes_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Function: void gcm_aes128_decrypt (struct gcm_aes128_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Function: void gcm_aes192_decrypt (struct gcm_aes192_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Function: void gcm_aes256_decrypt (struct gcm_aes256_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Function: void gcm_aes_decrypt (struct gcm_aes_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Encrypts or decrypts the data of a message. All but the last call for each message must use a length that is a multiple of the block size.

Function: void gcm_aes128_digest (struct gcm_aes128_ctx *ctx, size_t length, uint8_t *digest)

Function: void gcm_aes192_digest (struct gcm_aes192_ctx *ctx, size_t length, uint8_t *digest)

Function: void gcm_aes256_digest (struct gcm_aes256_ctx *ctx, size_t length, uint8_t *digest)

Function: void gcm_aes_digest (struct gcm_aes_ctx *ctx, size_t length, uint8_t *digest)

Extracts the message digest (also known “authentication tag”). This is the final operation when processing a message. It’s strongly recommended that length is GCM_DIGEST_SIZE, but if you provide a smaller value, only the first length octets of the digest are written.

6.4.2.4 GCM-Camellia interface

The following functions implement the case of GCM using Camellia as the underlying cipher.

Context struct: struct gcm_camellia128_ctx

Context struct: struct gcm_camellia256_ctx

Context structs, defined using GCM_CTX.

Function: void gcm_camellia128_set_key (struct gcm_camellia128_ctx *ctx, const uint8_t *key)

Function: void gcm_camellia256_set_key (struct gcm_camellia256_ctx *ctx, const uint8_t *key)

Initializes ctx using the given key.

Function: void gcm_camellia128_set_iv (struct gcm_camellia128_ctx *ctx, size_t length, const uint8_t *iv)

Function: void gcm_camellia256_set_iv (struct gcm_camellia256_ctx *ctx, size_t length, const uint8_t *iv)

Initializes the per-message state, using the given IV.

Function: void gcm_camellia128_update (struct gcm_camellia128_ctx *ctx, size_t length, const uint8_t *data)

Function: void gcm_camellia256_update (struct gcm_camellia256_ctx *ctx, size_t length, const uint8_t *data)

Provides associated data to be authenticated. If used, must be called before gcm_camellia_encrypt or gcm_camellia_decrypt. All but the last call for each message must use a length that is a multiple of the block size.

Function: void gcm_camellia128_encrypt (struct gcm_camellia128_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Function: void gcm_camellia256_encrypt (struct gcm_camellia256_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Function: void gcm_camellia128_decrypt (struct gcm_camellia128_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Function: void gcm_camellia256_decrypt (struct gcm_camellia256_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Encrypts or decrypts the data of a message. All but the last call for each message must use a length that is a multiple of the block size.

Function: void gcm_camellia128_digest (struct gcm_camellia128_ctx *ctx, size_t length, uint8_t *digest)

Function: void gcm_camellia192_digest (struct gcm_camellia192_ctx *ctx, size_t length, uint8_t *digest)

Function: void gcm_camellia256_digest (struct gcm_camellia256_ctx *ctx, size_t length, uint8_t *digest)

Function: void gcm_camellia_digest (struct gcm_camellia_ctx *ctx, size_t length, uint8_t *digest)

Extracts the message digest (also known “authentication tag”). This is the final operation when processing a message. It’s strongly recommended that length is GCM_DIGEST_SIZE, but if you provide a smaller value, only the first length octets of the digest are written.

6.4.3 Counter with CBC-MAC mode

CCM mode is a combination of counter mode with message authentication based on cipher block chaining, the same building blocks as EAX, see EAX. It is constructed on top of a block cipher which must have a block size of 128 bits. CCM mode is recommended by NIST in NIST Special Publication 800-38C. Nettle’s support for CCM consists of a low-level general interface, a message encryption and authentication interface, and specific functions for CCM using AES as the underlying block cipher. These interfaces are defined in <nettle/ccm.h>.

In CCM, the length of the message must be known before processing. The maximum message size depends on the size of the nonce, since the message size is encoded in a field which must fit in a single block, together with the nonce and a flag byte. E.g., with a nonce size of 12 octets, there are three octets left for encoding the message length, the maximum message length is 2^24 - 1 octets.

CCM mode encryption operates as follows:

• The nonce and message length are concatenated to create B_0 = flags | nonce | mlength
• The authenticated data and plaintext is formatted into the string B = L(adata) | adata | padding | plaintext | padding with padding being the shortest string of zero bytes such that the length of the string is a multiple of the block size, and L(adata) is an encoding of the length of adata.
• The string B is separated into blocks B_1 ... B_n
• The authentication tag T is calculated as T=0, for i=0 to n, do T = E_k(B_i XOR T)
• An initial counter is then initialized from the nonce to create IC = flags | nonce | padding, where padding is the shortest string of zero bytes such that IC is exactly one block in length.
• The authentication tag is encrypted using using CTR mode: MAC = E_k(IC) XOR T
• The plaintext is then encrypted using CTR mode with an initial counter of IC+1.

CCM mode decryption operates similarly, except that the ciphertext and MAC are first decrypted using CTR mode to retreive the plaintext and authentication tag. The authentication tag can then be recalucated from the authenticated data and plantext, and compared to the value in the message to check for authenticity.

6.4.3.1 General CCM interface

For all of the functions in the CCM interface, cipher is the context struct for the underlying cipher and f is the encryption function. The cipher’s encryption key must be set before calling any of the CCM functions. The cipher’s decryption function and key are never used.

Context struct: struct ccm_ctx

Holds state corresponding to a particular message.

Constant: CCM_BLOCK_SIZE

CCM’s block size, 16.

Constant: CCM_DIGEST_SIZE

Size of the CCM digest, 16.

Constant: CCM_MIN_NONCE_SIZE

Constant: CCM_MAX_NONCE_SIZE

The the minimum and maximum sizes for an CCM nonce, 7 and 14, respectively.

Macro: CCM_MAX_MSG_SIZE (nonce_size)

The largest allowed plaintext length, when using CCM with a nonce of the given size.

Function: void ccm_set_nonce (struct ccm_ctx *ctx, const void *cipher, nettle_cipher_func *f, size_t noncelen, const uint8_t *nonce, size_t authlen, size_t msglen, size_t taglen)

Initializes ctx using the given nonce and the sizes of the authenticated data, message, and MAC to be processed.

Function: void ccm_update (struct ccm_ctx *ctx, const void *cipher, nettle_cipher_func *f, size_t length, const uint8_t *data)

Provides associated data to be authenticated. Must be called after ccm_set_nonce, and before ccm_encrypt, ccm_decrypt, or ccm_digest.

Function: void ccm_encrypt (struct ccm_ctx *ctx, const void *cipher, nettle_cipher_func *f, size_t length, uint8_t *dst, const uint8_t *src)

Function: void ccm_decrypt (struct ccm_ctx *ctx, const void *cipher, nettle_cipher_func *f, size_t length, uint8_t *dst, const uint8_t *src)

Encrypts or decrypts the message data. Must be called after ccm_set_nonce and before ccm_digest. All but the last call for each message must use a length that is a multiple of the block size.

Function: void ccm_digest (struct ccm_ctx *ctx, const void *cipher, nettle_cipher_func *f, size_t length, uint8_t *digest)

Extracts the message digest (also known “authentication tag”). This is the final operation when processing a message. length is usually equal to the taglen parameter supplied to ccm_set_nonce, but if you provide a smaller value, only the first length octets of the digest are written.

To encrypt a message using the general CCM interface, set the message nonce and length using ccm_set_nonce and then call ccm_update to generate the digest of any authenticated data. After all of the authenticated data has been digested use ccm_encrypt to encrypt the plaintext. Finally, use ccm_digest to return the encrypted MAC.

To decrypt a message, use ccm_set_nonce and ccm_update the same as you would for encryption, and then call ccm_decrypt to decrypt the ciphertext. After decrypting the ciphertext ccm_digest will return the encrypted MAC which should be identical to the MAC in the received message.

6.4.3.2 CCM message interface

The CCM message fuctions provides a simple interface that will perform authentication and message encryption in a single function call. The length of the cleartext is given by mlength and the length of the ciphertext is given by clength, always exactly tlength bytes longer than the corresponding plaintext. The length argument passed to a function is always the size for the result, clength for the encryption functions, and mlength for the decryption functions.

Function: void ccm_encrypt_message (void *cipher, nettle_cipher_func *f, size_t nlength, const uint8_t *nonce, size_t alength, const uint8_t *adata, size_t tlength, size_t clength, uint8_t *dst, const uint8_t *src)

Computes the message digest from the adata and src parameters, encrypts the plaintext from src, appends the encrypted MAC to ciphertext and outputs it to dst.

Function: int ccm_decrypt_message (void *cipher, nettle_cipher_func *f, size_t nlength, const uint8_t *nonce, size_t alength, const uint8_t *adata, size_t tlength, size_t mlength, uint8_t *dst, const uint8_t *src)

Decrypts the ciphertext from src, outputs the plaintext to dst, recalculates the MAC from adata and the plaintext, and compares it to the final tlength bytes of src. If the values of the received and calculated MACs are equal, this will return 1 indicating a valid and authenticated message. Otherwise, this function will return zero.

6.4.3.3 CCM-AES interface

The AES CCM functions provide an API for using CCM mode with the AES block ciphers. The parameters all have the same meaning as the general and message interfaces, except that the cipher, f, and ctx parameters are replaced with an AES context structure, and a set-key function must be called before using any of the other functions in this interface.

