nettle.texinfo 126 KB
Newer Older
Niels Möller's avatar
Niels Möller committed
1 2 3
\input texinfo          @c -*-texinfo-*-
@c %**start of header
@setfilename nettle.info
4
@settitle Nettle: a low-level cryptographic library
5
@documentencoding ISO-8859-1
6
@footnotestyle separate
Niels Möller's avatar
Niels Möller committed
7
@syncodeindex fn cp
8
@c %**end of header
Niels Möller's avatar
Niels Möller committed
9

Niels Möller's avatar
Niels Möller committed
10
@set UPDATED-FOR 2.3
Niels Möller's avatar
Niels Möller committed
11
@set AUTHOR Niels Möller
12

13 14 15
@copying
This manual is for the Nettle library (version @value{UPDATED-FOR}), a
low-level cryptographic library.
Niels Möller's avatar
Niels Möller committed
16

Niels Möller's avatar
Niels Möller committed
17
Originally written 2001 by @value{AUTHOR}, updated 2011.
Niels Möller's avatar
Niels Möller committed
18

19
@quotation
20 21 22
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.
23 24
@end quotation
@end copying
Niels Möller's avatar
Niels Möller committed
25 26 27 28 29 30 31

@titlepage
@title Nettle Manual
@subtitle For the Nettle Library version @value{UPDATED-FOR}
@author @value{AUTHOR}
@page
@vskip 0pt plus 1filll
32
@insertcopying
Niels Möller's avatar
Niels Möller committed
33 34
@end titlepage

35 36 37 38 39
@dircategory Encryption
@direntry
* Nettle: (nettle).             A low-level cryptographic library.
@end direntry

40 41
@contents

Niels Möller's avatar
Niels Möller committed
42 43 44
@ifnottex
@node     Top, Introduction, (dir), (dir)
@comment  node-name,  next,  previous,  up
45
@top Nettle
Niels Möller's avatar
Niels Möller committed
46

47 48 49
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.
Niels Möller's avatar
Niels Möller committed
50

51
@insertcopying
Niels Möller's avatar
Niels Möller committed
52 53

@menu
Niels Möller's avatar
Niels Möller committed
54 55
* Introduction::                What is Nettle?
* Copyright::                   Your rights.
56 57
* Conventions::                 General interface conventions.
* Example::                     An example program.
58
* Linking::                     Linking with the libnettle and libhogweed.
Niels Möller's avatar
Niels Möller committed
59 60 61
* Reference::                   All Nettle functions and features.
* Nettle soup::                 For the serious nettle hacker.
* Installation::                How to install Nettle.
62
* Index::                       Function and concept index.
63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89

@detailmenu
 --- The Detailed Node Listing ---

Reference

* Hash functions::              
* Cipher functions::            
* Cipher modes::                
* Keyed hash functions::        
* Public-key algorithms::       
* Randomness::                  
* 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
Niels Möller's avatar
Niels Möller committed
90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105
@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.

106
And as the requirements of applications differ in subtle and not so
Niels Möller's avatar
Niels Möller committed
107 108 109 110 111 112 113 114 115 116
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
Niels Möller's avatar
Niels Möller committed
117
interfaces on top of Nettle, and share the code, test cases, benchmarks,
Niels Möller's avatar
Niels Möller committed
118 119 120
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.
Niels Möller's avatar
Niels Möller committed
121

122 123 124 125
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.

Niels Möller's avatar
Niels Möller committed
126 127 128 129
@node Copyright, Conventions, Introduction, Top
@comment  node-name,  next,  previous,  up
@chapter Copyright

130 131 132 133
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.
Niels Möller's avatar
Niels Möller committed
134

135 136 137 138 139
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.

Niels Möller's avatar
Niels Möller committed
140 141 142 143 144
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
145 146 147
by Rafael Sevilla. Assembler for x86 by Rafael Sevilla and
@value{AUTHOR}, Sparc assembler by @value{AUTHOR}. Released under the
LGPL.
Niels Möller's avatar
Niels Möller committed
148 149 150

@item ARCFOUR
The implementation of the ARCFOUR (also known as RC4) cipher is written
151
by @value{AUTHOR}. Released under the LGPL.
Niels Möller's avatar
Niels Möller committed
152

Niels Möller's avatar
Niels Möller committed
153 154 155 156 157
@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.

Niels Möller's avatar
Niels Möller committed
158 159
@item BLOWFISH
The implementation of the BLOWFISH cipher is written by Werner Koch,
160 161
copyright owned by the Free Software Foundation. Also hacked by Simon
Josefsson and Niels Möller. Released under the LGPL.
Niels Möller's avatar
Niels Möller committed
162

163
@item CAMELLIA
164
The C implementation is by Nippon Telegraph and Telephone Corporation
165 166
(NTT), heavily modified by @value{AUTHOR}. Assembler for x86 and x86_64
by @value{AUTHOR}. Released under the LGPL.
167

Niels Möller's avatar
Niels Möller committed
168 169 170 171 172 173 174 175
@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.

