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\input texinfo          @c -*-texinfo-*-
@c %**start of header
@setfilename nettle.info
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@settitle Nettle: a low-level cryptographic library
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@documentencoding UTF-8
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@footnotestyle separate
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@syncodeindex fn cp
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@c %**end of header
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@set UPDATED-FOR 2.6
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@set AUTHOR Niels Möller
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@copying
This manual is for the Nettle library (version @value{UPDATED-FOR}), a
low-level cryptographic library.
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Originally written 2001 by @value{AUTHOR}, updated 2012.
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@quotation
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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.
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@end quotation
@end copying
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@titlepage
@title Nettle Manual
@subtitle For the Nettle Library version @value{UPDATED-FOR}
@author @value{AUTHOR}
@page
@vskip 0pt plus 1filll
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@insertcopying
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@end titlepage

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@dircategory Encryption
@direntry
* Nettle: (nettle).             A low-level cryptographic library.
@end direntry

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@contents

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@ifnottex
@node     Top, Introduction, (dir), (dir)
@comment  node-name,  next,  previous,  up
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@top Nettle
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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.
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@insertcopying
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@menu
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* Introduction::                What is Nettle?
* Copyright::                   Your rights.
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* Conventions::                 General interface conventions.
* Example::                     An example program.
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* Linking::                     Linking with the libnettle and libhogweed.
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* Reference::                   All Nettle functions and features.
* Nettle soup::                 For the serious nettle hacker.
* Installation::                How to install Nettle.
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* Index::                       Function and concept index.
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@detailmenu
 --- The Detailed Node Listing ---

Reference

* Hash functions::              
* Cipher functions::            
* Cipher modes::                
* Keyed hash functions::        
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* Key derivation functions::    
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* Public-key algorithms::       
* Randomness::                  
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* Ascii encoding::              
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* Miscellaneous functions::     
* Compatibility functions::     

Cipher modes

* CBC::                         
* CTR::                         
* GCM::                         

Public-key algorithms

* RSA::                         The RSA public key algorithm.
* DSA::                         The DSA digital signature algorithm.

@end detailmenu
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@end menu

@end ifnottex

@node Introduction, Copyright, Top, Top
@comment  node-name,  next,  previous,  up
@chapter 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.

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And as the requirements of applications differ in subtle and not so
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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 @emph{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
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interfaces on top of Nettle, and share the code, test cases, benchmarks,
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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.
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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.

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@node Copyright, Conventions, Introduction, Top
@comment  node-name,  next,  previous,  up
@chapter Copyright

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Nettle is distributed under the GNU Lesser General Public License
(LGPL), see the file COPYING.LIB for details. A few of the individual
files are 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.
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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.

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A list of the supported algorithms, their origins and licenses:

@table @emph
@item AES
The implementation of the AES cipher (also known as rijndael) is written
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by Rafael Sevilla. Assembler for x86 by Rafael Sevilla and
@value{AUTHOR}, Sparc assembler by @value{AUTHOR}. Released under the
LGPL.
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@item ARCFOUR
The implementation of the ARCFOUR (also known as RC4) cipher is written
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by @value{AUTHOR}. Released under the LGPL.
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@item ARCTWO
The implementation of the ARCTWO (also known as RC2) cipher is written
by Nikos Mavroyanopoulos and modified by Werner Koch and Simon
Josefsson. Released under the LGPL.

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@item BLOWFISH
The implementation of the BLOWFISH cipher is written by Werner Koch,
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copyright owned by the Free Software Foundation. Also hacked by Simon
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Josefsson and Niels Möller. Released under the LGPL.
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@item CAMELLIA
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The C implementation is by Nippon Telegraph and Telephone Corporation
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(NTT), heavily modified by @value{AUTHOR}. Assembler for x86 and x86_64
by @value{AUTHOR}. Released under the LGPL.
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@item CAST128
The implementation of the CAST128 cipher is written by Steve Reid.
Released into the public domain.

@item DES
The implementation of the DES cipher is written by Dana L. How, and
released under the LGPL.

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@item MD2
The implementation of MD2 is written by Andrew Kuchling, and hacked
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some by Andreas Sigfridsson and @value{AUTHOR}. Python Cryptography
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Toolkit license (essentially public domain).

