nettle.texinfo 166 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
Niels Möller's avatar
Niels Möller committed
5
@documentencoding UTF-8
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

10
@set UPDATED-FOR 2.7
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

17
Originally written 2001 by @value{AUTHOR}, updated 2013.
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
@ifnottex
@macro pmod {m} 
(mod \m\)
@end macro
@end ifnottex

Niels Möller's avatar
Niels Möller committed
32
33
34
35
36
37
@titlepage
@title Nettle Manual
@subtitle For the Nettle Library version @value{UPDATED-FOR}
@author @value{AUTHOR}
@page
@vskip 0pt plus 1filll
38
@insertcopying
Niels Möller's avatar
Niels Möller committed
39
40
@end titlepage

41
42
43
44
45
@dircategory Encryption
@direntry
* Nettle: (nettle).             A low-level cryptographic library.
@end direntry

46
47
@contents

Niels Möller's avatar
Niels Möller committed
48
49
50
@ifnottex
@node     Top, Introduction, (dir), (dir)
@comment  node-name,  next,  previous,  up
51
@top Nettle
Niels Möller's avatar
Niels Möller committed
52

53
54
55
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
56

57
@insertcopying
Niels Möller's avatar
Niels Möller committed
58
59

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

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

Reference

* Hash functions::              
* Cipher functions::            
* Cipher modes::                
* Keyed hash functions::        
Simon Josefsson's avatar
Simon Josefsson committed
79
* Key derivation functions::    
80
81
* Public-key algorithms::       
* Randomness::                  
82
* ASCII encoding::              
83
84
85
86
87
88
89
90
91
92
93
94
95
* Miscellaneous functions::     
* Compatibility functions::     

Cipher modes

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

Public-key algorithms

* RSA::                         The RSA public key algorithm.
* DSA::                         The DSA digital signature algorithm.
96
* Elliptic curves::             Elliptic curves and ECDSA
97
98

@end detailmenu
Niels Möller's avatar
Niels Möller committed
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
@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.

115
And as the requirements of applications differ in subtle and not so
Niels Möller's avatar
Niels Möller committed
116
117
118
119
120
121
122
123
124
125
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
126
interfaces on top of Nettle, and share the code, test cases, benchmarks,
Niels Möller's avatar
Niels Möller committed
127
128
129
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
130

131
132
133
134
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
135
136
137
138
@node Copyright, Conventions, Introduction, Top
@comment  node-name,  next,  previous,  up
@chapter Copyright

Niels Möller's avatar
Niels Möller committed
139
140
141
142
143
144
145
Nettle is dual licenced under the GNU General Public License version 2
or later, and the GNU Lesser General Public License version 3 or later.
When using Nettle, you must comply fully with all conditions of at least
one of these licenses. A few of the individual files are licensed under
more permissive terms, or in the public domain. To find the current
status of particular files, you have to read the copyright notices at
the top of the files.
Niels Möller's avatar
Niels Möller committed
146

147
148
149
150
151
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
152
153
A list of the supported algorithms, their origins, and exceptions to the
above licensing:
Niels Möller's avatar
Niels Möller committed
154
155
156
157

@table @emph
@item AES
The implementation of the AES cipher (also known as rijndael) is written
158
by Rafael Sevilla. Assembler for x86 by Rafael Sevilla and
Niels Möller's avatar
Niels Möller committed
159
@value{AUTHOR}, Sparc assembler by @value{AUTHOR}.
Niels Möller's avatar
Niels Möller committed
160
161
162

@item ARCFOUR
The implementation of the ARCFOUR (also known as RC4) cipher is written
Niels Möller's avatar
Niels Möller committed
163
by @value{AUTHOR}.
Niels Möller's avatar
Niels Möller committed
164

Niels Möller's avatar
Niels Möller committed
165
166
167
@item ARCTWO
The implementation of the ARCTWO (also known as RC2) cipher is written
by Nikos Mavroyanopoulos and modified by Werner Koch and Simon
Niels Möller's avatar
Niels Möller committed
168
Josefsson.
Niels Möller's avatar
Niels Möller committed
169

Niels Möller's avatar
Niels Möller committed
170
171
@item BLOWFISH
The implementation of the BLOWFISH cipher is written by Werner Koch,
172
copyright owned by the Free Software Foundation. Also hacked by Simon
Niels Möller's avatar
Niels Möller committed
173
Josefsson and Niels Möller.
Niels Möller's avatar
Niels Möller committed
174

175
@item CAMELLIA
176
The C implementation is by Nippon Telegraph and Telephone Corporation
177
(NTT), heavily modified by @value{AUTHOR}. Assembler for x86 and x86_64
Niels Möller's avatar
Niels Möller committed
178
by @value{AUTHOR}.
179

Niels Möller's avatar
Niels Möller committed
180
181
182
183
@item CAST128
The implementation of the CAST128 cipher is written by Steve Reid.
Released into the public domain.