Context struct: struct ccm_aes128_ctx

Holds state corresponding to a particular message encrypted using the AES-128 block cipher.

Context struct: struct ccm_aes192_ctx

Holds state corresponding to a particular message encrypted using the AES-192 block cipher.

Context struct: struct ccm_aes256_ctx

Holds state corresponding to a particular message encrypted using the AES-256 block cipher.

Function: void ccm_aes128_set_key (struct ccm_aes128_ctx *ctx, const uint8_t *key)

Function: void ccm_aes192_set_key (struct ccm_aes192_ctx *ctx, const uint8_t *key)

Function: void ccm_aes256_set_key (struct ccm_aes256_ctx *ctx, const uint8_t *key)

Initializes the encryption key for the AES block cipher. One of these functions must be called before any of the other functions in the AES CCM interface.

Function: void ccm_aes128_set_nonce (struct ccm_aes128_ctx *ctx, size_t noncelen, const uint8_t *nonce, size_t authlen, size_t msglen, size_t taglen)

Function: void ccm_aes192_set_nonce (struct ccm_aes192_ctx *ctx, size_t noncelen, const uint8_t *nonce, size_t authlen, size_t msglen, size_t taglen)

Function: void ccm_aes256_set_nonce (struct ccm_aes256_ctx *ctx, size_t noncelen, const uint8_t *nonce, size_t authlen, size_t msglen, size_t taglen)

These are identical to ccm_set_nonce, except that cipher, f, and ctx are replaced with a context structure.

Function: void ccm_aes128_update (struct ccm_aes128_ctx *ctx, size_t length, const uint8_t *data)

Function: void ccm_aes192_update (struct ccm_aes192_ctx *ctx, size_t length, const uint8_t *data)

Function: void ccm_aes256_update (struct ccm_aes256_ctx *ctx, size_t length, const uint8_t *data)

These are identical to ccm_set_update, except that cipher, f, and ctx are replaced with a context structure.

Function: void ccm_aes128_encrypt (struct ccm_aes128_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Function: void ccm_aes192_encrypt (struct ccm_aes192_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Function: void ccm_aes256_encrypt (struct ccm_aes256_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Function: void ccm_aes128_decrypt (struct ccm_aes128_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Function: void ccm_aes192_decrypt (struct ccm_aes192_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Function: void ccm_aes256_decrypt (struct ccm_aes256_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

These are identical to ccm_set_encrypt and ccm_set_decrypt, except that cipher, f, and ctx are replaced with a context structure.

Function: void ccm_aes128_digest (struct ccm_aes128_ctx *ctx, size_t length, uint8_t *digest)

Function: void ccm_aes192_digest (struct ccm_aes192_ctx *ctx, size_t length, uint8_t *digest)

Function: void ccm_aes256_digest (struct ccm_aes256_ctx *ctx, size_t length, uint8_t *digest)

These are identical to ccm_set_digest, except that cipher, f, and ctx are replaced with a context structure.

Function: void ccm_aes128_encrypt_message (struct ccm_aes128_ctx *ctx, size_t nlength, const uint8_t *nonce, size_t alength, const uint8_t *adata, size_t tlength, size_t clength, uint8_t *dst, const uint8_t *src)

Function: void ccm_aes192_encrypt_message (struct ccm_aes192_ctx *ctx, size_t nlength, const uint8_t *nonce, size_t alength, const uint8_t *adata, size_t tlength, size_t clength, uint8_t *dst, const uint8_t *src)

Function: void ccm_aes256_encrypt_message (struct ccm_aes256_ctx *ctx, size_t nlength, const uint8_t *nonce, size_t alength, const uint8_t *adata, size_t tlength, size_t clength, uint8_t *dst, const uint8_t *src)

Function: int ccm_aes128_decrypt_message (struct ccm_aes128_ctx *ctx, size_t nlength, const uint8_t *nonce, size_t alength, const uint8_t *adata, size_t tlength, size_t mlength, uint8_t *dst, const uint8_t *src)

Function: int ccm_aes192_decrypt_message (struct ccm_aes192_ctx *ctx, size_t nlength, const uint8_t *nonce, size_t alength, const uint8_t *adata, size_t tlength, size_t mlength, uint8_t *dst, const uint8_t *src)

Function: int ccm_aes192_decrypt_message (struct ccm_aes256_ctx *ctx, size_t nlength, const uint8_t *nonce, size_t alength, const uint8_t *adata, size_t tlength, size_t mlength, uint8_t *dst, const uint8_t *src)

These are identical to ccm_encrypt_message and ccm_decrypt_message except that cipher and f are replaced with a context structure.

6.4.4 ChaCha-Poly1305

ChaCha-Poly1305 is a combination of the ChaCha stream cipher and the poly1305 message authentication code (see Poly1305). It originates from the NaCl cryptographic library by D. J. Bernstein et al, which defines a similar construction but with Salsa20 instead of ChaCha.

Nettle’s implementation ChaCha-Poly1305 should be considered experimental. At the time of this writing, there is no authoritative specification for ChaCha-Poly1305, and a couple of different incompatible variants. Nettle implements it using the original definition of ChaCha, with 64 bits (8 octets) each for the nonce and the block counter. Some protocols prefer to use nonces of 12 bytes, and it’s a small change to ChaCha to use the upper 32 bits of the block counter as a nonce, instead limiting message size to 2^32 blocks or 256 GBytes, but that variant is currently not supported.

For ChaCha-Poly1305, the ChaCha cipher is initialized with a key, of 256 bits, and a per-message nonce. The first block of the key stream (counter all zero) is set aside for the authentication subkeys. Of this 64-octet block, the first 16 octets specify the poly1305 evaluation point, and the next 16 bytes specify the value to add in for the final digest. The final 32 bytes of this block are unused. Note that unlike poly1305-aes, the evaluation point depends on the nonce. This is preferable, because it leaks less information in case the attacker for some reason is lucky enough to forge a valid authentication tag, and observe (from the receiver’s behaviour) that the forgery succeeded.

The ChaCha key stream, starting with counter value 1, is then used to encrypt the message. For authentication, poly1305 is applied to the concatenation of the associated data, the cryptotext, and the lengths of the associated data and the message, each a 64-bit number (eight octets, little-endian). Nettle defines ChaCha-Poly1305 in <nettle/chacha-poly1305.h>.

Constant: CHACHA_POLY1305_BLOCK_SIZE

Same as the ChaCha block size, 64.

Constant: CHACHA_POLY1305_KEY_SIZE

ChaCha-Poly1305 key size, 32.

Constant: CHACHA_POLY1305_NONCE_SIZE

Same as the ChaCha nonce size, 16.

Constant: CHACHA_POLY1305_DIGEST_SIZE

Digest size, 16.

Context struct: struct chacha_poly1305_ctx

Function: void chacha_poly1305_set_key (struct chacha_poly1305_ctx *ctx, const uint8_t *key)

Initializes ctx using the given key. Before using the context, you must also call chacha_poly1305_set_nonce, see below.

Function: void chacha_poly1305_set_nonce (struct chacha_poly1305_ctx *ctx, const uint8_t *nonce)

Initializes the per-message state, using the given nonce.

Function: void chacha_poly1305_update (struct chacha_poly1305_ctx *ctx, size_t length, const uint8_t *data)

Process associated data for authentication.

Function: void chacha_poly1305_encrypt (struct chacha_poly1305_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Function: void chacha_poly1305_decrypt (struct chacha_poly1305_ctx *ctx, size_t length, uint8_t *dst, const uint8_t *src)

Encrypts or decrypts the data of a message. All but the last call for each message must use a length that is a multiple of the block size.

Function: void chacha_poly1305_digest (struct chacha_poly1305_ctx *ctx, size_t length, uint8_t *digest)

Extracts the message digest (also known “authentication tag”). This is the final operation when processing a message. If length is smaller than CHACHA_POLY1305_DIGEST_SIZE, only the first length octets of the digest are written.

6.4.5 The struct nettle_aead abstraction

Nettle includes a struct including information about the supported hash functions. It is defined in <nettle/nettle-meta.h>.

Meta struct: struct nettle_aead name context_size block_size key_size nonce_size digest_size set_encrypt_key set_decrypt_key set_nonce update encrypt decrypt digest

The last seven attributes are function pointers.

Constant Struct: struct nettle_aead nettle_gcm_aes128

Constant Struct: struct nettle_aead nettle_gcm_aes192

Constant Struct: struct nettle_aead nettle_gcm_aes256

Constant Struct: struct nettle_aead nettle_gcm_camellia128

Constant Struct: struct nettle_aead nettle_gcm_camellia256

Constant Struct: struct nettle_aead nettle_eax_aes128

Constant Struct: struct nettle_aead nettle_chacha_poly1305

These are most of the AEAD constructions that Nettle implements. Note that CCM is missing; it requirement that the message size is specified in advance makes it incompatible with the nettle_aead abstraction.

Nettle also exports a list of all these constructions.

Function: const struct nettle_aead **nettle_get_aeads(void)

Returns a NULL-terminated list of pointers to supported algorithms.This list can be used to dynamically enumerate or search the supported algorithms.

Macro: nettle_aeads

A macro expanding to a call to nettle_get_aeads. In earlier versions, this was not a macro but the actual array of pointers.