176 177
@item MD2
The implementation of MD2 is written by Andrew Kuchling, and hacked
178
some by Andreas Sigfridsson and @value{AUTHOR}. Python Cryptography
179 180 181 182 183 184
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.

Niels Möller's avatar
Niels Möller committed
185 186
@item MD5
The implementation of the MD5 message digest is written by Colin Plumb.
187
It has been hacked some more by Andrew Kuchling and @value{AUTHOR}.
Niels Möller's avatar
Niels Möller committed
188 189
Released into the public domain.

190 191 192 193 194
@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.

Niels Möller's avatar
Niels Möller committed
195
@item SERPENT
196
The implementation of the SERPENT cipher is based on the code in libgcrypt,
197 198 199
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. Released under the LGPL.
Niels Möller's avatar
Niels Möller committed
200 201

@item SHA1
202 203 204 205
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.
Niels Möller's avatar
Niels Möller committed
206

207
@item SHA224, SHA256, SHA384, and SHA512
Niels Möller's avatar
Niels Möller committed
208 209 210
Written by @value{AUTHOR}, using Peter Gutmann's SHA1 code as a model. 
Released under the LGPL.

Niels Möller's avatar
Niels Möller committed
211 212 213
@item TWOFISH
The implementation of the TWOFISH cipher is written by Ruud de Rooij.
Released under the LGPL.
Niels Möller's avatar
Niels Möller committed
214 215 216 217 218 219 220 221

@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.
Niels Möller's avatar
Niels Möller committed
222 223 224 225 226 227 228 229
@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
230
operating on the context. The context struct encapsulates all information
Niels Möller's avatar
Niels Möller committed
231 232 233
needed by the algorithm, and it can be copied or moved in memory with no
unexpected effects.

234 235
For consistency, functions for different algorithms are very similar,
but there are some differences, for instance reflecting if the key setup
236
or encryption function differ for encryption and decryption, and whether
237 238 239 240 241
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.
Niels Möller's avatar
Niels Möller committed
242

243 244 245
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.
Niels Möller's avatar
Niels Möller committed
246 247 248 249 250 251 252

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
253
type @code{unsigned}, and a pointer of type @code{uint8_t *} or
Niels Möller's avatar
Niels Möller committed
254 255 256 257
@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,
258
but they @emph{must not} overlap in any other way.
Niels Möller's avatar
Niels Möller committed
259

260 261 262
Many of the functions lack return value and can never fail. Those
functions which can fail, return one on success and zero on failure.

263 264
@c FIXME: Say something about the name mangling.

265
@node Example, Linking, Conventions, Top
Niels Möller's avatar
Niels Möller committed
266 267 268
@comment  node-name,  next,  previous,  up
@chapter Example

269 270 271
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.
Niels Möller's avatar
Niels Möller committed
272 273

@example
274
@verbatiminclude sha-example.c
Niels Möller's avatar
Niels Möller committed
275 276
@end example

277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292
On a typical Unix system, this program can be compiled and linked with
the command line 
@example
cc sha-example.c -o sha-example -lnettle
@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.

293 294 295 296 297 298 299
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.
300 301

@node Reference, Nettle soup, Linking, Top
Niels Möller's avatar
Niels Möller committed
302 303 304 305 306 307 308 309
@comment  node-name,  next,  previous,  up
@chapter Reference

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

@menu
* Hash functions::              
* Cipher functions::            
310
* Cipher modes::                
Niels Möller's avatar
Niels Möller committed
311
* Keyed hash functions::        
312 313
* Public-key algorithms::       
* Randomness::                  
Niels Möller's avatar
Niels Möller committed
314
* Miscellaneous functions::     
315
* Compatibility functions::     
Niels Möller's avatar
Niels Möller committed
316 317 318 319 320
@end menu

@node Hash functions, Cipher functions, Reference, Reference
@comment  node-name,  next,  previous,  up
@section Hash functions
321
@cindex Hash function
Niels Möller's avatar
Niels Möller committed
322 323 324 325 326 327 328 329 330 331
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
332
@cindex One-way
Niels Möller's avatar
Niels Möller committed
333 334 335 336
Given a hash value @code{H(x)} it is hard to find a string @code{x}
that hashes to that value.

@item Collision-resistant
337
@cindex Collision-resistant
Niels Möller's avatar
Niels Möller committed
338 339 340 341 342 343
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,
344
message authentication codes, pseudo random generators, association of
345
unique ids to documents, and many other things.
Niels Möller's avatar
Niels Möller committed
346

Niels Möller's avatar
Niels Möller committed
347 348 349 350 351 352
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 function
for new applications is SHA256, even though it uses a structure similar
to MD5 and SHA1. Constructing better hash functions is an urgent research
problem.
353

Niels Möller's avatar
Niels Möller committed
354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380
@subsection @acronym{MD5}

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}
@end deftp

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

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

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

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

@deftypefun void md5_digest (struct md5_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{digest})
381 382 383 384
Performs final processing and extracts the message digest, writing it
to @var{digest}. @var{length} may be smaller than
@code{MD5_DIGEST_SIZE}, in which case only the first @var{length}
octets of the digest are written.
Niels Möller's avatar
Niels Möller committed
385

386 387
This function also resets the context in the same way as
@code{md5_init}.
Niels Möller's avatar
Niels Möller committed
388 389 390
@end deftypefun

The normal way to use MD5 is to call the functions in order: First
391 392 393 394
@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.
Niels Möller's avatar
Niels Möller committed
395 396 397

To start over, you can call @code{md5_init} at any time.