@item MD4
This is almost the same code as for MD5 below, with modifications by
Marcus Comstedt. Released into the public domain.

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@item MD5
The implementation of the MD5 message digest is written by Colin Plumb.
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It has been hacked some more by Andrew Kuchling and @value{AUTHOR}.
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Released into the public domain.

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@item RIPMED160
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. Released under the LGPL.

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@item SALSA20
The C implementation of SALSA20 is based on D. J. Bernstein's reference
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implementation (in the public domain), adapted to Nettle by Simon
Josefsson, and heavily modified by Niels Möller. Assembly for x86_64 by
Niels Möller. Released under the LGPL.
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@item PBKDF2
The C implementation of PBKDF2 is based on earlier work for Shishi and
GnuTLS by Simon Josefsson.  Released under the LGPL.

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@item SERPENT
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The implementation of the SERPENT cipher is based on the code in libgcrypt,
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copyright owned by the Free Software Foundation. Adapted to Nettle by
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Simon Josefsson and heavily modified by Niels Möller. Assembly for
x86_64 by Niels Möller. Released under the LGPL.
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@item SHA1
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The C implementation of the SHA1 message digest is written by Peter
Gutmann, and hacked some more by Andrew Kuchling and @value{AUTHOR}.
Released into the public domain. Assembler for x86 by @value{AUTHOR},
released under the LGPL.
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@item SHA2
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Written by @value{AUTHOR}, using Peter Gutmann's SHA1 code as a model. 
Released under the LGPL.

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@item SHA3
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Written by @value{AUTHOR}. Released under the LGPL.

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@item 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.

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@item TWOFISH
The implementation of the TWOFISH cipher is written by Ruud de Rooij.
Released under the LGPL.
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@item RSA
Written by @value{AUTHOR}, released under the LGPL. Uses the GMP library
for bignum operations.

@item DSA
Written by @value{AUTHOR}, released under the LGPL. Uses the GMP library
for bignum operations.
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@end table

@node Conventions, Example, Copyright, Top
@comment  node-name,  next,  previous,  up
@chapter Conventions

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

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For consistency, functions for different algorithms are very similar,
but there are some differences, for instance reflecting if the key setup
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or encryption function differ for encryption and decryption, and whether
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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.
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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.
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By convention, for an algorithm named @code{foo}, the struct tag for the
context struct is @code{foo_ctx}, constants and functions uses prefixes
like @code{FOO_BLOCK_SIZE} (a constant) and @code{foo_set_key} (a
function).

In all functions, strings are represented with an explicit length, of
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type @code{unsigned}, and a pointer of type @code{uint8_t *} or
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@code{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 of the same length. Source and
destination may be the same, so that you can process strings in place,
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but they @emph{must not} overlap in any other way.
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Many of the functions lack return value and can never fail. Those
functions which can fail, return one on success and zero on failure.

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@c FIXME: Say something about the name mangling.

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@node Example, Linking, Conventions, Top
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@comment  node-name,  next,  previous,  up
@chapter Example

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A simple example program that reads a file from standard input and
writes its SHA1 checksum on standard output should give the flavor of
Nettle.
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@example
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@verbatiminclude sha-example.c
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@end example

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On a typical Unix system, this program can be compiled and linked with
the command line 
@example
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gcc sha-example.c -o sha-example -lnettle
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@end example

@node Linking, Reference, Example, Top
@comment  node-name,  next,  previous,  up
@chapter Linking

Nettle actually consists of two libraries, @file{libnettle} and
@file{libhogweed}. The @file{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.

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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 @code{-lnettle}. If an application also uses public-key
algorithms, the recommended linker flags are @code{-lhogweed -lnettle
-lgmp}. If the involved libraries are installed as dynamic libraries, it
may be sufficient to link with just @code{-lhogweed}, and the loader
will resolve the dependencies automatically.
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@node Reference, Nettle soup, Linking, Top
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@comment  node-name,  next,  previous,  up
@chapter Reference

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

@menu
* Hash functions::              
* Cipher functions::            
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* Cipher modes::                
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* Keyed hash functions::        
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* Key derivation functions::    
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* Public-key algorithms::       
* Randomness::                  
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* Ascii encoding::              
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* Miscellaneous functions::     
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* Compatibility functions::     
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@end menu

@node Hash functions, Cipher functions, Reference, Reference
@comment  node-name,  next,  previous,  up
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@section Hash functions
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@cindex Hash function
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A cryptographic @dfn{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
@code{H} should have the following properties:

@table @emph

@item One-way
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@cindex One-way
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Given a hash value @code{H(x)} it is hard to find a string @code{x}
that hashes to that value.