6.5 Keyed Hash Functions

A keyed hash function, or Message Authentication Code (MAC) is a function that takes a key and a message, and produces fixed size MAC. It should be hard to compute a message and a matching MAC without knowledge of the key. It should also be hard to compute the key given only messages and corresponding MACs.

Keyed hash functions are useful primarily for message authentication, when Alice and Bob shares a secret: The sender, Alice, computes the MAC and attaches it to the message. The receiver, Bob, also computes the MAC of the message, using the same key, and compares that to Alice’s value. If they match, Bob can be assured that the message has not been modified on its way from Alice.

However, unlike digital signatures, this assurance is not transferable. Bob can’t show the message and the MAC to a third party and prove that Alice sent that message. Not even if he gives away the key to the third party. The reason is that the same key is used on both sides, and anyone knowing the key can create a correct MAC for any message. If Bob believes that only he and Alice knows the key, and he knows that he didn’t attach a MAC to a particular message, he knows it must be Alice who did it. However, the third party can’t distinguish between a MAC created by Alice and one created by Bob.

Keyed hash functions are typically a lot faster than digital signatures as well.

6.5.1 HMAC

One can build keyed hash functions from ordinary hash functions. Older constructions simply concatenate secret key and message and hashes that, but such constructions have weaknesses. A better construction is HMAC, described in RFC 2104.

For an underlying hash function H, with digest size l and internal block size b, HMAC-H is constructed as follows: From a given key k, two distinct subkeys k_i and k_o are constructed, both of length b. The HMAC-H of a message m is then computed as H(k_o | H(k_i | m)), where | denotes string concatenation.

HMAC keys can be of any length, but it is recommended to use keys of length l, the digest size of the underlying hash function H. Keys that are longer than b are shortened to length l by hashing with H, so arbitrarily long keys aren’t very useful.

Nettle’s HMAC functions are defined in <nettle/hmac.h>. There are abstract functions that use a pointer to a struct nettle_hash to represent the underlying hash function and void * pointers that point to three different context structs for that hash function. There are also concrete functions for HMAC-MD5, HMAC-RIPEMD160 HMAC-SHA1, HMAC-SHA256, and HMAC-SHA512. First, the abstract functions:

Function: void hmac_set_key (void *outer, void *inner, void *state, const struct nettle_hash *H, size_t length, const uint8_t *key)

Initializes the three context structs from the key. The outer and inner contexts corresponds to the subkeys k_o and k_i. state is used for hashing the message, and is initialized as a copy of the inner context.

Function: void hmac_update (void *state, const struct nettle_hash *H, size_t length, const uint8_t *data)

This function is called zero or more times to process the message. Actually, hmac_update(state, H, length, data) is equivalent to H->update(state, length, data), so if you wish you can use the ordinary update function of the underlying hash function instead.

Function: void hmac_digest (const void *outer, const void *inner, void *state, const struct nettle_hash *H, size_t length, uint8_t *digest)

Extracts the MAC of the message, writing it to digest. outer and inner are not modified. length is usually equal to H->digest_size, but if you provide a smaller value, only the first length octets of the MAC are written.

This function also resets the state context so that you can start over processing a new message (with the same key).

Like for CBC, there are some macros to help use these functions correctly.

Macro: HMAC_CTX (type)

Expands to

{
   type outer;
   type inner;
   type state;
}

It can be used to define a HMAC context struct, either directly,

struct HMAC_CTX(struct md5_ctx) ctx;

or to give it a struct tag,

struct hmac_md5_ctx HMAC_CTX (struct md5_ctx);

Macro: HMAC_SET_KEY (ctx, H, length, key)

ctx is a pointer to a context struct as defined by HMAC_CTX, H is a pointer to a const struct nettle_hash describing the underlying hash function (so it must match the type of the components of ctx). The last two arguments specify the secret key.

Macro: HMAC_DIGEST (ctx, H, length, digest)

ctx is a pointer to a context struct as defined by HMAC_CTX, H is a pointer to a const struct nettle_hash describing the underlying hash function. The last two arguments specify where the digest is written.

Note that there is no HMAC_UPDATE macro; simply call hmac_update function directly, or the update function of the underlying hash function.

6.5.2 Concrete HMAC functions

Now we come to the specialized HMAC functions, which are easier to use than the general HMAC functions.

6.5.2.1 HMAC-MD5

Context struct: struct hmac_md5_ctx

Function: void hmac_md5_set_key (struct hmac_md5_ctx *ctx, size_t key_length, const uint8_t *key)

Initializes the context with the key.

Function: void hmac_md5_update (struct hmac_md5_ctx *ctx, size_t length, const uint8_t *data)

Process some more data.

Function: void hmac_md5_digest (struct hmac_md5_ctx *ctx, size_t length, uint8_t *digest)

Extracts the MAC, writing it to digest. length may be smaller than MD5_DIGEST_SIZE, in which case only the first length octets of the MAC are written.

This function also resets the context for processing new messages, with the same key.

6.5.2.2 HMAC-RIPEMD160

Context struct: struct hmac_ripemd160_ctx

Function: void hmac_ripemd160_set_key (struct hmac_ripemd160_ctx *ctx, size_t key_length, const uint8_t *key)

Initializes the context with the key.

Function: void hmac_ripemd160_update (struct hmac_ripemd160_ctx *ctx, size_t length, const uint8_t *data)

Process some more data.

Function: void hmac_ripemd160_digest (struct hmac_ripemd160_ctx *ctx, size_t length, uint8_t *digest)

Extracts the MAC, writing it to digest. length may be smaller than RIPEMD160_DIGEST_SIZE, in which case only the first length octets of the MAC are written.

This function also resets the context for processing new messages, with the same key.

6.5.2.3 HMAC-SHA1

Context struct: struct hmac_sha1_ctx

Function: void hmac_sha1_set_key (struct hmac_sha1_ctx *ctx, size_t key_length, const uint8_t *key)

Initializes the context with the key.

Function: void hmac_sha1_update (struct hmac_sha1_ctx *ctx, size_t length, const uint8_t *data)

Process some more data.

Function: void hmac_sha1_digest (struct hmac_sha1_ctx *ctx, size_t length, uint8_t *digest)

Extracts the MAC, writing it to digest. length may be smaller than SHA1_DIGEST_SIZE, in which case only the first length octets of the MAC are written.

This function also resets the context for processing new messages, with the same key.

6.5.2.4 HMAC-SHA256

Context struct: struct hmac_sha256_ctx

Function: void hmac_sha256_set_key (struct hmac_sha256_ctx *ctx, size_t key_length, const uint8_t *key)

Initializes the context with the key.

Function: void hmac_sha256_update (struct hmac_sha256_ctx *ctx, size_t length, const uint8_t *data)

Process some more data.

Function: void hmac_sha256_digest (struct hmac_sha256_ctx *ctx, size_t length, uint8_t *digest)

Extracts the MAC, writing it to digest. length may be smaller than SHA256_DIGEST_SIZE, in which case only the first length octets of the MAC are written.

This function also resets the context for processing new messages, with the same key.

6.5.2.5 HMAC-SHA512

Context struct: struct hmac_sha512_ctx

Function: void hmac_sha512_set_key (struct hmac_sha512_ctx *ctx, size_t key_length, const uint8_t *key)

Initializes the context with the key.

Function: void hmac_sha512_update (struct hmac_sha512_ctx *ctx, size_t length, const uint8_t *data)

Process some more data.

Function: void hmac_sha512_digest (struct hmac_sha512_ctx *ctx, size_t length, uint8_t *digest)

Extracts the MAC, writing it to digest. length may be smaller than SHA512_DIGEST_SIZE, in which case only the first length octets of the MAC are written.

This function also resets the context for processing new messages, with the same key.

6.5.3 UMAC

UMAC is a message authentication code based on universal hashing, and designed for high performance on modern processors (in contrast to GCM, See GCM, which is designed primarily for hardware performance). On processors with good integer multiplication performance, it can be 10 times faster than SHA256 and SHA512. UMAC is specified in RFC 4418.

The secret key is always 128 bits (16 octets). The key is used as an encryption key for the AES block cipher. This cipher is used in counter mode to generate various internal subkeys needed in UMAC. Messages are of arbitrary size, and for each message, UMAC also needs a unique nonce. Nonce values must not be reused for two messages with the same key, but they need not be kept secret.

The nonce must be at least one octet, and at most 16; nonces shorter than 16 octets are zero-padded. Nettle’s implementation of UMAC increments the nonce automatically for each message, so explicitly setting the nonce for each message is optional. This auto-increment uses network byte order and it takes the length of the nonce into account. E.g., if the initial nonce is “abc” (3 octets), this value is zero-padded to 16 octets for the first message. For the next message, the nonce is incremented to “abd”, and this incremented value is zero-padded to 16 octets.

UMAC is defined in four variants, for different output sizes: 32 bits (4 octets), 64 bits (8 octets), 96 bits (12 octets) and 128 bits (16 octets), corresponding to different trade-offs between speed and security. Using a shorter output size sometimes (but not always!) gives the same result as using a longer output size and truncating the result. So it is important to use the right variant. For consistency with other hash and MAC functions, Nettle’s _digest functions for UMAC accept a length parameter so that the output can be truncated to any desired size, but it is recommended to stick to the specified output size and select the umac variant corresponding to the desired size.

The internal block size of UMAC is 1024 octets, and it also generates more than 1024 bytes of subkeys. This makes the size of the context struct quite a bit larger than other hash functions and MAC algorithms in Nettle.

Nettle defines UMAC in <nettle/umac.h>.