Niels Möller's avatar
Niels Möller committed
184
185
186
187
@item CHACHA
Implemented by Joachim Strömbergson, based on the implementation of
SALSA20 (see below). Assembly for x86_64 by Niels Möller.

Niels Möller's avatar
Niels Möller committed
188
189
@item DES
The implementation of the DES cipher is written by Dana L. How, and
Niels Möller's avatar
Niels Möller committed
190
released under the LGPL, version 2 or later.
Niels Möller's avatar
Niels Möller committed
191

192
193
194
195
196
@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.

197
198
@item MD2
The implementation of MD2 is written by Andrew Kuchling, and hacked
199
some by Andreas Sigfridsson and @value{AUTHOR}. Python Cryptography
200
201
202
203
204
205
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
206
207
@item MD5
The implementation of the MD5 message digest is written by Colin Plumb.
208
It has been hacked some more by Andrew Kuchling and @value{AUTHOR}.
Niels Möller's avatar
Niels Möller committed
209
210
Released into the public domain.

211
212
@item PBKDF2
The C implementation of PBKDF2 is based on earlier work for Shishi and
Niels Möller's avatar
Niels Möller committed
213
GnuTLS by Simon Josefsson.
214

215
@item RIPEMD160
216
217
The implementation of RIPEMD160 message digest is based on the code in
libgcrypt, copyright owned by the Free Software Foundation. Ported to
Niels Möller's avatar
Niels Möller committed
218
Nettle by Andres Mejia.
219

Niels Möller's avatar
Niels Möller committed
220
221
@item SALSA20
The C implementation of SALSA20 is based on D. J. Bernstein's reference
222
implementation (in the public domain), adapted to Nettle by Simon
223
Josefsson, and heavily modified by Niels Möller. Assembly for x86_64 and
Niels Möller's avatar
Niels Möller committed
224
ARM by Niels Möller.
Simon Josefsson's avatar
Simon Josefsson committed
225

Niels Möller's avatar
Niels Möller committed
226
@item SERPENT
227
The implementation of the SERPENT cipher is based on the code in libgcrypt,
228
copyright owned by the Free Software Foundation. Adapted to Nettle by
Niels Möller's avatar
Niels Möller committed
229
Simon Josefsson and heavily modified by Niels Möller. Assembly for
Niels Möller's avatar
Niels Möller committed
230
231
232
233
234
235
x86_64 by Niels Möller.

@item POLY1305
Based on the implementation by Andrew M. (floodyberry), modified by
Nikos Mavrogiannopoulos and Niels Möller. Assembly for x86_64 by Niels
Möller.
Niels Möller's avatar
Niels Möller committed
236
237

@item SHA1
238
239
The C implementation of the SHA1 message digest is written by Peter
Gutmann, and hacked some more by Andrew Kuchling and @value{AUTHOR}.
240
241
Released into the public domain. Assembler for x86, x86_64 and ARM by
@value{AUTHOR}, released under the LGPL.
Niels Möller's avatar
Niels Möller committed
242

Niels Möller's avatar
Niels Möller committed
243
@item SHA2
Niels Möller's avatar
Niels Möller committed
244
245
Written by @value{AUTHOR}, using Peter Gutmann's SHA1 code as a model. 

Niels Möller's avatar
Niels Möller committed
246
@item SHA3
Niels Möller's avatar
Niels Möller committed
247
Written by @value{AUTHOR}.
248

Niels Möller's avatar
Niels Möller committed
249
250
@item TWOFISH
The implementation of the TWOFISH cipher is written by Ruud de Rooij.
Niels Möller's avatar
Niels Möller committed
251

252
@item UMAC
Niels Möller's avatar
Niels Möller committed
253
Written by @value{AUTHOR}.
254

Niels Möller's avatar
Niels Möller committed
255
@item RSA
Niels Möller's avatar
Niels Möller committed
256
Written by @value{AUTHOR}. Uses the GMP library for bignum operations.
Niels Möller's avatar
Niels Möller committed
257
258

@item DSA
Niels Möller's avatar
Niels Möller committed
259
Written by @value{AUTHOR}. Uses the GMP library for bignum operations.
260
261

@item ECDSA
Niels Möller's avatar
Niels Möller committed
262
263
Written by @value{AUTHOR}. Uses the GMP library for bignum operations.
Development of Nettle's ECC support was funded by the .SE Internet Fund.
Niels Möller's avatar
Niels Möller committed
264
265
266
267
268
269
270
271
@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
272
operating on the context. The context struct encapsulates all information
Niels Möller's avatar
Niels Möller committed
273
274
275
needed by the algorithm, and it can be copied or moved in memory with no
unexpected effects.