@item Collision-resistant
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@cindex Collision-resistant
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It is hard to find two different strings, @code{x} and @code{y}, such
that @code{H(x)} = @code{H(y)}.

@end table

Hash functions are useful as building blocks for digital signatures,
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message authentication codes, pseudo random generators, association of
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unique ids to documents, and many other things.
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The most commonly used hash functions are MD5 and SHA1. Unfortunately,
both these fail the collision-resistance requirement; cryptologists have
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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 (December 2012), the winner of the NIST SHA3
competition has recently been announced, and the new SHA3 (earlier known
as Keccak) and other top SHA3 candidates may also be reasonable
alternatives.
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@menu
* Recommended hash functions::
* Legacy hash functions::
* nettle_hash abstraction::
@end menu
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@node Recommended hash functions, Legacy hash functions,, Hash functions
@comment  node-name,  next,  previous,  up
@subsection Recommended hash functions
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The following hash functions have no known weaknesses, and are suitable
for new applications. The SHA2 family of hash functions were specified
by @dfn{NIST}, intended as a replacement for @acronym{SHA1}.

@subsubsection @acronym{SHA256}

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

@deftp {Context struct} {struct sha256_ctx}
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@end deftp

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@defvr Constant SHA256_DIGEST_SIZE
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The size of a SHA256 digest, i.e. 32.
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@end defvr

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@defvr Constant SHA256_DATA_SIZE
The internal block size of SHA256. Useful for some special constructions,
in particular HMAC-SHA256.
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@end defvr

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@deftypefun void sha256_init (struct sha256_ctx *@var{ctx})
Initialize the SHA256 state.
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@end deftypefun

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@deftypefun void sha256_update (struct sha256_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{data})
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Hash some more data.
@end deftypefun

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@deftypefun void sha256_digest (struct sha256_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{digest})
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Performs final processing and extracts the message digest, writing it
to @var{digest}. @var{length} may be smaller than
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@code{SHA256_DIGEST_SIZE}, in which case only the first @var{length}
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octets of the digest are written.
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This function also resets the context in the same way as
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@code{sha256_init}.
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@end deftypefun

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Earlier versions of nettle defined SHA256 in the header file
@file{<nettle/sha.h>}, which is now deprecated, but kept for
compatibility.
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@subsubsection @acronym{SHA224}
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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
@file{<nettle/sha2.h>} (and in @file{<nettle/sha.h>}, for backwards
compatibility).
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@deftp {Context struct} {struct sha224_ctx}
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@end deftp

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@defvr Constant SHA224_DIGEST_SIZE
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The size of a SHA224 digest, i.e. 28.
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@end defvr

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@defvr Constant SHA224_DATA_SIZE
The internal block size of SHA224. Useful for some special constructions,
in particular HMAC-SHA224.
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@end defvr

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@deftypefun void sha224_init (struct sha224_ctx *@var{ctx})
Initialize the SHA224 state.
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@end deftypefun

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@deftypefun void sha224_update (struct sha224_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{data})
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Hash some more data.
@end deftypefun

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@deftypefun void sha224_digest (struct sha224_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{digest})
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Performs final processing and extracts the message digest, writing it
to @var{digest}. @var{length} may be smaller than
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@code{SHA224_DIGEST_SIZE}, in which case only the first @var{length}
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octets of the digest are written.