Context struct: struct umac32_ctx

Context struct: struct umac64_ctx

Context struct: struct umac96_ctx

Context struct: struct umac128_ctx

Each UMAC variant uses its own context struct.

Constant: UMAC_KEY_SIZE

The UMAC key size, 16.

Constant: UMAC_MIN_NONCE_SIZE

Constant: UMAC_MAX_NONCE_SIZE

The the minimum and maximum sizes for an UMAC nonce, 1 and 16, respectively.

Constant: UMAC32_DIGEST_SIZE

The size of an UMAC32 digest, 4.

Constant: UMAC64_DIGEST_SIZE

The size of an UMAC64 digest, 8.

Constant: UMAC96_DIGEST_SIZE

The size of an UMAC96 digest, 12.

Constant: UMAC128_DIGEST_SIZE

The size of an UMAC128 digest, 16.

Constant: UMAC_BLOCK_SIZE

The internal block size of UMAC.

Function: void umac32_set_key (struct umac32_ctx *ctx, const uint8_t *key)

Function: void umac64_set_key (struct umac64_ctx *ctx, const uint8_t *key)

Function: void umac96_set_key (struct umac96_ctx *ctx, const uint8_t *key)

Function: void umac128_set_key (struct umac128_ctx *ctx, const uint8_t *key)

These functions initialize the UMAC context struct. They also initialize the nonce to zero (with length 16, for auto-increment).

Function: void umac32_set_nonce (struct umac32_ctx *ctx, size_t length, const uint8_t *nonce)

Function: void umac64_set_nonce (struct umac64_ctx *ctx, size_t length, const uint8_t *nonce)

Function: void umac96_set_nonce (struct umac96_ctx *ctx, size_t length, const uint8_t *nonce)

Function: void umac128_set_nonce (struct umac128_ctx *ctx, size_t length, const uint8_t *nonce)

Sets the nonce to be used for the next message. In general, nonces should be set before processing of the message. This is not strictly required for UMAC (the nonce only affects the final processing generating the digest), but it is nevertheless recommended that this function is called before the first _update call for the message.

Function: void umac32_update (struct umac32_ctx *ctx, size_t length, const uint8_t *data)

Function: void umac64_update (struct umac64_ctx *ctx, size_t length, const uint8_t *data)

Function: void umac96_update (struct umac96_ctx *ctx, size_t length, const uint8_t *data)

Function: void umac128_update (struct umac128_ctx *ctx, size_t length, const uint8_t *data)

These functions are called zero or more times to process the message.

Function: void umac32_digest (struct umac32_ctx *ctx, size_t length, uint8_t *digest)

Function: void umac64_digest (struct umac64_ctx *ctx, size_t length, uint8_t *digest)

Function: void umac96_digest (struct umac96_ctx *ctx, size_t length, uint8_t *digest)

Function: void umac128_digest (struct umac128_ctx *ctx, size_t length, uint8_t *digest)

Extracts the MAC of the message, writing it to digest. length is usually equal to the specified output size, but if you provide a smaller value, only the first length octets of the MAC are written. These functions reset the context for processing of a new message with the same key. The nonce is incremented as described above, the new value is used unless you call the _set_nonce function explicitly for each message.

6.5.4 Poly1305

Poly1305-AES is a message authentication code designed by D. J. Bernstein. It treats the message as a polynomial modulo the prime number 2^130 - 5.

The key, 256 bits, consists of two parts, where the first half is an AES-128 key, and the second half specifies the point where the polynomial is evaluated. Of the latter half, 22 bits are set to zero, to enable high-performance implementation, leaving 106 bits for specifying an evaluation point r. For each message, one must also provide a 128-bit nonce. The nonce is encrypted using the AES key, and that’s the only thing AES is used for.

The message is split into 128-bit chunks (with final chunk possibly being shorter), each read as a little-endian integer. Each chunk has a one-bit appended at the high end. The resulting integers are treated as polynomial coefficients modulo 2^130 - 5, and the polynomial is evaluated at the point r. Finally, this value is reduced modulo 2^128, and added (also modulo 2^128) to the encrypted nonce, to produce an 128-bit authenticator for the message. See http://cr.yp.to/mac/poly1305-20050329.pdf for further details.

Clearly, variants using a different cipher than AES could be defined. Another variant is the ChaCha-Poly1305 AEAD construction (see ChaCha-Poly1305). Nettle defines Poly1305-AES in nettle/poly1305.h.

Constant: POLY1305_AES_KEY_SIZE

Key size, 32 octets.

Constant: POLY1305_AES_DIGEST_SIZE

Size of the digest or “authenticator”, 16 octets.

Constant: POLY1305_AES_NONCE_SIZE

Nonce size, 16 octets.

Context struct: struct poly1305_aes_ctx

The poly1305-aes context struct.

Function: void poly1305_aes_set_key (struct poly1305_aes_ctx *ctx, const uint8_t *key)

Initialize the context struct. Also sets the nonce to zero.

Function: void poly1305_aes_set_nonce (struct poly1305_aes_ctx *ctx, const uint8_t *nonce)

Sets the nonce. Calling this function is optional, since the nonce is incremented automatically for each message.

Function: void poly1305_aes_update (struct poly1305_aes_ctx *ctx, size_t length, const uint8_t *data)

Process more data.

Function: void poly1305_aes_digest (struct poly1305_aes_ctx *ctx, size_t length, uint8_t *digest)

Extracts the digest. If length is smaller than POLY1305_AES_DIGEST_SIZE, only the first length octets are written. Also increments the nonce, and prepares the context for processing a new message.

6.6 Key derivation Functions

A key derivation function (KDF) is a function that from a given symmetric key derives other symmetric keys. A sub-class of KDFs is the password-based key derivation functions (PBKDFs), which take as input a password or passphrase, and its purpose is typically to strengthen it and protect against certain pre-computation attacks by using salting and expensive computation.

6.6.1 HKDF: HMAC-based Extract-and-Expand

HKDF is a key derivation function used as a building block of higher-level protocols like TLS 1.3. It is a derivation function based on HMAC described in RFC 5869, and is split into two logical modules, called ’extract’ and ’expand’. The extract module takes an initial secret and a random salt to "extract" a fixed-length pseudorandom key (PRK). The second stage takes as input the previous PRK and some informational data (e.g., text) and expands them into multiple keys.

Nettle’s HKDF functions are defined in <nettle/hkdf.h>. There are two abstract functions for the extract and expand operations that operate on any HMAC implemented via the nettle_hash_update_func, and nettle_hash_digest_func interfaces.

Function: void hkdf_extract (void *mac_ctx, nettle_hash_update_func *update, nettle_hash_digest_func *digest, size_t digest_size,size_t secret_size, const uint8_t *secret, uint8_t *dst)

Extract a Pseudorandom Key (PRK) from a secret and a salt according to HKDF. The HMAC must have been initialized, with its key being the salt for the Extract operation. This function will call the update and digest functions passing the mac_ctx context parameter as an argument in order to compute digest of size digest_size. Inputs are the secret secret of length secret_length. The output length is fixed to digest_size octets, thus the output buffer dst must have room for at least digest_size octets.

Function: void hkdf_expand (void *mac_ctx, nettle_hash_update_func *update, nettle_hash_digest_func *digest, size_t digest_size, size_t info_size, const uint8_t *info, size_t length, uint8_t *dst)

Expand a Pseudorandom Key (PRK) to an arbitrary size according to HKDF. The HMAC must have been initialized, with its key being the PRK from the Extract operation. This function will call the update and digest functions passing the mac_ctx context parameter as an argument in order to compute digest of size digest_size. Inputs are the info info of length info_length, and the desired derived output length length. The output buffer is dst which must have room for at least length octets.

6.6.2 PBKDF2

The most well known PBKDF is the PKCS #5 PBKDF2 described in RFC 2898 which uses a pseudo-random function such as HMAC-SHA1.

Nettle’s PBKDF2 functions are defined in <nettle/pbkdf2.h>. There is an abstract function that operate on any PRF implemented via the nettle_hash_update_func, nettle_hash_digest_func interfaces. There is also helper macros and concrete functions PBKDF2-HMAC-SHA1 and PBKDF2-HMAC-SHA256. First, the abstract function:

Function: void pbkdf2 (void *mac_ctx, nettle_hash_update_func *update, nettle_hash_digest_func *digest, size_t digest_size, unsigned iterations, size_t salt_length, const uint8_t *salt, size_t length, uint8_t *dst)

Derive symmetric key from a password according to PKCS #5 PBKDF2. The PRF is assumed to have been initialized and this function will call the update and digest functions passing the mac_ctx context parameter as an argument in order to compute digest of size digest_size. Inputs are the salt salt of length salt_length, the iteration counter iterations (> 0), and the desired derived output length length. The output buffer is dst which must have room for at least length octets.

Like for CBC and HMAC, there is a macro to help use the function correctly.

Macro: PBKDF2 (ctx, update, digest, digest_size, iterations, salt_length, salt, length, dst)

ctx is a pointer to a context struct passed to the update and digest functions (of the types nettle_hash_update_func and nettle_hash_digest_func respectively) to implement the underlying PRF with digest size of digest_size. Inputs are the salt salt of length salt_length, the iteration counter iterations (> 0), and the desired derived output length length. The output buffer is dst which must have room for at least length octets.

6.6.3 Concrete PBKDF2 functions

Now we come to the specialized PBKDF2 functions, which are easier to use than the general PBKDF2 function.