276
277
For consistency, functions for different algorithms are very similar,
but there are some differences, for instance reflecting if the key setup
278
or encryption function differ for encryption and decryption, and whether
279
280
281
282
283
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
284

285
286
287
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
288
289
290
291
292
293
294

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
295
type @code{size_t}, and a pointer of type @code{uint8_t *} or
Niels Möller's avatar
Niels Möller committed
296
297
298
299
@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,
300
but they @emph{must not} overlap in any other way.
Niels Möller's avatar
Niels Möller committed
301

302
303
304
Many of the functions lack return value and can never fail. Those
functions which can fail, return one on success and zero on failure.

305
306
@c FIXME: Say something about the name mangling.

307
@node Example, Linking, Conventions, Top
Niels Möller's avatar
Niels Möller committed
308
309
310
@comment  node-name,  next,  previous,  up
@chapter Example

311
A simple example program that reads a file from standard input and
312
writes its SHA1 check-sum on standard output should give the flavor of
313
Nettle.
Niels Möller's avatar
Niels Möller committed
314
315

@example
316
@verbatiminclude sha-example.c
Niels Möller's avatar
Niels Möller committed
317
318
@end example

319
320
321
On a typical Unix system, this program can be compiled and linked with
the command line 
@example
322
gcc sha-example.c -o sha-example -lnettle
323
324
325
326
327
328
329
330
331
332
333
334
@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.

335
336
337
338
339
340
341
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.
342
343

@node Reference, Nettle soup, Linking, Top
Niels Möller's avatar
Niels Möller committed
344
345
346
347
348
349
350
351
@comment  node-name,  next,  previous,  up
@chapter Reference

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

@menu
* Hash functions::              
* Cipher functions::            
352
* Cipher modes::                
Niels Möller's avatar
Niels Möller committed
353
* Keyed hash functions::        
Simon Josefsson's avatar
Simon Josefsson committed
354
* Key derivation functions::    
355
356
* Public-key algorithms::       
* Randomness::                  
357
* ASCII encoding::              
Niels Möller's avatar
Niels Möller committed
358
* Miscellaneous functions::     
359
* Compatibility functions::     
Niels Möller's avatar
Niels Möller committed
360
361
362
363
@end menu

@node Hash functions, Cipher functions, Reference, Reference
@comment  node-name,  next,  previous,  up
364

Niels Möller's avatar
Niels Möller committed
365
@section Hash functions
366
@cindex Hash function
Niels Möller's avatar
Niels Möller committed
367
368
369
370
371
372
373
374
375
376
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
377
@cindex One-way
Niels Möller's avatar
Niels Möller committed
378
379
380
381
Given a hash value @code{H(x)} it is hard to find a string @code{x}
that hashes to that value.

@item Collision-resistant
382
@cindex Collision-resistant
Niels Möller's avatar
Niels Möller committed
383
384
385
386
387
388
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,
389
message authentication codes, pseudo random generators, association of
390
unique ids to documents, and many other things.
Niels Möller's avatar
Niels Möller committed
391

Niels Möller's avatar
Niels Möller committed
392
393
The most commonly used hash functions are MD5 and SHA1. Unfortunately,
both these fail the collision-resistance requirement; cryptologists have
394
395
396
397
398
399
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.
400

401
402
403
404
405
@menu
* Recommended hash functions::
* Legacy hash functions::
* nettle_hash abstraction::
@end menu
Niels Möller's avatar
Niels Möller committed
406

407
408
409
@node Recommended hash functions, Legacy hash functions,, Hash functions
@comment  node-name,  next,  previous,  up
@subsection Recommended hash functions
Niels Möller's avatar
Niels Möller committed
410

411
412
413
414
415
416
417
418
419
420
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}
Niels Möller's avatar
Niels Möller committed
421
422
@end deftp

423
@defvr Constant SHA256_DIGEST_SIZE
Niels Möller's avatar
Niels Möller committed
424
The size of a SHA256 digest, i.e. 32.
Niels Möller's avatar
Niels Möller committed
425
426
@end defvr

427
428
429
@defvr Constant SHA256_DATA_SIZE
The internal block size of SHA256. Useful for some special constructions,
in particular HMAC-SHA256.
Niels Möller's avatar
Niels Möller committed
430
431
@end defvr

432
433
@deftypefun void sha256_init (struct sha256_ctx *@var{ctx})
Initialize the SHA256 state.
Niels Möller's avatar
Niels Möller committed
434
435
@end deftypefun

436
@deftypefun void sha256_update (struct sha256_ctx *@var{ctx}, size_t @var{length}, const uint8_t *@var{data})
Niels Möller's avatar
Niels Möller committed
437
438
439
Hash some more data.
@end deftypefun

440
@deftypefun void sha256_digest (struct sha256_ctx *@var{ctx}, size_t @var{length}, uint8_t *@var{digest})
441
442
Performs final processing and extracts the message digest, writing it
to @var{digest}. @var{length} may be smaller than
443
@code{SHA256_DIGEST_SIZE}, in which case only the first @var{length}
444
octets of the digest are written.
Niels Möller's avatar
Niels Möller committed
445