This function also resets the context in the same way as
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@code{sha224_init}.
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@end deftypefun

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@subsubsection @acronym{SHA512}
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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 @file{<nettle/sha2.h>} (and in
@file{<nettle/sha.h>}, for backwards compatibility).
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@deftp {Context struct} {struct sha512_ctx}
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@end deftp

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@defvr Constant SHA512_DIGEST_SIZE
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The size of a SHA512 digest, i.e. 64.
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@end defvr

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@defvr Constant SHA512_DATA_SIZE
The internal block size of SHA512. Useful for some special constructions,
in particular HMAC-SHA512.
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@end defvr

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@deftypefun void sha512_init (struct sha512_ctx *@var{ctx})
Initialize the SHA512 state.
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@end deftypefun

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@deftypefun void sha512_update (struct sha512_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{data})
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Hash some more data.
@end deftypefun

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@deftypefun void sha512_digest (struct sha512_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{digest})
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Performs final processing and extracts the message digest, writing it
to @var{digest}. @var{length} may be smaller than
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@code{SHA512_DIGEST_SIZE}, in which case only the first @var{length}
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octets of the digest are written.

This function also resets the context in the same way as
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@code{sha512_init}.
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@end deftypefun

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@subsubsection @acronym{SHA384}
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SHA384 is a variant of SHA512, with a different initial state, and with
the output truncated to 384 bits, or 48 octets. Nettle defines SHA384 in
@file{<nettle/sha2.h>} (and in @file{<nettle/sha.h>}, for backwards
compatibility).
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@deftp {Context struct} {struct sha384_ctx}
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@end deftp

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@defvr Constant SHA384_DIGEST_SIZE
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The size of a SHA384 digest, i.e. 48.
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@end defvr

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@defvr Constant SHA384_DATA_SIZE
The internal block size of SHA384. Useful for some special constructions,
in particular HMAC-SHA384.
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@end defvr

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@deftypefun void sha384_init (struct sha384_ctx *@var{ctx})
Initialize the SHA384 state.
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@end deftypefun

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@deftypefun void sha384_update (struct sha384_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{data})
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Hash some more data.
@end deftypefun

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@deftypefun void sha384_digest (struct sha384_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{digest})
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Performs final processing and extracts the message digest, writing it
to @var{digest}. @var{length} may be smaller than
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@code{SHA384_DIGEST_SIZE}, in which case only the first @var{length}
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octets of the digest are written.

This function also resets the context in the same way as
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@code{sha384_init}.
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@end deftypefun

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@subsubsection @acronym{SHA3-224}
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The SHA3 hash functions were specified by NIST in repsonse to weaknesses
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in SHA1, and doubts about SHA2 hash functions which structurally are
very similar to SHA1. The standard 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).
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Nettle defines SHA3-224 in @file{<nettle/sha3.h>}.
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@deftp {Context struct} {struct sha3_224_ctx}
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@end deftp

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@defvr Constant SHA3_224_DIGEST_SIZE
The size of a SHA3_224 digest, i.e., 28.
@end defvr

@defvr Constant SHA3_224_DATA_SIZE
The internal block size of SHA3_224.
@end defvr

@deftypefun void sha3_224_init (struct sha3_224_ctx *@var{ctx})
Initialize the SHA3-224 state.
@end deftypefun

@deftypefun void sha3_224_update (struct sha3_224_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{data})
Hash some more data.
@end deftypefun

@deftypefun void sha3_224_digest (struct sha3_224_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{digest})
Performs final processing and extracts the message digest, writing it
to @var{digest}. @var{length} may be smaller than
@code{SHA3_224_DIGEST_SIZE}, in which case only the first @var{length}
octets of the digest are written.

This function also resets the context.
@end deftypefun

@subsubsection @acronym{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 @file{<nettle/sha3.h>}.

@deftp {Context struct} {struct sha3_256_ctx}
@end deftp

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@defvr Constant SHA3_256_DIGEST_SIZE
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The size of a SHA3_256 digest, i.e., 32.
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@end defvr

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@defvr Constant SHA3_256_DATA_SIZE
The internal block size of SHA3_256.
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@end defvr

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@deftypefun void sha3_256_init (struct sha3_256_ctx *@var{ctx})
Initialize the SHA3-256 state.
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@end deftypefun

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@deftypefun void sha3_256_update (struct sha3_256_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{data})
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Hash some more data.
@end deftypefun

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@deftypefun void sha3_256_digest (struct sha3_256_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{digest})
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Performs final processing and extracts the message digest, writing it
to @var{digest}. @var{length} may be smaller than
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@code{SHA3_256_DIGEST_SIZE}, in which case only the first @var{length}
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octets of the digest are written.