6.6.3.1 PBKDF2-HMAC-SHA1

Function: void pbkdf2_hmac_sha1 (size_t key_length, const uint8_t *key, unsigned iterations, size_t salt_length, const uint8_t *salt, size_t length, uint8_t *dst)

PBKDF2 with HMAC-SHA1. Derive length bytes of key into buffer dst using the password key of length key_length and salt salt of length salt_length, with iteration counter iterations (> 0). The output buffer is dst which must have room for at least length octets.

6.6.3.2 PBKDF2-HMAC-SHA256

Function: void pbkdf2_hmac_sha256 (size_t key_length, const uint8_t *key, unsigned iterations, size_t salt_length, const uint8_t *salt, size_t length, uint8_t *dst)

PBKDF2 with HMAC-SHA256. Derive length bytes of key into buffer dst using the password key of length key_length and salt salt of length salt_length, with iteration counter iterations (> 0). The output buffer is dst which must have room for at least length octets.

6.7 Public-key algorithms

Nettle uses GMP, the GNU bignum library, for all calculations with large numbers. In order to use the public-key features of Nettle, you must install GMP, at least version 3.0, before compiling Nettle, and you need to link your programs with -lhogweed -lnettle -lgmp.

The concept of Public-key encryption and digital signatures was discovered by Whitfield Diffie and Martin E. Hellman and described in a paper 1976. In traditional, “symmetric”, cryptography, sender and receiver share the same keys, and these keys must be distributed in a secure way. And if there are many users or entities that need to communicate, each pair needs a shared secret key known by nobody else.

Public-key cryptography uses trapdoor one-way functions. A one-way function is a function F such that it is easy to compute the value F(x) for any x, but given a value y, it is hard to compute a corresponding x such that y = F(x). Two examples are cryptographic hash functions, and exponentiation in certain groups.

A trapdoor one-way function is a function F that is one-way, unless one knows some secret information about F. If one knows the secret, it is easy to compute both F and it’s inverse. If this sounds strange, look at the RSA example below.

Two important uses for one-way functions with trapdoors are public-key encryption, and digital signatures. The public-key encryption functions in Nettle are not yet documented; the rest of this chapter is about digital signatures.

To use a digital signature algorithm, one must first create a key-pair: A public key and a corresponding private key. The private key is used to sign messages, while the public key is used for verifying that that signatures and messages match. Some care must be taken when distributing the public key; it need not be kept secret, but if a bad guy is able to replace it (in transit, or in some user’s list of known public keys), bad things may happen.

There are two operations one can do with the keys. The signature operation takes a message and a private key, and creates a signature for the message. A signature is some string of bits, usually at most a few thousand bits or a few hundred octets. Unlike paper-and-ink signatures, the digital signature depends on the message, so one can’t cut it out of context and glue it to a different message.

The verification operation takes a public key, a message, and a string that is claimed to be a signature on the message, and returns true or false. If it returns true, that means that the three input values matched, and the verifier can be sure that someone went through with the signature operation on that very message, and that the “someone” also knows the private key corresponding to the public key.

The desired properties of a digital signature algorithm are as follows: Given the public key and pairs of messages and valid signatures on them, it should be hard to compute the private key, and it should also be hard to create a new message and signature that is accepted by the verification operation.

Besides signing meaningful messages, digital signatures can be used for authorization. A server can be configured with a public key, such that any client that connects to the service is given a random nonce message. If the server gets a reply with a correct signature matching the nonce message and the configured public key, the client is granted access. So the configuration of the server can be understood as “grant access to whoever knows the private key corresponding to this particular public key, and to no others”.

6.7.1 RSA

The RSA algorithm was the first practical digital signature algorithm that was constructed. It was described 1978 in a paper by Ronald Rivest, Adi Shamir and L.M. Adleman, and the technique was also patented in the USA in 1983. The patent expired on September 20, 2000, and since that day, RSA can be used freely, even in the USA.

It’s remarkably simple to describe the trapdoor function behind RSA. The “one-way”-function used is

F(x) = x^e mod n

I.e. raise x to the e’th power, while discarding all multiples of n. The pair of numbers n and e is the public key. e can be quite small, even e = 3 has been used, although slightly larger numbers are recommended. n should be about 2000 bits or larger.

If n is large enough, and properly chosen, the inverse of F, the computation of e’th roots modulo n, is very difficult. But, where’s the trapdoor?

Let’s first look at how RSA key-pairs are generated. First n is chosen as the product of two large prime numbers p and q of roughly the same size (so if n is 2000 bits, p and q are about 1000 bits each). One also computes the number phi = (p-1)(q-1), in mathematical speak, phi is the order of the multiplicative group of integers modulo n.

Next, e is chosen. It must have no factors in common with phi (in particular, it must be odd), but can otherwise be chosen more or less randomly. e = 65537 is a popular choice, because it makes raising to the e’th power particularly efficient, and being prime, it usually has no factors common with phi.

Finally, a number d, d < n is computed such that e d mod phi = 1. It can be shown that such a number exists (this is why e and phi must have no common factors), and that for all x,

(x^e)^d mod n = x^(ed) mod n = (x^d)^e mod n = x

Using Euclid’s algorithm, d can be computed quite easily from phi and e. But it is still hard to get d without knowing phi, which depends on the factorization of n.

So d is the trapdoor, if we know d and y = F(x), we can recover x as y^d mod n. d is also the private half of the RSA key-pair.

The most common signature operation for RSA is defined in PKCS#1, a specification by RSA Laboratories. The message to be signed is first hashed using a cryptographic hash function, e.g. MD5 or SHA1. Next, some padding, the ASN.1 “Algorithm Identifier” for the hash function, and the message digest itself, are concatenated and converted to a number x. The signature is computed from x and the private key as s = x^d mod n¹. The signature, s is a number of about the same size of n, and it usually encoded as a sequence of octets, most significant octet first.

The verification operation is straight-forward, x is computed from the message in the same way as above. Then s^e mod n is computed, the operation returns true if and only if the result equals x.

The RSA algorithm can also be used for encryption. RSA encryption uses the public key (n,e) to compute the ciphertext m^e mod n. The PKCS#1 padding scheme will use at least 8 random and non-zero octets, using m of the form [00 02 padding 00 plaintext]. It is required that m < n, and therefor the plaintext must be smaller than the octet size of the modulo n, with some margin.

To decrypt the message, one needs the private key to compute m = c^e mod n followed by checking and removing the padding.

6.7.1.1 Nettle’s RSA support

Nettle represents RSA keys using two structures that contain large numbers (of type mpz_t).

Context struct: rsa_public_key size n e

size is the size, in octets, of the modulo, and is used internally. n and e is the public key.

Context struct: rsa_private_key size d p q a b c

size is the size, in octets, of the modulo, and is used internally. d is the secret exponent, but it is not actually used when signing. Instead, the factors p and q, and the parameters a, b and c are used. They are computed from p, q and e such that a e mod (p - 1) = 1, b e mod (q - 1) = 1, c q mod p = 1.

Before use, these structs must be initialized by calling one of

Function: void rsa_public_key_init (struct rsa_public_key *pub)

Function: void rsa_private_key_init (struct rsa_private_key *key)

Calls mpz_init on all numbers in the key struct.

and when finished with them, the space for the numbers must be deallocated by calling one of

Function: void rsa_public_key_clear (struct rsa_public_key *pub)

Function: void rsa_private_key_clear (struct rsa_private_key *key)

Calls mpz_clear on all numbers in the key struct.

In general, Nettle’s RSA functions deviates from Nettle’s “no memory allocation”-policy. Space for all the numbers, both in the key structs above, and temporaries, are allocated dynamically. For information on how to customize allocation, see See GMP Allocation in GMP Manual.

When you have assigned values to the attributes of a key, you must call

Function: int rsa_public_key_prepare (struct rsa_public_key *pub)

Function: int rsa_private_key_prepare (struct rsa_private_key *key)

Computes the octet size of the key (stored in the size attribute, and may also do other basic sanity checks. Returns one if successful, or zero if the key can’t be used, for instance if the modulo is smaller than the minimum size needed for RSA operations specified by PKCS#1.

For each operation using the private key, there are two variants, e.g., rsa_sha256_sign and rsa_sha256_sign_tr. The former function is older, and it should be avoided, because it provides no defenses against side-channel attacks. The latter function use randomized RSA blinding, which defends against timing attacks using chosen-ciphertext, and it also checks the correctness of the private key computation using the public key, which defends against software or hardware errors which could leak the private key.

Before signing or verifying a message, you first hash it with the appropriate hash function. You pass the hash function’s context struct to the RSA signature function, and it will extract the message digest and do the rest of the work. There are also alternative functions that take the hash digest as argument.

There is currently no support for using SHA224 or SHA384 with RSA signatures, since there’s no gain in either computation time nor message size compared to using SHA256 and SHA512, respectively.