398 399
@subsection @acronym{MD2}

Niels Möller's avatar
Niels Möller committed
400
MD2 is another hash function of Ronald Rivest's, described in
401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434
@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}
@end deftp

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

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

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

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

@deftypefun void md2_digest (struct md2_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{MD2_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{md2_init}.
@end deftypefun

@subsection @acronym{MD4}

Niels Möller's avatar
Niels Möller committed
435 436 437 438 439
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.
440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469

@deftp {Context struct} {struct md4_ctx}
@end deftp

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

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

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

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

@deftypefun void md4_digest (struct md4_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{MD4_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{md4_init}.
@end deftypefun

470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506
@subsection @acronym{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 @file{nettle/ripemd160.h}.

@deftp {Context struct} {struct ripemd160_ctx}
@end deftp

@defvr Constant RIPEMD160_DIGEST_SIZE
The size of an RIPEMD160 digest, i.e. 20.
@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

Niels Möller's avatar
Niels Möller committed
507 508 509
@subsection @acronym{SHA1}

SHA1 is a hash function specified by @dfn{NIST} (The U.S. National Institute
510 511
for Standards and Technology). It outputs hash values of 160 bits, or 20
octets. Nettle defines SHA1 in @file{<nettle/sha.h>}.
Niels Möller's avatar
Niels Möller committed
512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534

The functions are analogous to the MD5 ones.

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

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

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

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

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

535 536 537 538 539 540 541 542
@deftypefun void sha1_digest (struct sha1_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{SHA1_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{sha1_init}.
Niels Möller's avatar
Niels Möller committed
543 544
@end deftypefun

545 546 547 548 549 550 551 552 553 554 555 556 557
@subsection @acronym{SHA256}

SHA256 is another hash function specified by @dfn{NIST}, intended as a
replacement for @acronym{SHA1}, generating larger digests. It outputs
hash values of 256 bits, or 32 octets. Nettle defines SHA256 in
@file{<nettle/sha.h>}.

The functions are analogous to the MD5 ones.

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

@defvr Constant SHA256_DIGEST_SIZE
558
The size of an SHA256 digest, i.e. 32.
559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578
@end defvr

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

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

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

@deftypefun void sha256_digest (struct sha256_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{SHA256_DIGEST_SIZE}, in which case only the first @var{length}
octets of the digest are written.
Niels Möller's avatar
Niels Möller committed
579

580 581
This function also resets the context in the same way as
@code{sha256_init}.
Niels Möller's avatar
Niels Möller committed
582 583
@end deftypefun

584
@subsection @acronym{SHA224}
585

586 587
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
588 589 590 591
@file{<nettle/sha.h>}.

The functions are analogous to the MD5 ones.

592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631
@deftp {Context struct} {struct sha224_ctx}
@end deftp

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

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

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

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

@deftypefun void sha224_digest (struct sha224_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{SHA224_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{sha224_init}.
@end deftypefun

@subsection @acronym{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 @file{<nettle/sha.h>}.

The functions are analogous to the MD5 ones.

632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661
@deftp {Context struct} {struct sha512_ctx}
@end deftp

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

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

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

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

@deftypefun void sha512_digest (struct sha512_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{SHA512_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{sha512_init}.
@end deftypefun

662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699
@subsection @acronym{SHA384}

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/sha.h>}.

The functions are analogous to the MD5 ones.

@deftp {Context struct} {struct sha384_ctx}
@end deftp

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

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

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

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

@deftypefun void sha384_digest (struct sha384_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{SHA384_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{sha384_init}.
@end deftypefun

Niels Möller's avatar
Niels Möller committed
700 701 702 703
@subsection @code{struct nettle_hash}

Nettle includes a struct including information about the supported hash
functions. It is defined in @file{<nettle/nettle-meta.h>}, and is used
704 705
by Nettle's implementation of @acronym{HMAC} (@pxref{Keyed hash
functions}).
Niels Möller's avatar
Niels Möller committed
706 707 708 709 710

@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
711
@code{void *} pointer to a context struct, which is of size
Niels Möller's avatar
Niels Möller committed
712 713 714
@code{context_size}. 
@end deftp

715 716 717
@deftypevr {Constant Struct} {struct nettle_hash} nettle_md2
@deftypevrx {Constant Struct} {struct nettle_hash} nettle_md4
@deftypevrx {Constant Struct} {struct nettle_hash} nettle_md5
718
@deftypevrx {Constant Struct} {struct nettle_hash} nettle_ripemd160
719 720 721 722 723
@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
Niels Möller's avatar
Niels Möller committed
724 725

These are all the hash functions that Nettle implements.
726 727 728 729 730 731

Nettle also exports a list of all these hashes.  This list can be used
to dynamically enumerate or search the supported algorithms:

@deftypevrx {Constant Struct} {struct nettle_hash **} nettle_hashes

Niels Möller's avatar
Niels Möller committed
732 733
@end deftypevr

734
@node Cipher functions, Cipher modes, Hash functions, Reference
Niels Möller's avatar
Niels Möller committed
735 736
@comment  node-name,  next,  previous,  up
@section Cipher functions
737
@cindex Cipher
Niels Möller's avatar
Niels Möller committed
738 739 740 741

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
Niels Möller's avatar
Niels Möller committed
742
plaintext. Given matching pairs of plaintext and ciphertext, it should
Niels Möller's avatar
Niels Möller committed
743 744
be hard to find the key.