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This function also resets the context.
@end deftypefun

@subsubsection @acronym{SHA3-384}

This is SHA3 with 384-bit output size.

Nettle defines SHA3-384 in @file{<nettle/sha3.h>}.

@deftp {Context struct} {struct sha3_384_ctx}
@end deftp

@defvr Constant SHA3_384_DIGEST_SIZE
The size of a SHA3_384 digest, i.e., 48.
@end defvr

@defvr Constant SHA3_384_DATA_SIZE
The internal block size of SHA3_384.
@end defvr

@deftypefun void sha3_384_init (struct sha3_384_ctx *@var{ctx})
Initialize the SHA3-384 state.
@end deftypefun

@deftypefun void sha3_384_update (struct sha3_384_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{data})
Hash some more data.
@end deftypefun

@deftypefun void sha3_384_digest (struct sha3_384_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{digest})
Performs final processing and extracts the message digest, writing it
to @var{digest}. @var{length} may be smaller than
@code{SHA3_384_DIGEST_SIZE}, in which case only the first @var{length}
octets of the digest are written.

This function also resets the context.
@end deftypefun

@subsubsection @acronym{SHA3-512}

This is SHA3 with 512-bit output size.

Nettle defines SHA3-512 in @file{<nettle/sha3.h>}.

@deftp {Context struct} {struct sha3_512_ctx}
@end deftp

@defvr Constant SHA3_512_DIGEST_SIZE
The size of a SHA3_512 digest, i.e. 64.
@end defvr

@defvr Constant SHA3_512_DATA_SIZE
The internal block size of SHA3_512.
@end defvr

@deftypefun void sha3_512_init (struct sha3_512_ctx *@var{ctx})
Initialize the SHA3-512 state.
@end deftypefun

@deftypefun void sha3_512_update (struct sha3_512_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{data})
Hash some more data.
@end deftypefun

@deftypefun void sha3_512_digest (struct sha3_512_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{digest})
Performs final processing and extracts the message digest, writing it
to @var{digest}. @var{length} may be smaller than
@code{SHA3_512_DIGEST_SIZE}, in which case only the first @var{length}
octets of the digest are written.

This function also resets the context.
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@end deftypefun

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@node Legacy hash functions, nettle_hash abstraction, Recommended hash functions, Hash functions
@comment  node-name,  next,  previous,  up
@subsection Legacy hash functions
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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.
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@subsubsection @acronym{MD5}
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MD5 is a message digest function constructed by Ronald Rivest, and
described in @cite{RFC 1321}. It outputs message digests of 128 bits, or
16 octets. Nettle defines MD5 in @file{<nettle/md5.h>}.

@deftp {Context struct} {struct md5_ctx}
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@end deftp

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@defvr Constant MD5_DIGEST_SIZE
The size of an MD5 digest, i.e. 16.
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@end defvr

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@defvr Constant MD5_DATA_SIZE
The internal block size of MD5. Useful for some special constructions,
in particular HMAC-MD5.
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@end defvr

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@deftypefun void md5_init (struct md5_ctx *@var{ctx})
Initialize the MD5 state.
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@end deftypefun

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@deftypefun void md5_update (struct md5_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{data})
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Hash some more data.
@end deftypefun

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@deftypefun void md5_digest (struct md5_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{digest})
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Performs final processing and extracts the message digest, writing it
to @var{digest}. @var{length} may be smaller than
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@code{MD5_DIGEST_SIZE}, in which case only the first @var{length}
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octets of the digest are written.
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This function also resets the context in the same way as
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@code{md5_init}.
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@end deftypefun

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The normal way to use MD5 is to call the functions in order: First
@code{md5_init}, then @code{md5_update} zero or more times, and finally
@code{md5_digest}. After @code{md5_digest}, the context is reset to
its initial state, so you can start over calling @code{md5_update} to
hash new data.
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To start over, you can call @code{md5_init} at any time.
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@subsubsection @acronym{MD2}
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MD2 is another hash function of Ronald Rivest's, described in
@cite{RFC 1319}. It outputs message digests of 128 bits, or 16 octets.
Nettle defines MD2 in @file{<nettle/md2.h>}.