Creating an RSA signature is done with one of the following functions:

Function: int rsa_md5_sign_tr(const struct rsa_public_key *pub, const struct rsa_private_key *key, void *random_ctx, nettle_random_func *random, struct md5_ctx *hash, mpz_t signature)

Function: int rsa_sha1_sign_tr(const struct rsa_public_key *pub, const struct rsa_private_key *key, void *random_ctx, nettle_random_func *random, struct sha1_ctx *hash, mpz_t signature)

Function: int rsa_sha256_sign_tr(const struct rsa_public_key *pub, const struct rsa_private_key *key, void *random_ctx, nettle_random_func *random, struct sha256_ctx *hash, mpz_t signature)

Function: int rsa_sha512_sign_tr(const struct rsa_public_key *pub, const struct rsa_private_key *key, void *random_ctx, nettle_random_func *random, struct sha512_ctx *hash, mpz_t signature)

The signature is stored in signature (which must have been mpz_init’ed earlier). The hash context is reset so that it can be used for new messages. The random_ctx and random pointers are used to generate the RSA blinding. Returns one on success, or zero on failure. Signing fails if an error in the computation was detected, or if the key is too small for the given hash size, e.g., it’s not possible to create a signature using SHA512 and a 512-bit RSA key.

Function: int rsa_md5_sign_digest_tr(const struct rsa_public_key *pub, const struct rsa_private_key *key, void *random_ctx, nettle_random_func *random, const uint8_t *digest, mpz_t signature)

Function: int rsa_sha1_sign_digest_tr(const struct rsa_public_key *pub, const struct rsa_private_key *key, void *random_ctx, nettle_random_func *random, const uint8_t *digest, mpz_t signature)

Function: int rsa_sha256_sign_digest_tr(const struct rsa_public_key *pub, const struct rsa_private_key *key, void *random_ctx, nettle_random_func *random, const uint8_t *digest, mpz_t signature)

Function: int rsa_sha512_sign_digest_tr(const struct rsa_public_key *pub, const struct rsa_private_key *key, void *random_ctx, nettle_random_func *random, const uint8_t *digest, mpz_t signature)

Creates a signature from the given hash digest. digest should point to a digest of size MD5_DIGEST_SIZE, SHA1_DIGEST_SIZE, SHA256_DIGEST_SIZE, or SHA512_DIGEST_SIZErespectively. The signature is stored in signature (which must have been mpz_init:ed earlier). Returns one on success, or zero on failure.

Function: int rsa_pkcs1_sign_tr(const struct rsa_public_key *pub, const struct rsa_private_key *key, void *random_ctx, nettle_random_func *random, size_t length, const uint8_t *digest_info, mpz_t signature)

Similar to the above _sign_digest_tr functions, but the input is not the plain hash digest, but a PKCS#1 “DigestInfo”, an ASN.1 DER-encoding of the digest together with an object identifier for the used hash algorithm.

Function: int rsa_md5_sign (const struct rsa_private_key *key, struct md5_ctx *hash, mpz_t signature)

Function: int rsa_sha1_sign (const struct rsa_private_key *key, struct sha1_ctx *hash, mpz_t signature)

Function: int rsa_sha256_sign (const struct rsa_private_key *key, struct sha256_ctx *hash, mpz_t signature)

Function: int rsa_sha512_sign (const struct rsa_private_key *key, struct sha512_ctx *hash, mpz_t signature)

The signature is stored in signature (which must have been mpz_init’ed earlier). The hash context is reset so that it can be used for new messages. Returns one on success, or zero on failure. Signing fails if the key is too small for the given hash size, e.g., it’s not possible to create a signature using SHA512 and a 512-bit RSA key.

Function: int rsa_md5_sign_digest (const struct rsa_private_key *key, const uint8_t *digest, mpz_t signature)

Function: int rsa_sha1_sign_digest (const struct rsa_private_key *key, const uint8_t *digest, mpz_t signature);

Function: int rsa_sha256_sign_digest (const struct rsa_private_key *key, const uint8_t *digest, mpz_t signature);

Function: int rsa_sha512_sign_digest (const struct rsa_private_key *key, const uint8_t *digest, mpz_t signature);

Creates a signature from the given hash digest; otherwise analoguous to the above signing functions. digest should point to a digest of size MD5_DIGEST_SIZE, SHA1_DIGEST_SIZE, SHA256_DIGEST_SIZE, or SHA512_DIGEST_SIZE, respectively. The signature is stored in signature (which must have been mpz_init:ed earlier). Returns one on success, or zero on failure.

Function: int rsa_pkcs1_sign(const struct rsa_private_key *key, size_t length, const uint8_t *digest_info, mpz_t s)

Similar to the above _sign_digest functions, but the input is not the plain hash digest, but a PKCS#1 “DigestInfo”, an ASN.1 DER-encoding of the digest together with an object identifier for the used hash algorithm.

Verifying an RSA signature is done with one of the following functions:

Function: int rsa_md5_verify (const struct rsa_public_key *key, struct md5_ctx *hash, const mpz_t signature)

Function: int rsa_sha1_verify (const struct rsa_public_key *key, struct sha1_ctx *hash, const mpz_t signature)

Function: int rsa_sha256_verify (const struct rsa_public_key *key, struct sha256_ctx *hash, const mpz_t signature)

Function: int rsa_sha512_verify (const struct rsa_public_key *key, struct sha512_ctx *hash, const mpz_t signature)

Returns 1 if the signature is valid, or 0 if it isn’t. In either case, the hash context is reset so that it can be used for new messages.

Function: int rsa_md5_verify_digest (const struct rsa_public_key *key, const uint8_t *digest, const mpz_t signature)

Function: int rsa_sha1_verify_digest (const struct rsa_public_key *key, const uint8_t *digest, const mpz_t signature)

Function: int rsa_sha256_verify_digest (const struct rsa_public_key *key, const uint8_t *digest, const mpz_t signature)

Function: int rsa_sha512_verify_digest (const struct rsa_public_key *key, const uint8_t *digest, const mpz_t signature)

Returns 1 if the signature is valid, or 0 if it isn’t. digest should point to a digest of size MD5_DIGEST_SIZE, SHA1_DIGEST_SIZE, SHA256_DIGEST_SIZE, or SHA512_DIGEST_SIZE respectively.

Function: int rsa_pkcs1_verify(const struct rsa_public_key *key, size_t length, const uint8_t *digest_info, const mpz_t signature)

Similar to the above _verify_digest functions, but the input is not the plain hash digest, but a PKCS#1 “DigestInfo”, and ASN.1 DER-encoding of the digest together with an object identifier for the used hash algorithm.

While the above functions for the RSA signature operations use the PKCS#1 padding scheme, Nettle also provides the variants based on the PSS padding scheme, specified in RFC 3447. These variants take advantage of a randomly choosen salt value, which could enhance the security by causing output to be different for equivalent inputs. However, assuming the same security level as inverting the RSA algorithm, a longer salt value does not always mean a better security http://www.iacr.org/archive/eurocrypt2002/23320268/coron.pdf. The typical choices of the length are between 0 and the digest size of the underlying hash function.

Creating an RSA signature with the PSS padding scheme is done with one of the following functions:

Function: int rsa_pss_sha256_sign_digest_tr(const struct rsa_public_key *pub, const struct rsa_private_key *key, void *random_ctx, nettle_random_func *random, size_t salt_length, const uint8_t *salt, const uint8_t *digest, mpz_t signature)

Function: int rsa_pss_sha384_sign_digest_tr(const struct rsa_public_key *pub, const struct rsa_private_key *key, void *random_ctx, nettle_random_func *random, size_t salt_length, const uint8_t *salt, const uint8_t *digest, mpz_t signature)

Function: int rsa_pss_sha512_sign_digest_tr(const struct rsa_public_key *pub, const struct rsa_private_key *key, void *random_ctx, nettle_random_func *random, size_t salt_length, const uint8_t *salt, const uint8_t *digest, mpz_t signature)

Creates a signature using the PSS padding scheme. salt should point to a salt string of size salt_length. digest should point to a digest of size SHA256_DIGEST_SIZE, SHA384_DIGEST_SIZE, or SHA512_DIGEST_SIZErespectively. The signature is stored in signature (which must have been mpz_init:ed earlier). Returns one on success, or zero on failure.

Verifying an RSA signature with the PSS padding scheme is done with one of the following functions:

Function: int rsa_pss_sha256_verify_digest (const struct rsa_public_key *key, size_t salt_length, const uint8_t *digest, const mpz_t signature)

Function: int rsa_pss_sha384_verify_digest (const struct rsa_public_key *key, size_t salt_length, const uint8_t *digest, const mpz_t signature)

Function: int rsa_pss_sha512_verify_digest (const struct rsa_public_key *key, size_t salt_length, const uint8_t *digest, const mpz_t signature)

Returns 1 if the signature is valid, or 0 if it isn’t. digest should point to a digest of size SHA256_DIGEST_SIZE, SHA384_DIGEST_SIZE, or SHA512_DIGEST_SIZE respectively.

The following function is used to encrypt a clear text message using RSA.

Function: int rsa_encrypt (const struct rsa_public_key *key, void *random_ctx, nettle_random_func *random, size_t length, const uint8_t *cleartext, mpz_t ciphertext)

Returns 1 on success, 0 on failure. If the message is too long then this will lead to a failure.

The following function is used to decrypt a cipher text message using RSA.

Function: int rsa_decrypt (const struct rsa_private_key *key, size_t *length, uint8_t *cleartext, const mpz_t ciphertext)

Returns 1 on success, 0 on failure. Causes of failure include decryption failing or the resulting message being to large. The message buffer pointed to by cleartext must be of size *length. After decryption, *length will be updated with the size of the message.

There is also a timing resistant version of decryption that utilizes randomized RSA blinding.

Function: int rsa_decrypt_tr (const struct rsa_public_key *pub, const struct rsa_private_key *key, void *random_ctx, nettle_random_func *random, size_t *length, uint8_t *message, const mpz_t ciphertext)

Returns 1 on success, 0 on failure.