745 746 747
@cindex Block Cipher
@cindex Stream Cipher

Niels Möller's avatar
Niels Möller committed
748 749 750 751 752 753 754 755
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.
756
However, using @acronym{ECB} is usually a bad idea. For a start, plaintext blocks
Niels Möller's avatar
Niels Möller committed
757 758
that are equal are transformed to ciphertext blocks that are equal; that
leaks information about the plaintext. Usually you should apply the
759 760 761 762
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.
Niels Möller's avatar
Niels Möller committed
763

Niels Möller's avatar
Niels Möller committed
764
A stream cipher can be used for messages of arbitrary length. A typical
Niels Möller's avatar
Niels Möller committed
765
stream cipher is a keyed pseudo-random generator. To encrypt a plaintext
Niels Möller's avatar
Niels Möller committed
766
message of @var{n} octets, you key the generator, generate @var{n}
Niels Möller's avatar
Niels Möller committed
767
octets of pseudo-random data, and XOR it with the plaintext. To decrypt,
Niels Möller's avatar
Niels Möller committed
768 769 770 771 772 773 774 775
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
Niels Möller's avatar
Niels Möller committed
776
receive an encrypted message. You apply the key, and get a plaintext
777
message that makes sense to you. Can you then be sure that it really was
Niels Möller's avatar
Niels Möller committed
778
your friend that wrote the message you're reading? The answer is no. For
Niels Möller's avatar
Niels Möller committed
779 780 781 782
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
Niels Möller's avatar
Niels Möller committed
783 784 785 786
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.
Niels Möller's avatar
Niels Möller committed
787 788 789

It is recommended to @emph{always} use an authentication mechanism in
addition to encrypting the messages. Popular choices are Message
790 791
Authentication Codes like @acronym{HMAC-SHA1} (@pxref{Keyed hash
functions}), or digital signatures like @acronym{RSA}.
Niels Möller's avatar
Niels Möller committed
792

793
Some ciphers have so called ``weak keys'', keys that results in
Niels Möller's avatar
Niels Möller committed
794
undesirable structure after the key setup processing, and should be
795 796 797 798
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
Niels Möller's avatar
Niels Möller committed
799 800 801
have weak keys. There are several good ciphers that don't have any weak
keys.

802 803 804
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
805
as @dfn{key setup}. With Nettle, it is recommended to use each
806 807 808 809
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.

Niels Möller's avatar
Niels Möller committed
810
@subsection AES
811
AES is a block cipher, specified by NIST as a replacement for
Niels Möller's avatar
Niels Möller committed
812
the older DES standard. The standard is the result of a competition
813 814
between cipher designers. The winning design, also known as RIJNDAEL,
was constructed by Joan Daemen and Vincent Rijnmen.
Niels Möller's avatar
Niels Möller committed
815 816

Like all the AES candidates, the winning design uses a block size of 128
Niels Möller's avatar
Niels Möller committed
817
bits, or 16 octets, and variable key-size, 128, 192 and 256 bits (16, 24
Niels Möller's avatar
Niels Möller committed
818 819 820 821 822 823 824
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
Niels Möller's avatar
Niels Möller committed
825
The AES block-size, 16
Niels Möller's avatar
Niels Möller committed
826 827 828 829 830 831 832 833 834 835 836 837
@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

838 839 840
@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.
Niels Möller's avatar
Niels Möller committed
841 842
@end deftypefun

843 844 845 846 847 848 849 850 851 852 853
@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

854
@deftypefun void aes_encrypt (struct aes_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
Niels Möller's avatar
Niels Möller committed
855 856 857 858 859 860
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

861
@deftypefun void aes_decrypt (struct aes_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
Niels Möller's avatar
Niels Möller committed
862 863 864 865 866 867 868 869
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
870
``secret:1'', ``secret:2'', @enddots{}. If you have keys that don't look
Niels Möller's avatar
Niels Möller committed
871
like random bit strings, and you want to use ARCFOUR, always hash the
Niels Möller's avatar
Niels Möller committed
872 873 874
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.
Niels Möller's avatar
Niels Möller committed
875 876 877 878

@example
/* A more robust key setup function for ARCFOUR */
void
879 880
arcfour_set_key_hashed(struct arcfour_ctx *ctx,
                       unsigned length, const uint8_t *key)
Niels Möller's avatar
Niels Möller committed
881
@{
Niels Möller's avatar
Niels Möller committed
882 883 884
  struct sha256_ctx hash;
  uint8_t digest[SHA256_DIGEST_SIZE];
  uint8_t buffer[0x200];
Niels Möller's avatar
Niels Möller committed
885