@deftp {Context struct} {struct md2_ctx}
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@end deftp

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@defvr Constant MD2_DIGEST_SIZE
The size of an MD2 digest, i.e. 16.
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@end defvr

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@defvr Constant MD2_DATA_SIZE
The internal block size of MD2.
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@end defvr

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@deftypefun void md2_init (struct md2_ctx *@var{ctx})
Initialize the MD2 state.
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@end deftypefun

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@deftypefun void md2_update (struct md2_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{data})
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Hash some more data.
@end deftypefun

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@deftypefun void md2_digest (struct md2_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{digest})
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Performs final processing and extracts the message digest, writing it
to @var{digest}. @var{length} may be smaller than
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@code{MD2_DIGEST_SIZE}, in which case only the first @var{length}
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octets of the digest are written.

This function also resets the context in the same way as
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@code{md2_init}.
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@end deftypefun

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@subsubsection @acronym{MD4}
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MD4 is a predecessor of MD5, described in @cite{RFC 1320}. Like MD5, it
is constructed by Ronald Rivest. It outputs message digests of 128 bits,
or 16 octets. Nettle defines MD4 in @file{<nettle/md4.h>}. Use of MD4 is
not recommended, but it is sometimes needed for compatibility with
existing applications and protocols.
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@deftp {Context struct} {struct md4_ctx}
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@end deftp

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@defvr Constant MD4_DIGEST_SIZE
The size of an MD4 digest, i.e. 16.
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@end defvr

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@defvr Constant MD4_DATA_SIZE
The internal block size of MD4.
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@end defvr

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@deftypefun void md4_init (struct md4_ctx *@var{ctx})
Initialize the MD4 state.
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@end deftypefun

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@deftypefun void md4_update (struct md4_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{data})
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Hash some more data.
@end deftypefun

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@deftypefun void md4_digest (struct md4_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{digest})
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Performs final processing and extracts the message digest, writing it
to @var{digest}. @var{length} may be smaller than
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@code{MD4_DIGEST_SIZE}, in which case only the first @var{length}
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octets of the digest are written.

This function also resets the context in the same way as
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@code{md4_init}.
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@end deftypefun

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@subsubsection @acronym{RIPEMD160}
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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 @file{nettle/ripemd160.h}.
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@deftp {Context struct} {struct ripemd160_ctx}
@end deftp
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@defvr Constant RIPEMD160_DIGEST_SIZE
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The size of a RIPEMD160 digest, i.e. 20.
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@end defvr

@defvr Constant RIPEMD160_DATA_SIZE
The internal block size of RIPEMD160.
@end defvr

@deftypefun void ripemd160_init (struct ripemd160_ctx *@var{ctx})
Initialize the RIPEMD160 state.
@end deftypefun

@deftypefun void ripemd160_update (struct ripemd160_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{data})
Hash some more data.
@end deftypefun

@deftypefun void ripemd160_digest (struct ripemd160_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{digest})
Performs final processing and extracts the message digest, writing it
to @var{digest}. @var{length} may be smaller than
@code{RIPEMD160_DIGEST_SIZE}, in which case only the first @var{length}
octets of the digest are written.

This function also resets the context in the same way as
@code{ripemd160_init}.
@end deftypefun

@subsubsection @acronym{SHA1}

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

@deftp {Context struct} {struct sha1_ctx}
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@end deftp

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@defvr Constant SHA1_DIGEST_SIZE
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The size of a SHA1 digest, i.e. 20.
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@end defvr

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@defvr Constant SHA1_DATA_SIZE
The internal block size of SHA1. Useful for some special constructions,
in particular HMAC-SHA1.
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@end defvr

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@deftypefun void sha1_init (struct sha1_ctx *@var{ctx})
Initialize the SHA1 state.
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@end deftypefun

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@deftypefun void sha1_update (struct sha1_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{data})
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Hash some more data.
@end deftypefun

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@deftypefun void sha1_digest (struct sha1_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{digest})
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Performs final processing and extracts the message digest, writing it
to @var{digest}. @var{length} may be smaller than
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@code{SHA1_DIGEST_SIZE}, in which case only the first @var{length}
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octets of the digest are written.

This function also resets the context in the same way as
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@code{sha1_init}.
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@end deftypefun

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

@deftp {Context struct} {struct gosthash94_ctx}
@end deftp

@defvr Constant GOSTHASH94_DIGEST_SIZE
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The size of a GOSTHASH94 digest, i.e. 32.
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@end defvr

@defvr Constant GOSTHASH94_DATA_SIZE
The internal block size of GOSTHASH94, i.e., 32.
@end defvr

@deftypefun void gosthash94_init (struct gosthash94_ctx *@var{ctx})
Initialize the GOSTHASH94 state.
@end deftypefun

@deftypefun void gosthash94_update (struct gosthash94_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{data})
Hash some more data.
@end deftypefun

@deftypefun void gosthash94_digest (struct gosthash94_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{digest})
Performs final processing and extracts the message digest, writing it
to @var{digest}. @var{length} may be smaller than
@code{GOSTHASH94_DIGEST_SIZE}, in which case only the first @var{length}
octets of the digest are written.

This function also resets the context in the same way as
@code{gosthash94_init}.
@end deftypefun

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@node nettle_hash abstraction,, Legacy hash functions, Hash functions
@comment  node-name,  next,  previous,  up
@subsection The nettle_hash abstraction
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Nettle includes a struct including information about the supported hash
functions. It is defined in @file{<nettle/nettle-meta.h>}, and is used
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by Nettle's implementation of @acronym{HMAC} (@pxref{Keyed hash
functions}).
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@deftp {Meta struct} @code{struct nettle_hash} name context_size digest_size block_size init update digest
The last three attributes are function pointers, of types
@code{nettle_hash_init_func}, @code{nettle_hash_update_func}, and
@code{nettle_hash_digest_func}. The first argument to these functions is
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@code{void *} pointer to a context struct, which is of size
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@code{context_size}. 
@end deftp

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@deftypevr {Constant Struct} {struct nettle_hash} nettle_md2
@deftypevrx {Constant Struct} {struct nettle_hash} nettle_md4
@deftypevrx {Constant Struct} {struct nettle_hash} nettle_md5
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@deftypevrx {Constant Struct} {struct nettle_hash} nettle_ripemd160
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@deftypevrx {Constant Struct} {struct nettle_hash} nettle_sha1
@deftypevrx {Constant Struct} {struct nettle_hash} nettle_sha224
@deftypevrx {Constant Struct} {struct nettle_hash} nettle_sha256
@deftypevrx {Constant Struct} {struct nettle_hash} nettle_sha384
@deftypevrx {Constant Struct} {struct nettle_hash} nettle_sha512
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@deftypevrx {Constant Struct} {struct nettle_hash} nettle_sha3_256
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@deftypevrx {Constant Struct} {struct nettle_hash} nettle_gosthash94
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These are all the hash functions that Nettle implements.
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@end deftypevr
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Nettle also exports a list of all these hashes.
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@deftypevr {Constant Array} {struct nettle_hash **} nettle_hashes
This list can be used to dynamically enumerate or search the supported
algorithms. NULL-terminated.
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@end deftypevr

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@node Cipher functions, Cipher modes, Hash functions, Reference
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@comment  node-name,  next,  previous,  up
@section Cipher functions
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@cindex Cipher
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A @dfn{cipher} is a function that takes a message or @dfn{plaintext}
and a secret @dfn{key} and transforms it to a @dfn{ciphertext}. Given
only the ciphertext, but not the key, it should be hard to find the
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plaintext. Given matching pairs of plaintext and ciphertext, it should
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be hard to find the key.

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@cindex Block Cipher
@cindex Stream Cipher

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

A block cipher can process data only in fixed size chunks, called
@dfn{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 @dfn{ECB}, Electronic Code Book mode.
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However, using @acronym{ECB} is usually a bad idea. For a start, plaintext blocks
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that are equal are transformed to ciphertext blocks that are equal; that
leaks information about the plaintext. Usually you should apply the
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cipher is some ``feedback mode'', @dfn{CBC} (Cipher Block Chaining) and
@dfn{CTR} (Counter mode) being two of
of the most popular. See @xref{Cipher modes}, for information on
how to apply @acronym{CBC} and @acronym{CTR} with Nettle.
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A stream cipher can be used for messages of arbitrary length. A typical
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stream cipher is a keyed pseudo-random generator. To encrypt a plaintext
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message of @var{n} octets, you key the generator, generate @var{n}
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octets of pseudo-random data, and XOR it with the plaintext. To decrypt,
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regenerate the same stream using the key, XOR it to the ciphertext, and
the plaintext is recovered.

@strong{Caution:} The first rule for this kind of cipher is the
same as for a One Time Pad: @emph{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
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receive an encrypted message. You apply the key, and get a plaintext
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message that makes sense to you. Can you then be sure that it really was
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your friend that wrote the message you're reading? The answer is no. For
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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
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wrote. If you are using a block cipher in @acronym{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.
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It is recommended to @emph{always} use an authentication mechanism in
addition to encrypting the messages. Popular choices are Message
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Authentication Codes like @acronym{HMAC-SHA1} (@pxref{Keyed hash
functions}), or digital signatures like @acronym{RSA}.
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Some ciphers have so called ``weak keys'', keys that results in
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undesirable structure after the key setup processing, and should be
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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
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have weak keys. There are several good ciphers that don't have any weak
keys.

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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
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as @dfn{key setup}. With Nettle, it is recommended to use each
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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.

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@subsection AES
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AES is a block cipher, specified by NIST as a replacement for
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the older DES standard. The standard is the result of a competition
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between cipher designers. The winning design, also known as RIJNDAEL,
was constructed by Joan Daemen and Vincent Rijnmen.
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Like all the AES candidates, the winning design uses a block size of 128
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bits, or 16 octets, and variable key-size, 128, 192 and 256 bits (16, 24
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and 32 octets) being the allowed key sizes. It does not have any weak
keys. Nettle defines AES in @file{<nettle/aes.h>}.
 
@deftp {Context struct} {struct aes_ctx}
@end deftp

@defvr Constant AES_BLOCK_SIZE
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The AES block-size, 16
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@end defvr

@defvr Constant AES_MIN_KEY_SIZE
@end defvr

@defvr Constant AES_MAX_KEY_SIZE
@end defvr

@defvr Constant AES_KEY_SIZE
Default AES key size, 32
@end defvr

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@deftypefun void aes_set_encrypt_key (struct aes_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{key})
@deftypefunx void aes_set_decrypt_key (struct aes_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{key})
Initialize the cipher, for encryption or decryption, respectively.
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@end deftypefun

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@deftypefun void aes_invert_key (struct aes_ctx *@var{dst}, const struct aes_ctx *@var{src})
Given a context @var{src} initialized for encryption, initializes the
context struct @var{dst} for decryption, using the same key. If the same
context struct is passed for both @code{src} and @code{dst}, it is
converted in place. Calling @code{aes_set_encrypt_key} and
@code{aes_invert_key} is more efficient than calling
@code{aes_set_encrypt_key} and @code{aes_set_decrypt_key}. This function
is mainly useful for applications which needs to both encrypt and
decrypt using the @emph{same} key.
@end deftypefun

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@deftypefun void aes_encrypt (struct aes_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
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Encryption function. @var{length} must be an integral multiple of the
block size. If it is more than one block, the data is processed in ECB
mode. @code{src} and @code{dst} may be equal, but they must not overlap
in any other way.
@end deftypefun

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@deftypefun void aes_decrypt (struct aes_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
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Analogous to @code{aes_encrypt}
@end deftypefun

@subsection 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
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``secret:1'', ``secret:2'', @enddots{}. If you have keys that don't look
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like random bit strings, and you want to use ARCFOUR, always hash the
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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.
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@example
/* A more robust key setup function for ARCFOUR */
void
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arcfour_set_key_hashed(struct arcfour_ctx *ctx,
                       unsigned length, const uint8_t *key)
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@{
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  struct sha256_ctx hash;
  uint8_t digest[SHA256_DIGEST_SIZE];
  uint8_t buffer[0x200];
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  sha256_init(&hash);
  sha256_update(&hash, length, key);
  sha256_digest(&hash, SHA256_DIGEST_SIZE, digest);
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