If you need to use the RSA trapdoor, the private key, in a way that isn’t supported by the above functions Nettle also includes a function that computes x^d mod n and nothing more, using the CRT optimization.

Function: int rsa_compute_root_tr(const struct rsa_public_key *pub, const struct rsa_private_key *key, void *random_ctx, nettle_random_func *random, mpz_t x, const mpz_t m)

Computes x = m^d. Returns one on success, or zero if a failure in the computation was detected.

Function: void rsa_compute_root (struct rsa_private_key *key, mpz_t x, const mpz_t m)

Computes x = m^d.

At last, how do you create new keys?

Function: int rsa_generate_keypair (struct rsa_public_key *pub, struct rsa_private_key *key, void *random_ctx, nettle_random_func random, void *progress_ctx, nettle_progress_func progress, unsigned n_size, unsigned e_size);

There are lots of parameters. pub and key is where the resulting key pair is stored. The structs should be initialized, but you don’t need to call rsa_public_key_prepare or rsa_private_key_prepare after key generation.

random_ctx and random is a randomness generator. random(random_ctx, length, dst) should generate length random octets and store them at dst. For advice, see See Randomness.

progress and progress_ctx can be used to get callbacks during the key generation process, in order to uphold an illusion of progress. progress can be NULL, in that case there are no callbacks.

size_n is the desired size of the modulo, in bits. If size_e is non-zero, it is the desired size of the public exponent and a random exponent of that size is selected. But if e_size is zero, it is assumed that the caller has already chosen a value for e, and stored it in pub. Returns one on success, and zero on failure. The function can fail for example if if n_size is too small, or if e_size is zero and pub->e is an even number.

6.7.2 DSA

The DSA digital signature algorithm is more complex than RSA. It was specified during the early 1990s, and in 1994 NIST published FIPS 186 which is the authoritative specification. Sometimes DSA is referred to using the acronym DSS, for Digital Signature Standard. The most recent revision of the specification, FIPS186-3, was issued in 2009, and it adds support for larger hash functions than sha1.

For DSA, the underlying mathematical problem is the computation of discrete logarithms. The public key consists of a large prime p, a small prime q which is a factor of p-1, a number g which generates a subgroup of order q modulo p, and an element y in that subgroup.

In the original DSA, the size of q is fixed to 160 bits, to match with the SHA1 hash algorithm. The size of p is in principle unlimited, but the standard specifies only nine specific sizes: 512 + l*64, where l is between 0 and 8. Thus, the maximum size of p is 1024 bits, and sizes less than 1024 bits are considered obsolete and not secure.

The subgroup requirement means that if you compute

g^t mod p

for all possible integers t, you will get precisely q distinct values.

The private key is a secret exponent x, such that

g^x = y mod p

In mathematical speak, x is the discrete logarithm of y mod p, with respect to the generator g. The size of x will also be about the same size as q. The security of the DSA algorithm relies on the difficulty of the discrete logarithm problem. Current algorithms to compute discrete logarithms in this setting, and hence crack DSA, are of two types. The first type works directly in the (multiplicative) group of integers mod p. The best known algorithm of this type is the Number Field Sieve, and it’s complexity is similar to the complexity of factoring numbers of the same size as p. The other type works in the smaller q-sized subgroup generated by g, which has a more difficult group structure. One good algorithm is Pollard-rho, which has complexity sqrt(q).

The important point is that security depends on the size of both p and q, and they should be chosen so that the difficulty of both discrete logarithm methods are comparable. Today, the security margin of the original DSA may be uncomfortably small. Using a p of 1024 bits implies that cracking using the number field sieve is expected to take about the same time as factoring a 1024-bit RSA modulo, and using a q of size 160 bits implies that cracking using Pollard-rho will take roughly 2^80 group operations. With the size of q fixed, tied to the SHA1 digest size, it may be tempting to increase the size of p to, say, 4096 bits. This will provide excellent resistance against attacks like the number field sieve which works in the large group. But it will do very little to defend against Pollard-rho attacking the small subgroup; the attacker is slowed down at most by a single factor of 10 due to the more expensive group operation. And the attacker will surely choose the latter attack.

The signature generation algorithm is randomized; in order to create a DSA signature, you need a good source for random numbers (see Randomness). Let us describe the common case of a 160-bit q.

To create a signature, one starts with the hash digest of the message, h, which is a 160 bit number, and a random number k, 0<k<q, also 160 bits. Next, one computes

r = (g^k mod p) mod q
s = k^-1 (h + x r) mod q

The signature is the pair (r, s), two 160 bit numbers. Note the two different mod operations when computing r, and the use of the secret exponent x.

To verify a signature, one first checks that 0 < r,s < q, and then one computes backwards,

w = s^-1 mod q
v = (g^(w h) y^(w r) mod p) mod q

The signature is valid if v = r. This works out because w = s^-1 mod q = k (h + x r)^-1 mod q, so that

g^(w h) y^(w r) = g^(w h) (g^x)^(w r) = g^(w (h + x r)) = g^k 

When reducing mod q this yields r. Note that when verifying a signature, we don’t know either k or x: those numbers are secret.

If you can choose between RSA and DSA, which one is best? Both are believed to be secure. DSA gained popularity in the late 1990s, as a patent free alternative to RSA. Now that the RSA patents have expired, there’s no compelling reason to want to use DSA. Today, the original DSA key size does not provide a large security margin, and it should probably be phased out together with RSA keys of 1024 bits. Using the revised DSA algorithm with a larger hash function, in particular, SHA256, a 256-bit q, and p of size 2048 bits or more, should provide for a more comfortable security margin, but these variants are not yet in wide use.

DSA signatures are smaller than RSA signatures, which is important for some specialized applications.

From a practical point of view, DSA’s need for a good randomness source is a serious disadvantage. If you ever use the same k (and r) for two different message, you leak your private key.

6.7.2.1 Nettle’s DSA support

Like for RSA, Nettle represents DSA keys using two structures, containing values of type mpz_t. For information on how to customize allocation, see See GMP Allocation in GMP Manual. Nettle’s DSA interface is defined in <nettle/dsa.h>.

A DSA group is represented using the following struct.

Context struct: dsa_params p q g

Parameters of the DSA group.

Function: void dsa_params_init (struct dsa_params *params)

Calls mpz_init on all numbers in the struct.

Function: void dsa_params_clear (struct dsa_params *paramsparams)

Calls mpz_clear on all numbers in the struct.

Function: int dsa_generate_params (struct dsa_params *params, void *random_ctx, nettle_random_func *random, void *progress_ctx, nettle_progress_func *progress, unsigned p_bits, unsigned q_bits)

Generates paramaters of a new group. The params struct should be initialized before you call this function.

random_ctx and random is a randomness generator. random(random_ctx, length, dst) should generate length random octets and store them at dst. For advice, see See Randomness.

progress and progress_ctx can be used to get callbacks during the key generation process, in order to uphold an illusion of progress. progress can be NULL, in that case there are no callbacks.

p_bits and q_bits are the desired sizes of p and q. To generate keys that conform to the original DSA standard, you must use q_bits = 160 and select p_bits of the form p_bits = 512 + l*64, for 0 <= l <= 8, where the smaller sizes are no longer recommended, so you should most likely stick to p_bits = 1024. Non-standard sizes are possible, in particular p_bits larger than 1024, although DSA implementations can not in general be expected to support such keys. Also note that using very large p_bits, with q_bits fixed at 160, doesn’t make much sense, because the security is also limited by the size of the smaller prime. To generate DSA keys for use with SHA256, use q_bits = 256 and, e.g., p_bits = 2048.

Returns one on success, and zero on failure. The function will fail if q_bits is too small, or too close to p_bits.

Signatures are represented using the structure below.

Context struct: dsa_signature r s

Function: void dsa_signature_init (struct dsa_signature *signature)

Function: void dsa_signature_clear (struct dsa_signature *signature)

You must call dsa_signature_init before creating or using a signature, and call dsa_signature_clear when you are finished with it.

Keys are represented as bignums, of type mpz_t. A public keys represent a group element, and is of the same size as p, while a private key is an exponent, of the same size as q.

Function: int dsa_sign (const struct dsa_params *params, const mpz_t x, void *random_ctx, nettle_random_func *random, size_t digest_size, const uint8_t *digest, struct dsa_signature *signature)

Creates a signature from the given hash digest, using the private key x. random_ctx and random is a randomness generator. random(random_ctx, length, dst) should generate length random octets and store them at dst. For advice, see See Randomness. Returns one on success, or zero on failure. Signing can fail only if the key is invalid, so that inversion modulo q fails.

Function: int dsa_verify (const struct dsa_params *params, const mpz_t y, size_t digest_size, const uint8_t *digest, const struct dsa_signature *signature)

Verifies a signature, using the public key y. Returns 1 if the signature is valid, otherwise 0.

To generate a keypair, first generate a DSA group using dsa_generate_params. A keypair in this group is then created using

Function: void dsa_generate_keypair (const struct dsa_params *params, mpz_t pub, mpz_t key, void *random_ctx, nettle_random_func *random)

Generates a new keypair, using the group params. The public key is stored in pub, and the private key in key. Both variables must be initialized using mpz_init before this call.

random_ctx and random is a randomness generator. random(random_ctx, length, dst) should generate length random octets and store them at dst. For advice, see See Randomness.