446
This function also resets the context in the same way as
447
@code{sha256_init}.
Niels Möller's avatar
Niels Möller committed
448
449
@end deftypefun

450
451
452
Earlier versions of nettle defined SHA256 in the header file
@file{<nettle/sha.h>}, which is now deprecated, but kept for
compatibility.
Niels Möller's avatar
Niels Möller committed
453

454
@subsubsection @acronym{SHA224}
455

456
457
458
459
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).
460

461
@deftp {Context struct} {struct sha224_ctx}
462
463
@end deftp

464
@defvr Constant SHA224_DIGEST_SIZE
Niels Möller's avatar
Niels Möller committed
465
The size of a SHA224 digest, i.e. 28.
466
467
@end defvr

468
469
470
@defvr Constant SHA224_DATA_SIZE
The internal block size of SHA224. Useful for some special constructions,
in particular HMAC-SHA224.
471
472
@end defvr

473
474
@deftypefun void sha224_init (struct sha224_ctx *@var{ctx})
Initialize the SHA224 state.
475
476
@end deftypefun

477
@deftypefun void sha224_update (struct sha224_ctx *@var{ctx}, size_t @var{length}, const uint8_t *@var{data})
478
479
480
Hash some more data.
@end deftypefun

481
@deftypefun void sha224_digest (struct sha224_ctx *@var{ctx}, size_t @var{length}, uint8_t *@var{digest})
482
483
Performs final processing and extracts the message digest, writing it
to @var{digest}. @var{length} may be smaller than
484
@code{SHA224_DIGEST_SIZE}, in which case only the first @var{length}
485
486
487
octets of the digest are written.

This function also resets the context in the same way as
488
@code{sha224_init}.
489
490
@end deftypefun

491
@subsubsection @acronym{SHA512}
492

493
494
495
496
497
498
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).
499

500
@deftp {Context struct} {struct sha512_ctx}
501
502
@end deftp

503
@defvr Constant SHA512_DIGEST_SIZE
Niels Möller's avatar
Niels Möller committed
504
The size of a SHA512 digest, i.e. 64.
505
506
@end defvr

507
508
509
@defvr Constant SHA512_DATA_SIZE
The internal block size of SHA512. Useful for some special constructions,
in particular HMAC-SHA512.
510
511
@end defvr

512
513
@deftypefun void sha512_init (struct sha512_ctx *@var{ctx})
Initialize the SHA512 state.
514
515
@end deftypefun

516
@deftypefun void sha512_update (struct sha512_ctx *@var{ctx}, size_t @var{length}, const uint8_t *@var{data})
517
518
519
Hash some more data.
@end deftypefun

520
@deftypefun void sha512_digest (struct sha512_ctx *@var{ctx}, size_t @var{length}, uint8_t *@var{digest})
521
522
Performs final processing and extracts the message digest, writing it
to @var{digest}. @var{length} may be smaller than
523
@code{SHA512_DIGEST_SIZE}, in which case only the first @var{length}
524
525
526
octets of the digest are written.

This function also resets the context in the same way as
527
@code{sha512_init}.
528
529
@end deftypefun

530
@subsubsection @acronym{SHA384}
531

532
533
534
535
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).
536

537
@deftp {Context struct} {struct sha384_ctx}
538
539
@end deftp

540
@defvr Constant SHA384_DIGEST_SIZE
Niels Möller's avatar
Niels Möller committed
541
The size of a SHA384 digest, i.e. 48.
542
543
@end defvr

544
545
546
@defvr Constant SHA384_DATA_SIZE
The internal block size of SHA384. Useful for some special constructions,
in particular HMAC-SHA384.
547
548
@end defvr

549
550
@deftypefun void sha384_init (struct sha384_ctx *@var{ctx})
Initialize the SHA384 state.
551
552
@end deftypefun

553
@deftypefun void sha384_update (struct sha384_ctx *@var{ctx}, size_t @var{length}, const uint8_t *@var{data})
554
555
556
Hash some more data.
@end deftypefun

557
@deftypefun void sha384_digest (struct sha384_ctx *@var{ctx}, size_t @var{length}, uint8_t *@var{digest})
558
559
Performs final processing and extracts the message digest, writing it
to @var{digest}. @var{length} may be smaller than
560
@code{SHA384_DIGEST_SIZE}, in which case only the first @var{length}
561
562
563
octets of the digest are written.

This function also resets the context in the same way as
564
@code{sha384_init}.
565
566
@end deftypefun

567
@subsubsection @acronym{SHA3-224}
Niels Möller's avatar
Niels Möller committed
568

569
The SHA3 hash functions were specified by NIST in response to weaknesses
570
571
572
573
574
575
576
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).
Niels Möller's avatar
Niels Möller committed
577

578
Nettle defines SHA3-224 in @file{<nettle/sha3.h>}.
Niels Möller's avatar
Niels Möller committed
579

580
@deftp {Context struct} {struct sha3_224_ctx}
Niels Möller's avatar
Niels Möller committed
581
582
@end deftp

583
584
585
586
587
588
589
590
591
592
593
594
@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

595
@deftypefun void sha3_224_update (struct sha3_224_ctx *@var{ctx}, size_t @var{length}, const uint8_t *@var{data})
596
597
598
Hash some more data.
@end deftypefun

599
@deftypefun void sha3_224_digest (struct sha3_224_ctx *@var{ctx}, size_t @var{length}, uint8_t *@var{digest})
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
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

618
@defvr Constant SHA3_256_DIGEST_SIZE
619
The size of a SHA3_256 digest, i.e., 32.
Niels Möller's avatar
Niels Möller committed
620
621
@end defvr

622
623
@defvr Constant SHA3_256_DATA_SIZE
The internal block size of SHA3_256.
Niels Möller's avatar
Niels Möller committed
624
625
@end defvr

626
627
@deftypefun void sha3_256_init (struct sha3_256_ctx *@var{ctx})
Initialize the SHA3-256 state.
Niels Möller's avatar
Niels Möller committed
628
629
@end deftypefun

630
@deftypefun void sha3_256_update (struct sha3_256_ctx *@var{ctx}, size_t @var{length}, const uint8_t *@var{data})
Niels Möller's avatar
Niels Möller committed
631
632
633
Hash some more data.
@end deftypefun

634
@deftypefun void sha3_256_digest (struct sha3_256_ctx *@var{ctx}, size_t @var{length}, uint8_t *@var{digest})
635
636
Performs final processing and extracts the message digest, writing it
to @var{digest}. @var{length} may be smaller than
637
@code{SHA3_256_DIGEST_SIZE}, in which case only the first @var{length}
638
639
octets of the digest are written.

640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
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

664
@deftypefun void sha3_384_update (struct sha3_384_ctx *@var{ctx}, size_t @var{length}, const uint8_t *@var{data})
665
666
667
Hash some more data.
@end deftypefun

668
@deftypefun void sha3_384_digest (struct sha3_384_ctx *@var{ctx}, size_t @var{length}, uint8_t *@var{digest})
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
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

698
@deftypefun void sha3_512_update (struct sha3_512_ctx *@var{ctx}, size_t @var{length}, const uint8_t *@var{data})
699
700
701
Hash some more data.
@end deftypefun

702
@deftypefun void sha3_512_digest (struct sha3_512_ctx *@var{ctx}, size_t @var{length}, uint8_t *@var{digest})
703
704
705
706
707
708
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.
Niels Möller's avatar
Niels Möller committed
709
710
@end deftypefun

711
712
713
@node Legacy hash functions, nettle_hash abstraction, Recommended hash functions, Hash functions
@comment  node-name,  next,  previous,  up
@subsection Legacy hash functions
714

715
716
717
718
719
720
721
722
723
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.
724

725
@subsubsection @acronym{MD5}
726

727
728
729
730
731
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}
732
733
@end deftp

734
735
@defvr Constant MD5_DIGEST_SIZE
The size of an MD5 digest, i.e. 16.
736
737
@end defvr

738
739
740
@defvr Constant MD5_DATA_SIZE
The internal block size of MD5. Useful for some special constructions,
in particular HMAC-MD5.
741
742
@end defvr

743
744
@deftypefun void md5_init (struct md5_ctx *@var{ctx})
Initialize the MD5 state.
745
746
@end deftypefun

747
@deftypefun void md5_update (struct md5_ctx *@var{ctx}, size_t @var{length}, const uint8_t *@var{data})
748
749
750
Hash some more data.
@end deftypefun

751
@deftypefun void md5_digest (struct md5_ctx *@var{ctx}, size_t @var{length}, uint8_t *@var{digest})
752
753
Performs final processing and extracts the message digest, writing it
to @var{digest}. @var{length} may be smaller than
754
@code{MD5_DIGEST_SIZE}, in which case only the first @var{length}
755
octets of the digest are written.
Niels Möller's avatar
Niels Möller committed
756

757
This function also resets the context in the same way as
758
@code{md5_init}.
Niels Möller's avatar
Niels Möller committed
759
760
@end deftypefun

761
762
763
764
765
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.
766

767
To start over, you can call @code{md5_init} at any time.
768

769
@subsubsection @acronym{MD2}
770

771
772
773
774
775
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}
776
777
@end deftp

778
779
@defvr Constant MD2_DIGEST_SIZE
The size of an MD2 digest, i.e. 16.
780
781
@end defvr

782
783
@defvr Constant MD2_DATA_SIZE
The internal block size of MD2.
784
785
@end defvr

786
787
@deftypefun void md2_init (struct md2_ctx *@var{ctx})
Initialize the MD2 state.
788
789
@end deftypefun

790
@deftypefun void md2_update (struct md2_ctx *@var{ctx}, size_t @var{length}, const uint8_t *@var{data})
791
792
793
Hash some more data.
@end deftypefun

794
@deftypefun void md2_digest (struct md2_ctx *@var{ctx}, size_t @var{length}, uint8_t *@var{digest})
795
796
Performs final processing and extracts the message digest, writing it
to @var{digest}. @var{length} may be smaller than
797
@code{MD2_DIGEST_SIZE}, in which case only the first @var{length}
798
799
800
octets of the digest are written.

This function also resets the context in the same way as
801
@code{md2_init}.
802
803
@end deftypefun

804
@subsubsection @acronym{MD4}
805

806
807
808
809
810
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.
811

812
@deftp {Context struct} {struct md4_ctx}
813
814
@end deftp

815
816
@defvr Constant MD4_DIGEST_SIZE
The size of an MD4 digest, i.e. 16.
817
818
@end defvr

819
820
@defvr Constant MD4_DATA_SIZE
The internal block size of MD4.
821
822
@end defvr

823
824
@deftypefun void md4_init (struct md4_ctx *@var{ctx})
Initialize the MD4 state.
825
826
@end deftypefun

827
@deftypefun void md4_update (struct md4_ctx *@var{ctx}, size_t @var{length}, const uint8_t *@var{data})
828
829
830
Hash some more data.
@end deftypefun

831
@deftypefun void md4_digest (struct md4_ctx *@var{ctx}, size_t @var{length}, uint8_t *@var{digest})
832
833
Performs final processing and extracts the message digest, writing it
to @var{digest}. @var{length} may be smaller than
834
@code{MD4_DIGEST_SIZE}, in which case only the first @var{length}
835
836
837
octets of the digest are written.

This function also resets the context in the same way as
838
@code{md4_init}.
839
840
@end deftypefun

841
@subsubsection @acronym{RIPEMD160}
842

843
844
845
846
847
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}.
848

849
850
@deftp {Context struct} {struct ripemd160_ctx}
@end deftp
851

852
@defvr Constant RIPEMD160_DIGEST_SIZE
Niels Möller's avatar
Niels Möller committed
853
The size of a RIPEMD160 digest, i.e. 20.
854
855
856
857
858
859
860
861
862
863
@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

864
@deftypefun void ripemd160_update (struct ripemd160_ctx *@var{ctx}, size_t @var{length}, const uint8_t *@var{data})
865
866
867
Hash some more data.
@end deftypefun

868
@deftypefun void ripemd160_digest (struct ripemd160_ctx *@var{ctx}, size_t @var{length}, uint8_t *@var{digest})
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
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}
886
887
@end deftp

888
@defvr Constant SHA1_DIGEST_SIZE
Niels Möller's avatar
Niels Möller committed
889
The size of a SHA1 digest, i.e. 20.
890
891
@end defvr

892
893
894
@defvr Constant SHA1_DATA_SIZE
The internal block size of SHA1. Useful for some special constructions,
in particular HMAC-SHA1.
895
896
@end defvr

897
898
@deftypefun void sha1_init (struct sha1_ctx *@var{ctx})
Initialize the SHA1 state.
899
900
@end deftypefun

901
@deftypefun void sha1_update (struct sha1_ctx *@var{ctx}, size_t @var{length}, const uint8_t *@var{data})
902
903
904
Hash some more data.
@end deftypefun

905
@deftypefun void sha1_digest (struct sha1_ctx *@var{ctx}, size_t @var{length}, uint8_t *@var{digest})
906
907
Performs final processing and extracts the message digest, writing it
to @var{digest}. @var{length} may be smaller than
908
@code{SHA1_DIGEST_SIZE}, in which case only the first @var{length}
909
910
911
octets of the digest are written.

This function also resets the context in the same way as
912
@code{sha1_init}.
913
914
@end deftypefun

915
916

@subsubsection @acronym{GOSTHASH94}
917
918
919
920
921
922
923
924
925
926

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
Niels Möller's avatar
Niels Möller committed
927
The size of a GOSTHASH94 digest, i.e. 32.
928
929
930
931
932
933
934
935
936
937
@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

938
@deftypefun void gosthash94_update (struct gosthash94_ctx *@var{ctx}, size_t @var{length}, const uint8_t *@var{data})
939
940
941
Hash some more data.
@end deftypefun

942
@deftypefun void gosthash94_digest (struct gosthash94_ctx *@var{ctx}, size_t @var{length}, uint8_t *@var{digest})
943
944
945
946
947
948
949
950
951
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

952
953
954
@node nettle_hash abstraction,, Legacy hash functions, Hash functions
@comment  node-name,  next,  previous,  up
@subsection The nettle_hash abstraction
Niels Möller's avatar
Niels Möller committed
955
956
957

Nettle includes a struct including information about the supported hash
functions. It is defined in @file{<nettle/nettle-meta.h>}, and is used
958
959
by Nettle's implementation of @acronym{HMAC} (@pxref{Keyed hash
functions}).
Niels Möller's avatar
Niels Möller committed
960
961
962
963
964

@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
965
@code{void *} pointer to a context struct, which is of size
Niels Möller's avatar
Niels Möller committed
966
967
968
@code{context_size}. 
@end deftp

969
970
971
@deftypevr {Constant Struct} {struct nettle_hash} nettle_md2
@deftypevrx {Constant Struct} {struct nettle_hash} nettle_md4
@deftypevrx {Constant Struct} {struct nettle_hash} nettle_md5
972
@deftypevrx {Constant Struct} {struct nettle_hash} nettle_ripemd160
973
974
975
976
977
@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
978
@deftypevrx {Constant Struct} {struct nettle_hash} nettle_sha3_256
979
@deftypevrx {Constant Struct} {struct nettle_hash} nettle_gosthash94
Niels Möller's avatar
Niels Möller committed
980
These are all the hash functions that Nettle implements.
981
@end deftypevr
982

983
Nettle also exports a list of all these hashes.
984

985
986
987
@deftypevr {Constant Array} {struct nettle_hash **} nettle_hashes
This list can be used to dynamically enumerate or search the supported
algorithms. NULL-terminated.
Niels Möller's avatar
Niels Möller committed
988
989
@end deftypevr

990
@node Cipher functions, Cipher modes, Hash functions, Reference
Niels Möller's avatar
Niels Möller committed
991
992
@comment  node-name,  next,  previous,  up
@section Cipher functions
993
@cindex Cipher
Niels Möller's avatar
Niels Möller committed
994
995
996
997

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
998
plaintext. Given matching pairs of plaintext and ciphertext, it should
Niels Möller's avatar
Niels Möller committed
999
1000
be hard to find the key.

1001
1002
1003
@cindex Block Cipher
@cindex Stream Cipher

Niels Möller's avatar
Niels Möller committed
1004
1005
1006
1007
1008
1009
1010
1011
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.
1012
However, using @acronym{ECB} is usually a bad idea. For a start, plaintext blocks
Niels Möller's avatar
Niels Möller committed
1013
1014
that are equal are transformed to ciphertext blocks that are equal; that
leaks information about the plaintext. Usually you should apply the
1015
1016
1017
1018
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
1019

Niels Möller's avatar
Niels Möller committed
1020
A stream cipher can be used for messages of arbitrary length. A typical
Niels Möller's avatar
Niels Möller committed
1021
stream cipher is a keyed pseudo-random generator. To encrypt a plaintext
Niels Möller's avatar
Niels Möller committed
1022
message of @var{n} octets, you key the generator, generate @var{n}
Niels Möller's avatar
Niels Möller committed
1023
octets of pseudo-random data, and XOR it with the plaintext. To decrypt,
Niels Möller's avatar
Niels Möller committed
1024
1025
1026
1027
1028
1029
1030
1031
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
1032
receive an encrypted message. You apply the key, and get a plaintext
1033
message that makes sense to you. Can you then be sure that it really was
Niels Möller's avatar
Niels Möller committed
1034
your friend that wrote the message you're reading? The answer is no. For
Niels Möller's avatar
Niels Möller committed
1035
1036
1037
1038
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
1039
1040
1041
1042
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
1043
1044
1045

It is recommended to @emph{always} use an authentication mechanism in
addition to encrypting the messages. Popular choices are Message
1046
1047
Authentication Codes like @acronym{HMAC-SHA1} (@pxref{Keyed hash
functions}), or digital signatures like @acronym{RSA}.
Niels Möller's avatar
Niels Möller committed
1048

1049
Some ciphers have so called ``weak keys'', keys that results in
Niels Möller's avatar
Niels Möller committed
1050
undesirable structure after the key setup processing, and should be
1051
1052
1053
1054
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
1055
1056
1057
have weak keys. There are several good ciphers that don't have any weak
keys.

1058
1059
1060
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
1061
as @dfn{key setup}. With Nettle, it is recommended to use each
1062
1063
1064
1065
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
1066
@subsection AES
1067
AES is a block cipher, specified by NIST as a replacement for
Niels Möller's avatar
Niels Möller committed
1068
the older DES standard. The standard is the result of a competition
1069
1070
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
1071
1072

Like all the AES candidates, the winning design uses a block size of 128
1073
1074
1075
1076
1077
1078
bits, or 16 octets, and three possible key-size, 128, 192 and 256 bits
(16, 24 and 32 octets) being the allowed key sizes. It does not have any
weak keys. Nettle defines AES in @file{<nettle/aes.h>}, and there is one
context struct for each key size. (Earlier versions of Nettle used a
single context struct, @code{struct aes_ctx}, for all key sizes. This
interface kept for backwards compatibility).
Niels Möller's avatar
Niels Möller committed
1079
 
1080
1081
1082
1083
1084
@deftp {Context struct} {struct aes128_ctx}
@deftpx {Context struct} {struct aes192_ctx}
@deftpx {Context struct} {struct aes256_ctx}
@end deftp

Niels Möller's avatar
Niels Möller committed
1085
@deftp {Context struct} {struct aes_ctx}
1086
Alternative struct, for the old AES interface.
Niels Möller's avatar
Niels Möller committed
1087
1088
1089
@end deftp

@defvr Constant AES_BLOCK_SIZE
Niels Möller's avatar
Niels Möller committed
1090
The AES block-size, 16.
Niels Möller's avatar
Niels Möller committed
1091
1092
@end defvr

1093
1094
1095
1096
1097
@defvr Constant AES128_KEY_SIZE
@defvrx Constant AES192_KEY_SIZE
@defvrx Constant AES256_KEY_SIZE
@defvrx Constant AES_MIN_KEY_SIZE
@defvrx Constant AES_MAX_KEY_SIZE
Niels Möller's avatar
Niels Möller committed
1098
1099
1100
@end defvr

@defvr Constant AES_KEY_SIZE
Niels Möller's avatar
Niels Möller committed
1101
Default AES key size, 32.
Niels Möller's avatar
Niels Möller committed
1102
1103
@end defvr

1104
1105
1106
1107
1108
1109
1110
1111
@deftypefun void aes128_set_encrypt_key (struct aes128_ctx *@var{ctx}, const uint8_t *@var{key})
@deftypefunx void aes128_set_decrypt_key (struct aes128_ctx *@var{ctx}, const uint8_t *@var{key})
@deftypefunx void aes192_set_encrypt_key (struct aes192_ctx *@var{ctx}, const uint8_t *@var{key})
@deftypefunx void aes192_set_decrypt_key (struct aes192_ctx *@var{ctx}, const uint8_t *@var{key})
@deftypefunx void aes256_set_encrypt_key (struct aes256_ctx *@var{ctx}, const uint8_t *@var{key})
@deftypefunx void aes256_set_decrypt_key (struct aes256_ctx *@var{ctx}, const uint8_t *@var{key})
@deftypefunx void aes_set_encrypt_key (struct aes_ctx *@var{ctx}, size_t @var{length}, const uint8_t *@var{key})
@deftypefunx void aes_set_decrypt_key (struct aes_ctx *@var{ctx}, size_t @var{length}, const uint8_t *@var{key})
1112
Initialize the cipher, for encryption or decryption, respectively.
Niels Möller's avatar
Niels Möller committed
1113
1114
@end deftypefun

1115
1116
1117
1118
@deftypefun void aes128_invert_key (struct aes128_ctx *@var{dst}, const struct aes128_ctx *@var{src})
@deftypefunx void aes192_invert_key (struct aes192_ctx *@var{dst}, const struct aes192_ctx *@var{src})
@deftypefunx void aes256_invert_key (struct aes256_ctx *@var{dst}, const struct aes256_ctx *@var{src})
@deftypefunx void aes_invert_key (struct aes_ctx *@var{dst}, const struct aes_ctx *@var{src})
1119
1120
1121
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
1122
1123
1124
1125
1126
converted in place. These functions are mainly useful for applications
which needs to both encrypt and decrypt using the @emph{same} key,
because calling, e.g., @code{aes128_set_encrypt_key} and
@code{aes128_invert_key}, is more efficient than calling
@code{aes128_set_encrypt_key} and @code{aes128_set_decrypt_key}.
1127
1128
@end deftypefun

1129
1130
1131
1132
@deftypefun void aes128_encrypt (struct aes128_ctx *@var{ctx}, size_t @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
@deftypefunx void aes192_encrypt (struct aes192_ctx *@var{ctx}, size_t @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
@deftypefunx void aes256_encrypt (struct aes256_ctx *@var{ctx}, size_t @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
@deftypefunx void aes_encrypt (struct aes_ctx *@var{ctx}, size_t @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
Niels Möller's avatar
Niels Möller committed
1133
1134
1135
1136
1137
1138
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

1139
1140
1141
1142
1143
@deftypefun void aes128_decrypt (struct aes128_ctx *@var{ctx}, size_t @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
@deftypefunx void aes192_decrypt (struct aes192_ctx *@var{ctx}, size_t @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
@deftypefunx void aes256_decrypt (struct aes256_ctx *@var{ctx}, size_t @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
@deftypefunx void aes_decrypt (struct aes_ctx *@var{ctx}, size_t @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})
Analogous to the encryption functions above.
Niels Möller's avatar
Niels Möller committed
1144
1145
1146
1147
1148
1149
1150
@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
1151