Niels Möller's avatar
Niels Möller committed
886 887 888
  sha256_init(&hash);
  sha256_update(&hash, length, key);
  sha256_digest(&hash, SHA256_DIGEST_SIZE, digest);
Niels Möller's avatar
Niels Möller committed
889

Niels Möller's avatar
Niels Möller committed
890 891
  arcfour_set_key(ctx, SHA256_DIGEST_SIZE, digest);
  arcfour_crypt(ctx, sizeof(buffer), buffer, buffer);
Niels Möller's avatar
Niels Möller committed
892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916
@}
@end example

Nettle defines ARCFOUR in @file{<nettle/arcfour.h>}.

@deftp {Context struct} {struct arcfour_ctx}
@end deftp

@defvr Constant ARCFOUR_MIN_KEY_SIZE
Minimum key size, 1
@end defvr

@defvr Constant ARCFOUR_MAX_KEY_SIZE
Maximum key size, 256
@end defvr

@defvr Constant ARCFOUR_KEY_SIZE
Default ARCFOUR key size, 16
@end defvr

@deftypefun void arcfour_set_key (struct arcfour_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{key})
Initialize the cipher. The same function is used for both encryption and
decryption. 
@end deftypefun

917
@deftypefun void arcfour_crypt (struct arcfour_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
Niels Möller's avatar
Niels Möller committed
918 919 920 921 922 923 924
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
@code{arcfour_crypt} only once with all the data.
@end deftypefun

Niels Möller's avatar
Niels Möller committed
925 926 927 928
@subsection 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
929
reverse engineered RC2 cipher description, as described in a Usenet
Niels Möller's avatar
Niels Möller committed
930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946
post to @code{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, @code{ekb}.
This parameter can be used to artificially reduce the key size. In
practice, @code{ekb} is usually set equal to the input key size.
Nettle defines ARCTWO in @file{<nettle/arctwo.h>}.

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

@deftp {Context struct} {struct arctwo_ctx}
@end deftp

@defvr Constant ARCTWO_BLOCK_SIZE
947
The ARCTWO block-size, 8
Niels Möller's avatar
Niels Möller committed
948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974
@end defvr

@defvr Constant ARCTWO_MIN_KEY_SIZE
@end defvr

@defvr Constant ARCTWO_MAX_KEY_SIZE
@end defvr

@defvr Constant ARCTWO_KEY_SIZE
Default ARCTWO key size, 8
@end defvr

@deftypefun void arctwo_set_key_ekb (struct arctwo_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{key}, unsigned @var{ekb})
@deftypefunx void arctwo_set_key (struct arctwo_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{key})
@deftypefunx void arctwo_set_key_gutmann (struct arctwo_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{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 @var{ekb} is 1024, and for
convenience, @code{ekb = 0} has the same effect as @code{ekb = 1024}.

@code{arctwo_set_key(ctx, length, key)} is equivalent to
@code{arctwo_set_key_ekb(ctx, length, key, 8*length)}, and
@code{arctwo_set_key_gutmann(ctx, length, key)} is equivalent to
@code{arctwo_set_key_ekb(ctx, length, key, 1024)}
@end deftypefun

975
@deftypefun void arctwo_encrypt (struct arctwo_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
Niels Möller's avatar
Niels Möller committed
976 977 978 979 980 981
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

982
@deftypefun void arctwo_decrypt (struct arctwo_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
Niels Möller's avatar
Niels Möller committed
983 984 985
Analogous to @code{arctwo_encrypt}
@end deftypefun

Niels Möller's avatar
Niels Möller committed
986 987
@subsection BLOWFISH

988 989 990 991 992 993 994 995
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 @file{<nettle/blowfish.h>}.

@deftp {Context struct} {struct blowfish_ctx}
@end deftp

@defvr Constant BLOWFISH_BLOCK_SIZE
Niels Möller's avatar
Niels Möller committed
996
The BLOWFISH block-size, 8
997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012
@end defvr

@defvr Constant BLOWFISH_MIN_KEY_SIZE
Minimum BLOWFISH key size, 8
@end defvr

@defvr Constant BLOWFISH_MAX_KEY_SIZE
Maximum BLOWFISH key size, 56
@end defvr

@defvr Constant BLOWFISH_KEY_SIZE
Default BLOWFISH key size, 16
@end defvr

@deftypefun int blowfish_set_key (struct blowfish_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{key})
Initialize the cipher. The same function is used for both encryption and
1013 1014 1015 1016
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.

1017 1018 1019 1020
@code{blowfish_encrypt} or @code{blowfish_decrypt} with a weak key will
crash with an assert violation.
@end deftypefun

1021
@deftypefun void blowfish_encrypt (struct blowfish_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
1022 1023 1024 1025 1026 1027
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

1028
@deftypefun void blowfish_decrypt (struct blowfish_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
1029
Analogous to @code{blowfish_encrypt}
1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080
@end deftypefun

@subsection Camellia

Camellia is a block cipher developed by Mitsubishi and Nippon Telegraph
and Telephone Corporation, described in @cite{RFC3713}, and recommended
by some Japanese and European authorities as an alternative to AES. 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
@url{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. Nettle defines Camellia in @file{<nettle/camellia.h>}.

@deftp {Context struct} {struct camellia_ctx}
@end deftp

@defvr Constant CAMELLIA_BLOCK_SIZE
The CAMELLIA block-size, 16
@end defvr

@defvr Constant CAMELLIA_MIN_KEY_SIZE
@end defvr

@defvr Constant CAMELLIA_MAX_KEY_SIZE
@end defvr

@defvr Constant CAMELLIA_KEY_SIZE
Default CAMELLIA key size, 32
@end defvr

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

@deftypefun void camellia_invert_key (struct camellia_ctx *@var{dst}, const struct camellia_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{camellia_set_encrypt_key} and
@code{camellia_invert_key} is more efficient than calling
@code{camellia_set_encrypt_key} and @code{camellia_set_decrypt_key}. This function
is mainly useful for applications which needs to both encrypt and
decrypt using the @emph{same} key.
@end deftypefun

1081
@deftypefun void camellia_crypt (struct camellia_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117
The same function is used for both encryption and decryption.
@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

@subsection CAST128

CAST-128 is a block cipher, specified in @cite{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 @file{<nettle/cast128.h>}.

@deftp {Context struct} {struct cast128_ctx}
@end deftp

@defvr Constant CAST128_BLOCK_SIZE
The CAST128 block-size, 8
@end defvr

@defvr Constant CAST128_MIN_KEY_SIZE
Minimum CAST128 key size, 5
@end defvr

@defvr Constant CAST128_MAX_KEY_SIZE
Maximum CAST128 key size, 16
@end defvr

@defvr Constant CAST128_KEY_SIZE
Default CAST128 key size, 16
@end defvr

@deftypefun void cast128_set_key (struct cast128_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{key})
Initialize the cipher. The same function is used for both encryption and
decryption. 
@end deftypefun

1118
@deftypefun void cast128_encrypt (struct cast128_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
1119 1120 1121 1122 1123 1124
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

1125
@deftypefun void cast128_decrypt (struct cast128_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
1126
Analogous to @code{cast128_encrypt}
1127 1128
@end deftypefun

Niels Möller's avatar
Niels Möller committed
1129
@subsection DES
1130 1131 1132
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
1133
bit of each octet may be used for parity. A common way to use DES is to
1134 1135 1136 1137 1138 1139
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
Niels Möller's avatar
Niels Möller committed
1140
parallel. One shouldn't be using plain DES at all today, if one uses
Niels Möller's avatar
Niels Möller committed
1141
DES at all one should be using ``triple DES'', see DES3 below.
1142 1143 1144 1145 1146 1147 1148

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

@deftp {Context struct} {struct des_ctx}
@end deftp

@defvr Constant DES_BLOCK_SIZE
Niels Möller's avatar
Niels Möller committed
1149
The DES block-size, 8
1150 1151 1152 1153 1154 1155
@end defvr

@defvr Constant DES_KEY_SIZE
DES key size, 8
@end defvr

1156
@deftypefun int des_set_key (struct des_ctx *@var{ctx}, const uint8_t *@var{key})
1157
Initialize the cipher. The same function is used for both encryption and
1158 1159 1160
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.
1161 1162
@end deftypefun

1163
@deftypefun void des_encrypt (struct des_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
1164 1165 1166 1167 1168 1169
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

1170
@deftypefun void des_decrypt (struct des_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
1171 1172
Analogous to @code{des_encrypt}
@end deftypefun
Niels Möller's avatar
Niels Möller committed
1173

1174 1175 1176 1177 1178
@deftypefun int des_check_parity (unsigned @var{length}, const uint8_t *@var{key});
Checks that the given key has correct, odd, parity. Returns 1 for
correct parity, and 0 for bad parity.
@end deftypefun

1179 1180 1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195 1196 1197
@deftypefun void des_fix_parity (unsigned @var{length}, uint8_t *@var{dst}, const uint8_t *@var{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. @var{dst} and @var{src} may
be equal.
@end deftypefun

@subsection 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 @emph{decrypt} it using the
middle 56 bits of the key, and finally encrypt it again using the last
1198 1199
56 bits of the key. This is known as ``ede'' triple-DES, for
``encrypt-decrypt-encrypt''.
1200

1201
The ``ede'' construction provides some backward compatibility, as you get
1202 1203 1204 1205 1206 1207 1208
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
1209
impractical, some applications uses a ``two-key'' variant of triple-DES.
1210 1211 1212 1213 1214 1215
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
1216
functions. Nettle includes an implementation of three-key ``ede''
1217 1218 1219 1220 1221 1222 1223
triple-DES, it is defined in the same place as plain DES,
@file{<nettle/des.h>}.

@deftp {Context struct} {struct des3_ctx}
@end deftp

@defvr Constant DES3_BLOCK_SIZE
Niels Möller's avatar
Niels Möller committed
1224
The DES3 block-size is the same as DES_BLOCK_SIZE, 8
1225 1226 1227 1228 1229 1230 1231 1232
@end defvr

@defvr Constant DES3_KEY_SIZE
DES key size, 24
@end defvr

@deftypefun int des3_set_key (struct des3_ctx *@var{ctx}, const uint8_t *@var{key})
Initialize the cipher. The same function is used for both encryption and
1233 1234 1235 1236
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.
1237 1238 1239 1240 1241
@end deftypefun

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

1242
@deftypefun void des3_encrypt (struct des3_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
1243 1244 1245 1246 1247 1248
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

1249
@deftypefun void des3_decrypt (struct des3_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
1250 1251 1252
Analogous to @code{des_encrypt}
@end deftypefun

Niels Möller's avatar
Niels Möller committed
1253
@subsection SERPENT
1254 1255
SERPENT is one of the AES finalists, designed by Ross Anderson, Eli
Biham and Lars Knudsen. Thus, the interface and properties are similar
Niels Möller's avatar
Niels Möller committed
1256
to AES'. One peculiarity is that it is quite pointless to use it with
1257 1258 1259 1260 1261 1262 1263
anything but the maximum key size, smaller keys are just padded to
larger ones. Nettle defines SERPENT in @file{<nettle/serpent.h>}.

@deftp {Context struct} {struct serpent_ctx}
@end deftp

@defvr Constant SERPENT_BLOCK_SIZE
Niels Möller's avatar
Niels Möller committed
1264
The SERPENT block-size, 16
1265 1266 1267
@end defvr

@defvr Constant SERPENT_MIN_KEY_SIZE
Niels Möller's avatar
Niels Möller committed
1268
Minimum SERPENT key size, 16
1269 1270 1271 1272 1273 1274 1275 1276 1277 1278 1279 1280 1281 1282 1283
@end defvr

@defvr Constant SERPENT_MAX_KEY_SIZE
Maximum SERPENT key size, 32
@end defvr

@defvr Constant SERPENT_KEY_SIZE
Default SERPENT key size, 32
@end defvr

@deftypefun void serpent_set_key (struct serpent_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{key})
Initialize the cipher. The same function is used for both encryption and
decryption. 
@end deftypefun

1284
@deftypefun void serpent_encrypt (struct serpent_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
1285 1286 1287 1288 1289 1290
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

1291
@deftypefun void serpent_decrypt (struct serpent_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
1292 1293 1294
Analogous to @code{serpent_encrypt}
@end deftypefun

Niels Möller's avatar
Niels Möller committed
1295 1296

@subsection TWOFISH
1297 1298 1299 1300 1301 1302 1303
Another AES finalist, this one designed by Bruce Schneier and others.
Nettle defines it in @file{<nettle/twofish.h>}.

@deftp {Context struct} {struct twofish_ctx}
@end deftp

@defvr Constant TWOFISH_BLOCK_SIZE
Niels Möller's avatar
Niels Möller committed
1304
The TWOFISH block-size, 16
1305 1306 1307
@end defvr

@defvr Constant TWOFISH_MIN_KEY_SIZE
Niels Möller's avatar
Niels Möller committed
1308
Minimum TWOFISH key size, 16
1309 1310 1311 1312 1313 1314 1315 1316 1317 1318 1319 1320 1321 1322 1323
@end defvr

@defvr Constant TWOFISH_MAX_KEY_SIZE
Maximum TWOFISH key size, 32
@end defvr

@defvr Constant TWOFISH_KEY_SIZE
Default TWOFISH key size, 32
@end defvr

@deftypefun void twofish_set_key (struct twofish_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{key})
Initialize the cipher. The same function is used for both encryption and
decryption. 
@end deftypefun

1324
@deftypefun void twofish_encrypt (struct twofish_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
1325 1326 1327 1328 1329 1330
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

1331
@deftypefun void twofish_decrypt (struct twofish_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
1332 1333 1334
Analogous to @code{twofish_encrypt}
@end deftypefun

Niels Möller's avatar
Niels Möller committed
1335 1336 1337 1338 1339 1340 1341 1342 1343 1344 1345 1346 1347 1348 1349 1350 1351 1352 1353 1354 1355
@c @node nettle_cipher, Cipher Block Chaining, Cipher functions, Reference
@c @comment  node-name,  next,  previous,  up
@subsection @code{struct nettle_cipher}

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

@deftp {Meta struct} @code{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
@code{nettle_set_key_func} and @code{nettle_crypt_func}. The first
argument to these functions is a @code{void *} pointer to a context
struct, which is of size @code{context_size}.
@end deftp

@deftypevr {Constant Struct} {struct nettle_cipher} nettle_aes128
@deftypevrx {Constant Struct} {struct nettle_cipher} nettle_aes192
@deftypevrx {Constant Struct} {struct nettle_cipher} nettle_aes256

1356 1357 1358 1359 1360
@deftypevrx {Constant Struct} {struct nettle_cipher} nettle_arctwo40;
@deftypevrx {Constant Struct} {struct nettle_cipher} nettle_arctwo64;
@deftypevrx {Constant Struct} {struct nettle_cipher} nettle_arctwo128;
@deftypevrx {Constant Struct} {struct nettle_cipher} nettle_arctwo_gutmann128;

Niels Möller's avatar
Niels Möller committed
1361
@deftypevrx {Constant Struct} {struct nettle_cipher} nettle_arcfour128
1362 1363 1364 1365 1366

@deftypevrx {Constant Struct} {struct nettle_cipher} nettle_camellia128
@deftypevrx {Constant Struct} {struct nettle_cipher} nettle_camellia192
@deftypevrx {Constant Struct} {struct nettle_cipher} nettle_camellia256

Niels Möller's avatar
Niels Möller committed
1367 1368 1369 1370 1371 1372 1373 1374 1375 1376 1377
@deftypevrx {Constant Struct} {struct nettle_cipher} nettle_cast128

@deftypevrx {Constant Struct} {struct nettle_cipher} nettle_serpent128
@deftypevrx {Constant Struct} {struct nettle_cipher} nettle_serpent192
@deftypevrx {Constant Struct} {struct nettle_cipher} nettle_serpent256

@deftypevrx {Constant Struct} {struct nettle_cipher} nettle_twofish128
@deftypevrx {Constant Struct} {struct nettle_cipher} nettle_twofish192
@deftypevrx {Constant Struct} {struct nettle_cipher} nettle_twofish256

Nettle includes such structs for all the @emph{regular} ciphers, i.e.
Niels Möller's avatar
Niels Möller committed
1378
ones without weak keys or other oddities.
1379 1380 1381 1382 1383 1384 1385

Nettle also exports a list of all these ciphers without weak keys or
other oddities.  This list can be used to dynamically enumerate or
search the supported algorithms:

@deftypevrx {Constant Struct} {struct nettle_cipher **} nettle_ciphers

Niels Möller's avatar
Niels Möller committed
1386 1387
@end deftypevr

1388
@node Cipher modes, Keyed hash functions, Cipher functions, Reference
1389
@comment  node-name,  next,  previous,  up
1390 1391
@section Cipher modes

1392 1393 1394 1395
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
@xref{Cipher functions}, splitting the message into blocks and
processing them independently with the block cipher (Electronic Code
1396
Book mode, @acronym{ECB}) leaks information. Besides @acronym{ECB},
1397 1398 1399
Nettle provides three other modes of operation: Cipher Block Chaining
(@acronym{CBC}), Counter mode (@acronym{CTR}), and Galois/Counter mode
(@acronym{gcm}). @acronym{CBC} is widely used, but there are a few
1400 1401 1402
subtle issues of information leakage, see, e.g.,
@uref{http://www.kb.cert.org/vuls/id/958563, @acronym{SSH} @acronym{CBC}
vulnerability}. @acronym{CTR} and @acronym{GCM}
1403 1404 1405 1406 1407 1408 1409 1410 1411 1412
were standardized more recently, and are believed to be more secure.
@acronym{GCM} includes message authentication; for the other modes, one
should always use a @acronym{MAC} (@pxref{Keyed hash functions}) or
signature to authenticate the message.

@menu
* CBC::                         
* CTR::                         
* GCM::                         
@end menu
1413

1414 1415 1416

@node CBC, CTR, Cipher modes, Cipher modes
@comment  node-name,  next,  previous,  up
1417
@subsection Cipher Block Chaining
1418

1419 1420 1421
@cindex Cipher Block Chaining
@cindex CBC Mode

Niels Möller's avatar
Niels Möller committed
1422
When using @acronym{CBC} mode, plaintext blocks are not encrypted
Niels Möller's avatar
Niels Möller committed
1423 1424
independently of each other, like in Electronic Cook Book mode. Instead,
when encrypting a block in @acronym{CBC} mode, the previous ciphertext
1425
block is XORed with the plaintext before it is fed to the block cipher.
Niels Möller's avatar
Niels Möller committed
1426 1427 1428 1429
When encrypting the first block, a random block called an @dfn{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.
1430

Niels Möller's avatar
Niels Möller committed
1431 1432
In symbols, if @code{E_k} is the encryption function of a block cipher,
and @code{IV} is the initialization vector, then @code{n} plaintext blocks
1433 1434 1435 1436 1437 1438 1439 1440 1441 1442 1443 1444
@code{M_1},@dots{} @code{M_n} are transformed into @code{n} ciphertext blocks
@code{C_1},@dots{} @code{C_n} as follows:

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

@dots{}

C_n = E_k(C_(n-1) XOR M_n)
@end example

1445 1446
Nettle's includes two functions for applying a block cipher in Cipher
Block Chaining (@acronym{CBC}) mode, one for encryption and one for
Niels Möller's avatar
Niels Möller committed
1447
decryption. These functions uses @code{void *} to pass cipher contexts
1448
around.
1449

1450
@deftypefun {void} cbc_encrypt (void *@var{ctx}, nettle_crypt_func @var{f}, unsigned @var{block_size}, uint8_t *@var{iv}, unsigned @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
1451 1452