6.7.2.2 Old, deprecated, DSA interface

Versions before nettle-3.0 used a different interface for DSA signatures, where the group parameters and the public key was packed together as struct dsa_public_key. Most of this interface is kept for backwards compatibility, and declared in nettle/dsa-compat.h. Below is the old documentation. The old and new interface use distinct names and don’t confict, with one exception: The key generation function. The nettle/dsa-compat.h redefines dsa_generate_keypair as an alias for dsa_compat_generate_keypair, compatible with the old interface and documented below.

The old DSA functions are very similar to the corresponding RSA functions, but there are a few differences pointed out below. For a start, there are no functions corresponding to rsa_public_key_prepare and rsa_private_key_prepare.

Context struct: dsa_public_key p q g y

The public parameters described above.

Context struct: dsa_private_key x

The private key x.

Before use, these structs must be initialized by calling one of

Function: void dsa_public_key_init (struct dsa_public_key *pub)

Function: void dsa_private_key_init (struct dsa_private_key *key)

Calls mpz_init on all numbers in the key struct.

When finished with them, the space for the numbers must be deallocated by calling one of

Function: void dsa_public_key_clear (struct dsa_public_key *pub)

Function: void dsa_private_key_clear (struct dsa_private_key *key)

Calls mpz_clear on all numbers in the key struct.

Signatures are represented using struct dsa_signature, described earlier.

For signing, you need to provide both the public and the private key (unlike RSA, where the private key struct includes all information needed for signing), and a source for random numbers. Signatures can use the SHA1 or the SHA256 hash function, although the implementation of DSA with SHA256 should be considered somewhat experimental due to lack of official test vectors and interoperability testing.

Function: int dsa_sha1_sign (const struct dsa_public_key *pub, const struct dsa_private_key *key, void *random_ctx, nettle_random_func random, struct sha1_ctx *hash, struct dsa_signature *signature)

Function: int dsa_sha1_sign_digest (const struct dsa_public_key *pub, const struct dsa_private_key *key, void *random_ctx, nettle_random_func random, const uint8_t *digest, struct dsa_signature *signature)

Function: int dsa_sha256_sign (const struct dsa_public_key *pub, const struct dsa_private_key *key, void *random_ctx, nettle_random_func random, struct sha256_ctx *hash, struct dsa_signature *signature)

Function: int dsa_sha256_sign_digest (const struct dsa_public_key *pub, const struct dsa_private_key *key, void *random_ctx, nettle_random_func random, const uint8_t *digest, struct dsa_signature *signature)

Creates a signature from the given hash context or digest. random_ctx and random is a randomness generator. random(random_ctx, length, dst) should generate length random octets and store them at dst. For advice, see See Randomness. Returns one on success, or zero on failure. Signing fails if the key size and the hash size don’t match.

Verifying signatures is a little easier, since no randomness generator is needed. The functions are

Function: int dsa_sha1_verify (const struct dsa_public_key *key, struct sha1_ctx *hash, const struct dsa_signature *signature)

Function: int dsa_sha1_verify_digest (const struct dsa_public_key *key, const uint8_t *digest, const struct dsa_signature *signature)

Function: int dsa_sha256_verify (const struct dsa_public_key *key, struct sha256_ctx *hash, const struct dsa_signature *signature)

Function: int dsa_sha256_verify_digest (const struct dsa_public_key *key, const uint8_t *digest, const struct dsa_signature *signature)

Verifies a signature. Returns 1 if the signature is valid, otherwise 0.

Key generation uses mostly the same parameters as the corresponding RSA function.

Function: int dsa_compat_generate_keypair (struct dsa_public_key *pub, struct dsa_private_key *key, void *random_ctx, nettle_random_func random, void *progress_ctx, nettle_progress_func progress, unsigned p_bits, unsigned q_bits)

pub and key is where the resulting key pair is stored. The structs should be initialized before you call this function.

random_ctx and random is a randomness generator. random(random_ctx, length, dst) should generate length random octets and store them at dst. For advice, see See Randomness.

progress and progress_ctx can be used to get callbacks during the key generation process, in order to uphold an illusion of progress. progress can be NULL, in that case there are no callbacks.

p_bits and q_bits are the desired sizes of p and q. See dsa_generate_keypair for details.

6.7.3 Elliptic curves

For cryptographic purposes, an elliptic curve is a mathematical group of points, and computing logarithms in this group is computationally difficult problem. Nettle uses additive notation for elliptic curve groups. If P and Q are two points, and k is an integer, the point sum, P + Q, and the multiple k P can be computed efficiently, but given only two points P and Q, finding an integer k such that Q = k P is the elliptic curve discrete logarithm problem.

Nettle supports standard curves which are all of the form y^2 = x^3 - 3 x + b (mod p), i.e., the points have coordinates (x,y), both considered as integers modulo a specified prime p. Curves are represented as a struct ecc_curve. It also supports curve25519, which uses a different form of curve. Supported curves are declared in <nettle/ecc-curve.h>, e.g., call nettle_get_secp_256r1 for a standardized curve using the 256-bit prime p = 2^{256} - 2^{224} + 2^{192} + 2^{96} - 1. The contents of these structs is not visible to nettle users. The “bitsize of the curve” is used as a shorthand for the bitsize of the curve’s prime p, e.g., 256 bits for the SECP 256R1 curve.

6.7.3.1 Side-channel silence

Nettle’s implementation of the elliptic curve operations is intended to be side-channel silent. The side-channel attacks considered are:

• Timing attacks If the timing of operations depends on secret values, an attacker interacting with your system can measure the response time, and infer information about your secrets, e.g., a private signature key.
• Attacks using memory caches Assume you have some secret data on a multi-user system, and that this data is properly protected so that other users get no direct access to it. If you have a process operating on the secret data, and this process does memory accesses depending on the data, e.g, an internal lookup table in some cryptographic algorithm, an attacker running a separate process on the same system may use behavior of internal CPU caches to get information about your secrets. This type of attack can even cross virtual machine boundaries.

Nettle’s ECC implementation is designed to be side-channel silent, and not leak any information to these attacks. Timing and memory accesses depend only on the size of the input data and its location in memory, not on the actual data bits. This implies a performance penalty in several of the building blocks.

6.7.3.2 ECDSA

ECDSA is a variant of the DSA digital signature scheme (see DSA), which works over an elliptic curve group rather than over a (subgroup of) integers modulo p. Like DSA, creating a signature requires a unique random nonce (repeating the nonce with two different messages reveals the private key, and any leak or bias in the generation of the nonce also leaks information about the key).

Unlike DSA, signatures are in general not tied to any particular hash function or even hash size. Any hash function can be used, and the hash value is truncated or padded as needed to get a size matching the curve being used. It is recommended to use a strong cryptographic hash function with digest size close to the bit size of the curve, e.g., SHA256 is a reasonable choice when using ECDSA signature over the curve secp256r1. A protocol or application using ECDSA has to specify which curve and which hash function to use, or provide some mechanism for negotiating.

Nettle defines ECDSA in <nettle/ecdsa.h>. We first need to define the data types used to represent public and private keys.

struct: struct ecc_point

Represents a point on an elliptic curve. In particular, it is used to represent an ECDSA public key.

Function: void ecc_point_init (struct ecc_point *p, const struct ecc_curve *ecc)

Initializes p to represent points on the given curve ecc. Allocates storage for the coordinates, using the same allocation functions as GMP.

Function: void ecc_point_clear (struct ecc_point *p)

Deallocate storage.

Function: int ecc_point_set (struct ecc_point *p, const mpz_t x, const mpz_t y)

Check that the given coordinates represent a point on the curve. If so, the coordinates are copied and converted to internal representation, and the function returns 1. Otherwise, it returns 0. Currently, the infinity point (or zero point, with additive notation) is not allowed.

Function: void ecc_point_get (const struct ecc_point *p, mpz_t x, mpz_t y)

Extracts the coordinate of the point p. The output parameters x or y may be NULL if the caller doesn’t want that coordinate.

struct: struct ecc_scalar

Represents an integer in the range 0 < x < group order, where the “group order” refers to the order of an ECC group. In particular, it is used to represent an ECDSA private key.

Function: void ecc_scalar_init (struct ecc_scalar *s, const struct ecc_curve *ecc)

Initializes s to represent a scalar suitable for the given curve ecc. Allocates storage using the same allocation functions as GMP.

Function: void ecc_scalar_clear (struct ecc_scalar *s)

Deallocate storage.

Function: int ecc_scalar_set (struct ecc_scalar *s, const mpz_t z)

Check that z is in the correct range. If so, copies the value to s and returns 1, otherwise returns 0.

Function: void ecc_scalar_get (const struct ecc_scalar *s, mpz_t z)

Extracts the scalar, in GMP mpz_t representation.

To create and verify ECDSA signatures, the following functions are used.

Function: void ecdsa_sign (const struct ecc_scalar *key, void *random_ctx, nettle_random_func *random, size_t digest_length, const uint8_t *digest, struct dsa_signature *signature)

Uses the private key key to create a signature on digest. random_ctx and random is a randomness generator. random(random_ctx, length, dst) should generate length random octets and store them at dst. The signature is stored in signature, in the same was as for plain DSA.

Function: int ecdsa_verify (const struct ecc_point *pub, size_t length, const uint8_t *digest, const struct dsa_signature *signature)

Uses the public key pub to verify that signature is a valid signature for the message digest digest (of length octets). Returns 1 if the signature is valid, otherwise 0.

Finally, generating a new ECDSA key pair: