nettle.texinfo 99.8 KB
Newer Older
Niels Möller's avatar
Niels Möller committed
1 2 3 4 5 6 7
\input texinfo          @c -*-texinfo-*-

@c %**start of header
@setfilename nettle.info
@settitle The Nettle low-level cryptographic library.
@c %**end of header

8 9
@documentencoding ISO-8859-1

10
@footnotestyle end
Niels Möller's avatar
Niels Möller committed
11 12 13 14 15 16 17
@syncodeindex fn cp

@dircategory GNU Libraries
@direntry
* Nettle: (nettle).           A low-level cryptographics library.
@end direntry

18
@set COPYRIGHT-YEARS 2001, 2004, 2005
Niels Möller's avatar
Niels Möller committed
19
@set UPDATED-FOR 1.15
Niels Möller's avatar
Niels Möller committed
20

Niels Möller's avatar
Niels Möller committed
21
@c Latin-1 doesn't work with TeX output.
Niels Möller's avatar
Niels Möller committed
22 23
@c Also lookout for é characters.

24 25 26 27
@iftex
@set AUTHOR Niels M@"oller
@end iftex
@ifnottex
Niels Möller's avatar
Niels Möller committed
28
@set AUTHOR Niels Möller
29 30
@end ifnottex

Niels Möller's avatar
Niels Möller committed
31
@ifinfo
32
Manual for the Nettle library. This manual corresponds to version
Niels Möller's avatar
Niels Möller committed
33 34
@value{UPDATED-FOR}.

35
Copyright @value{COPYRIGHT-YEARS} @value{AUTHOR}
Niels Möller's avatar
Niels Möller committed
36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77

Permission is granted to make and distribute verbatim
copies of this manual provided the copyright notice and
this permission notice are preserved on all copies.

@ignore
Permission is granted to process this file through TeX
and print the results, provided the printed document
carries a copying permission notice identical to this
one except for the removal of this paragraph (this
paragraph not being relevant to the printed manual).

@end ignore
Permission is granted to copy and distribute modified
versions of this manual under the conditions for
verbatim copying, provided also that the sections
entitled ``Copying'' and ``GNU General Public License''
are included exactly as in the original, and provided
that the entire resulting derived work is distributed
under the terms of a permission notice identical to this
one.

Permission is granted to copy and distribute
translations of this manual into another language,
under the above conditions for modified versions,
except that this permission notice may be stated in a
translation approved by the Free Software Foundation.

@end ifinfo

@titlepage
@sp 10
@c @center @titlefont{Nettle Manual}

@title Nettle Manual
@subtitle For the Nettle Library version @value{UPDATED-FOR}

@author @value{AUTHOR}

@c The following two commands start the copyright page.
@page
@vskip 0pt plus 1filll
78
Copyright @copyright{} @value{COPYRIGHT-YEARS} @value{AUTHOR}
Niels Möller's avatar
Niels Möller committed
79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100

Permission is granted to make and distribute verbatim
copies of this manual provided the copyright notice and
this permission notice are preserved on all copies.

Permission is granted to copy and distribute modified
versions of this manual under the conditions for
verbatim copying, provided also that the sections
entitled ``Copying'' and ``GNU General Public License''
are included exactly as in the original, and provided
that the entire resulting derived work is distributed
under the terms of a permission notice identical to this
one.

Permission is granted to copy and distribute
translations of this manual into another language,
under the above conditions for modified versions,
except that this permission notice may be stated in a
translation approved by the Free Software Foundation.

@end titlepage

101 102
@contents

Niels Möller's avatar
Niels Möller committed
103 104 105
@ifnottex
@node     Top, Introduction, (dir), (dir)
@comment  node-name,  next,  previous,  up
106
@top Nettle
Niels Möller's avatar
Niels Möller committed
107 108

This document describes the nettle low-level cryptographic library. You
Niels Möller's avatar
Niels Möller committed
109
can use the library directly from your C programs, or (recommended)
Niels Möller's avatar
Niels Möller committed
110
write or use an object-oriented wrapper for your favorite language or
Niels Möller's avatar
Niels Möller committed
111 112
application.

Niels Möller's avatar
Niels Möller committed
113
This manual corresponds to version @value{UPDATED-FOR} of the library.
Niels Möller's avatar
Niels Möller committed
114 115

@menu
Niels Möller's avatar
Niels Möller committed
116 117
* Introduction::                What is Nettle?
* Copyright::                   Your rights.
Niels Möller's avatar
Niels Möller committed
118 119
* Conventions::                 
* Example::                     
Niels Möller's avatar
Niels Möller committed
120 121 122
* Reference::                   All Nettle functions and features.
* Nettle soup::                 For the serious nettle hacker.
* Installation::                How to install Nettle.
Niels Möller's avatar
Niels Möller committed
123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139
* Index::                       
@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.

140
And as the requirements of applications differ in subtle and not so
Niels Möller's avatar
Niels Möller committed
141 142 143 144 145 146 147 148 149 150
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
151
interfaces on top of Nettle, and share the code, test cases, benchmarks,
Niels Möller's avatar
Niels Möller committed
152 153 154
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
155

156 157 158 159
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
160 161 162 163
@node Copyright, Conventions, Introduction, Top
@comment  node-name,  next,  previous,  up
@chapter Copyright

164 165 166 167 168 169 170 171
Nettle is distributed under the GNU General Public License (GPL) (see
the file COPYING for details). However, most of the individual files
are dual licensed under less restrictive licenses like the GNU Lesser
General Public License (LGPL), or are in the public domain. This means
that if you don't use the parts of nettle that are GPL-only, you have
the option to use the Nettle library just as if it were licensed under
the LGPL. 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
172 173 174 175 176 177

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
178 179 180
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
181 182 183

@item ARCFOUR
The implementation of the ARCFOUR (also known as RC4) cipher is written
184
by @value{AUTHOR}. Released under the LGPL.
Niels Möller's avatar
Niels Möller committed
185

Niels Möller's avatar
Niels Möller committed
186 187 188 189 190
@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
191 192 193
@item BLOWFISH
The implementation of the BLOWFISH cipher is written by Werner Koch,
copyright owned by the Free Software Foundation. Also hacked by Ray
194
Dassen and @value{AUTHOR}. Released under the GPL.
Niels Möller's avatar
Niels Möller committed
195 196 197 198 199 200 201 202 203

@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.

204 205
@item MD2
The implementation of MD2 is written by Andrew Kuchling, and hacked
206
some by Andreas Sigfridsson and @value{AUTHOR}. Python Cryptography
207 208 209 210 211 212
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
213 214
@item MD5
The implementation of the MD5 message digest is written by Colin Plumb.
215
It has been hacked some more by Andrew Kuchling and @value{AUTHOR}.
Niels Möller's avatar
Niels Möller committed
216 217 218 219 220
Released into the public domain.

@item SERPENT
The implementation of the SERPENT cipher is written by Ross Anderson,
Eli Biham, and Lars Knudsen, adapted to LSH by Rafael Sevilla, and to
221
Nettle by @value{AUTHOR}. Released under the GPL.
Niels Möller's avatar
Niels Möller committed
222 223

@item SHA1
224 225 226 227
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
228

Niels Möller's avatar
Niels Möller committed
229 230 231 232
@item SHA256
Written by @value{AUTHOR}, using Peter Gutmann's SHA1 code as a model. 
Released under the LGPL.

Niels Möller's avatar
Niels Möller committed
233 234 235
@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
236 237 238 239 240 241 242 243

@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
244 245 246 247 248 249 250 251
@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
252
operating on the context. The context struct encapsulates all information
Niels Möller's avatar
Niels Möller committed
253 254 255
needed by the algorithm, and it can be copied or moved in memory with no
unexpected effects.

256 257 258 259 260 261 262 263
For consistency, functions for different algorithms are very similar,
but there are some differences, for instance reflecting if the key setup
or encryption function differ for encryption and encryption, and whether
or not key setup can fail. There are also differences between algorithms
that don't show in function prototypes, but which the application must
nevertheless be aware of. There is no big difference between the
functions for stream ciphers and for block ciphers, although they should
be used quite differently by the application.
Niels Möller's avatar
Niels Möller committed
264 265 266 267 268 269 270 271 272 273 274

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

By convention, for an algorithm named @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
275
type @code{unsigned}, and a pointer of type @code{uint8_t *} or
Niels Möller's avatar
Niels Möller committed
276 277 278 279
@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,
280
but they @emph{must not} overlap in any other way.
Niels Möller's avatar
Niels Möller committed
281

282 283
@c FIXME: Say something about the name mangling.

Niels Möller's avatar
Niels Möller committed
284 285 286 287 288
@node Example, Reference, Conventions, Top
@comment  node-name,  next,  previous,  up
@chapter Example

A simple example program that reads a file from standard in and writes
Niels Möller's avatar
Niels Möller committed
289
its SHA1 checksum on standard output should give the flavor of Nettle.
Niels Möller's avatar
Niels Möller committed
290 291

@example
292
@verbatiminclude sha-example.c
Niels Möller's avatar
Niels Möller committed
293 294
@end example

Niels Möller's avatar
Niels Möller committed
295
@node Reference, Nettle soup, Example, Top
Niels Möller's avatar
Niels Möller committed
296 297 298 299 300 301 302 303
@comment  node-name,  next,  previous,  up
@chapter Reference

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

@menu
* Hash functions::              
* Cipher functions::            
304
* Cipher modes::                
Niels Möller's avatar
Niels Möller committed
305
* Keyed hash functions::        
306 307
* Public-key algorithms::       
* Randomness::                  
Niels Möller's avatar
Niels Möller committed
308
* Miscellaneous functions::     
309
* Compatibility functions::     
Niels Möller's avatar
Niels Möller committed
310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335
@end menu

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

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

@item Collision-resistant
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,
336
message authentication codes, pseudo random generators, association of
Niels Möller's avatar
Niels Möller committed
337 338
unique id:s to documents, and many other things.

Niels Möller's avatar
Niels Möller committed
339 340 341 342 343 344
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.
345

Niels Möller's avatar
Niels Möller committed
346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372
@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})
373 374 375 376
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
377

378 379
This function also resets the context in the same way as
@code{md5_init}.
Niels Möller's avatar
Niels Möller committed
380 381 382
@end deftypefun

The normal way to use MD5 is to call the functions in order: First
383 384 385 386
@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
387 388 389

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

390 391
@subsection @acronym{MD2}

Niels Möller's avatar
Niels Möller committed
392
MD2 is another hash function of Ronald Rivest's, described in
393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426
@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
427 428 429 430 431
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.
432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461

@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

Niels Möller's avatar
Niels Möller committed
462 463 464
@subsection @acronym{SHA1}

SHA1 is a hash function specified by @dfn{NIST} (The U.S. National Institute
465 466
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
467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489

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

490 491 492 493 494 495 496 497
@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
498 499
@end deftypefun

500 501 502 503 504 505 506 507 508 509 510 511 512
@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
513
The size of an SHA256 digest, i.e. 32.
514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533
@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
534

535 536
This function also resets the context in the same way as
@code{sha256_init}.
Niels Möller's avatar
Niels Möller committed
537 538
@end deftypefun

Niels Möller's avatar
Niels Möller committed
539 540 541 542 543 544 545 546 547 548 549 550 551 552 553
@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
by Nettle's implementation of @acronym{HMAC} @pxref{Keyed hash
functions}.

@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
@code{void *} pointer so a context struct, which is of size
@code{context_size}. 
@end deftp

Niels Möller's avatar
Niels Möller committed
554 555 556
@deftypevr {Constant Struct} {struct nettle_cipher} nettle_md2
@deftypevrx {Constant Struct} {struct nettle_cipher} nettle_md4
@deftypevrx {Constant Struct} {struct nettle_cipher} nettle_md5
Niels Möller's avatar
Niels Möller committed
557 558 559 560 561 562
@deftypevrx {Constant Struct} {struct nettle_cipher} nettle_sha1
@deftypevrx {Constant Struct} {struct nettle_cipher} nettle_sha256

These are all the hash functions that Nettle implements.
@end deftypevr

563
@node Cipher functions, Cipher modes, Hash functions, Reference
Niels Möller's avatar
Niels Möller committed
564 565 566 567 568 569
@comment  node-name,  next,  previous,  up
@section Cipher functions

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
570
plaintext. Given matching pairs of plaintext and ciphertext, it should
Niels Möller's avatar
Niels Möller committed
571 572 573 574 575 576 577 578 579 580
be hard to find the key.

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

A block cipher can process data only in fixed size chunks, called
@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.
581
However, using @acronym{ECB} is usually a bad idea. For a start, plaintext blocks
Niels Möller's avatar
Niels Möller committed
582 583
that are equal are transformed to ciphertext blocks that are equal; that
leaks information about the plaintext. Usually you should apply the
584 585 586 587
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
588

Niels Möller's avatar
Niels Möller committed
589
A stream cipher can be used for messages of arbitrary length. A typical
Niels Möller's avatar
Niels Möller committed
590
stream cipher is a keyed pseudo-random generator. To encrypt a plaintext
Niels Möller's avatar
Niels Möller committed
591
message of @var{n} octets, you key the generator, generate @var{n}
Niels Möller's avatar
Niels Möller committed
592
octets of pseudo-random data, and XOR it with the plaintext. To decrypt,
Niels Möller's avatar
Niels Möller committed
593 594 595 596 597 598 599 600
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
601
receive an encrypted message. You apply the key, and get a plaintext
602
message that makes sense to you. Can you then be sure that it really was
Niels Möller's avatar
Niels Möller committed
603
your friend that wrote the message you're reading? The answer is no. For
Niels Möller's avatar
Niels Möller committed
604 605 606 607
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
608 609 610 611
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
612 613 614

It is recommended to @emph{always} use an authentication mechanism in
addition to encrypting the messages. Popular choices are Message
Niels Möller's avatar
Niels Möller committed
615
Authentication Codes like @acronym{HMAC-SHA1} @pxref{Keyed hash
616
functions}, or digital signatures like @acronym{RSA}.
Niels Möller's avatar
Niels Möller committed
617

618
Some ciphers have so called ``weak keys'', keys that results in
Niels Möller's avatar
Niels Möller committed
619 620 621 622 623 624 625 626
undesirable structure after the key setup processing, and should be
avoided. In Nettle, the presence of weak keys for a cipher mean that the
key setup function can fail, so you have to check its return value. In
addition, the context struct has a field @code{status}, that is set to a
non-zero value if key setup fails. When possible, avoid algorithm that
have weak keys. There are several good ciphers that don't have any weak
keys.

627 628 629 630 631 632 633 634
To encrypt a message, you first initialize a cipher context for
encryption or decryption with a particular key. You then use the context
to process plaintext or ciphertext messages. The initialization is known
as called @dfn{key setup}. With Nettle, it is recommended to use each
context struct for only one direction, even if some of the ciphers use a
single key setup function that can be used for both encryption and
decryption.

Niels Möller's avatar
Niels Möller committed
635 636
@subsection AES
AES is a quite new block cipher, specified by NIST as a replacement for
Niels Möller's avatar
Niels Möller committed
637
the older DES standard. The standard is the result of a competition
638 639
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
640 641

Like all the AES candidates, the winning design uses a block size of 128
Niels Möller's avatar
Niels Möller committed
642
bits, or 16 octets, and variable key-size, 128, 192 and 256 bits (16, 24
Niels Möller's avatar
Niels Möller committed
643 644 645 646 647 648 649
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
650
The AES block-size, 16
Niels Möller's avatar
Niels Möller committed
651 652 653 654 655 656 657 658 659 660 661 662
@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

663 664 665
@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
666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683
@end deftypefun

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

@deftypefun void aes_decrypt (struct aes_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{dst}, uint8_t *@var{src})
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
684
``secret:1'', ``secret:2'', @enddots{}. If you have keys that don't look
Niels Möller's avatar
Niels Möller committed
685
like random bit strings, and you want to use ARCFOUR, always hash the
Niels Möller's avatar
Niels Möller committed
686 687 688
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
689 690 691 692

@example
/* A more robust key setup function for ARCFOUR */
void
693 694
arcfour_set_key_hashed(struct arcfour_ctx *ctx,
                       unsigned length, const uint8_t *key)
Niels Möller's avatar
Niels Möller committed
695
@{
Niels Möller's avatar
Niels Möller committed
696 697 698
  struct sha256_ctx hash;
  uint8_t digest[SHA256_DIGEST_SIZE];
  uint8_t buffer[0x200];
Niels Möller's avatar
Niels Möller committed
699

Niels Möller's avatar
Niels Möller committed
700 701 702
  sha256_init(&hash);
  sha256_update(&hash, length, key);
  sha256_digest(&hash, SHA256_DIGEST_SIZE, digest);
Niels Möller's avatar
Niels Möller committed
703

Niels Möller's avatar
Niels Möller committed
704 705
  arcfour_set_key(ctx, SHA256_DIGEST_SIZE, digest);
  arcfour_crypt(ctx, sizeof(buffer), buffer, buffer);
Niels Möller's avatar
Niels Möller committed
706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730
@}
@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

731
@deftypefun void arcfour_crypt (struct arcfour_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{dst}, uint8_t *@var{src})
Niels Möller's avatar
Niels Möller committed
732 733 734 735 736 737 738
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
739 740 741 742
@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
743
reverse engineered RC2 cipher description, as described in a Usenet
Niels Möller's avatar
Niels Möller committed
744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799
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
The AES block-size, 8
@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

@deftypefun void arctwo_encrypt (struct arctwo_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{dst}, uint8_t *@var{src})
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

@deftypefun void arctwo_decrypt (struct arctwo_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{dst}, uint8_t *@var{src})
Analogous to @code{arctwo_encrypt}
@end deftypefun

Niels Möller's avatar
Niels Möller committed
800 801
@subsection CAST128

802 803 804 805 806 807 808 809
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
Niels Möller's avatar
Niels Möller committed
810
The CAST128 block-size, 8
811 812 813
@end defvr

@defvr Constant CAST128_MIN_KEY_SIZE
Niels Möller's avatar
Niels Möller committed
814
Minimum CAST128 key size, 5
815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840
@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

@deftypefun void cast128_encrypt (struct cast128_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{dst}, uint8_t *@var{src})
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

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

Niels Möller's avatar
Niels Möller committed
841 842
@subsection BLOWFISH

843 844 845 846 847 848 849 850
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
851
The BLOWFISH block-size, 8
852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883
@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
decryption. Returns 1 on success, and 0 if the key was weak. Calling
@code{blowfish_encrypt} or @code{blowfish_decrypt} with a weak key will
crash with an assert violation.
@end deftypefun

@deftypefun void blowfish_encrypt (struct blowfish_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{dst}, uint8_t *@var{src})
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

@deftypefun void blowfish_decrypt (struct blowfish_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{dst}, uint8_t *@var{src})
Analogous to @code{blowfish_encrypt}
@end deftypefun

Niels Möller's avatar
Niels Möller committed
884
@subsection DES
885 886 887 888 889 890 891 892 893 894
DES is the old Data Encryption Standard, specified by NIST. It uses a
block size of 64 bits (8 octets), and a key size of 56 bits. However,
the key bits are distributed over 8 octets, where the least significant
bit of each octet is used for parity. A common way to use DES is to
generate 8 random octets in some way, then set the least significant bit
of each octet to get odd parity, and initialize DES with the resulting
key.

The key size of DES is so small that keys can be found by brute force,
using specialized hardware or lots of ordinary work stations in
Niels Möller's avatar
Niels Möller committed
895
parallel. One shouldn't be using plain DES at all today, if one uses
Niels Möller's avatar
Niels Möller committed
896
DES at all one should be using ``triple DES'', see DES3 below.
897 898 899 900 901 902 903

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
904
The DES block-size, 8
905 906 907 908 909 910
@end defvr

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

911
@deftypefun int des_set_key (struct des_ctx *@var{ctx}, const uint8_t *@var{key})
912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927
Initialize the cipher. The same function is used for both encryption and
decryption. Returns 1 on success, and 0 if the key was weak or had bad
parity. Calling @code{des_encrypt} or @code{des_decrypt} with a bad key
will crash with an assert violation.
@end deftypefun

@deftypefun void des_encrypt (struct des_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{dst}, uint8_t *@var{src})
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

@deftypefun void des_decrypt (struct des_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{dst}, uint8_t *@var{src})
Analogous to @code{des_encrypt}
@end deftypefun
Niels Möller's avatar
Niels Möller committed
928

929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947
@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
948 949
56 bits of the key. This is known as ``ede'' triple-DES, for
``encrypt-decrypt-encrypt''.
950

951
The ``ede'' construction provides some backward compatibility, as you get
952 953 954 955 956 957 958
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
959
impractical, some applications uses a ``two-key'' variant of triple-DES.
960 961 962 963 964 965
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
966
functions. Nettle includes an implementation of three-key ``ede''
967 968 969 970 971 972 973
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
974
The DES3 block-size is the same as DES_BLOCK_SIZE, 8
975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001
@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
decryption. Returns 1 on success, and 0 if the key was weak or had bad
parity. Calling @code{des_encrypt} or @code{des_decrypt} with a bad key
will crash with an assert violation.
@end deftypefun

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

@deftypefun void des3_encrypt (struct des3_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{dst}, uint8_t *@var{src})
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

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

Niels Möller's avatar
Niels Möller committed
1002
@subsection SERPENT
1003 1004
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
1005
to AES'. One peculiarity is that it is quite pointless to use it with
1006 1007 1008 1009 1010 1011 1012
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
1013
The SERPENT block-size, 16
1014 1015 1016
@end defvr

@defvr Constant SERPENT_MIN_KEY_SIZE
Niels Möller's avatar
Niels Möller committed
1017
Minimum SERPENT key size, 16
1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043
@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

@deftypefun void serpent_encrypt (struct serpent_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{dst}, uint8_t *@var{src})
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

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

Niels Möller's avatar
Niels Möller committed
1044 1045

@subsection TWOFISH
1046 1047 1048 1049 1050 1051 1052
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
1053
The TWOFISH block-size, 16
1054 1055 1056
@end defvr

@defvr Constant TWOFISH_MIN_KEY_SIZE
Niels Möller's avatar
Niels Möller committed
1057
Minimum TWOFISH key size, 16
1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083
@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

@deftypefun void twofish_encrypt (struct twofish_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{dst}, uint8_t *@var{src})
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

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

Niels Möller's avatar
Niels Möller committed
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
@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

@deftypevrx {Constant Struct} {struct nettle_cipher} nettle_arcfour128
@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

Niels Möller's avatar
Niels Möller committed
1116 1117 1118 1119 1120
@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
1121
Nettle includes such structs for all the @emph{regular} ciphers, i.e.
Niels Möller's avatar
Niels Möller committed
1122
ones without weak keys or other oddities.
Niels Möller's avatar
Niels Möller committed
1123 1124
@end deftypevr

1125
@node Cipher modes, Keyed hash functions, Cipher functions, Reference
1126
@comment  node-name,  next,  previous,  up
1127 1128
@section Cipher modes

Niels Möller's avatar
Niels Möller committed
1129
Cipher modes of operation specifies the procedure to use when
1130 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140
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
Book mode, @acronym{ECB}) leaks information. Besides @acronym{ECB},
Nettle provides two other modes of operation: Cipher Block Chaining
(@acronym{CBC}) and Counter mode (@acronym{CTR}). @acronym{CBC} is
widely used, but there are a few subtle issues of information leakage.
@acronym{CTR} was standardized more recently, and is believed to be more
secure.

@subsection Cipher Block Chaining
1141

Niels Möller's avatar
Niels Möller committed
1142
When using @acronym{CBC} mode, plaintext blocks are not encrypted
Niels Möller's avatar
Niels Möller committed
1143 1144
independently of each other, like in Electronic Cook Book mode. Instead,
when encrypting a block in @acronym{CBC} mode, the previous ciphertext
1145
block is XORed with the plaintext before it is fed to the block cipher.
Niels Möller's avatar
Niels Möller committed
1146 1147 1148 1149
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.
1150

Niels Möller's avatar
Niels Möller committed
1151 1152
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
1153 1154 1155 1156 1157 1158 1159 1160 1161 1162 1163 1164
@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

1165 1166
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
1167
decryption. These functions uses @code{void *} to pass cipher contexts
1168
around.
1169

1170
@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})
1171 1172
@deftypefunx {void} cbc_decrypt (void *@var{ctx}, void (*@var{f})(), unsigned @var{block_size}, uint8_t *@var{iv}, unsigned @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})

Niels Möller's avatar
Niels Möller committed
1173
Applies the encryption or decryption function @var{f} in @acronym{CBC}
Niels Möller's avatar
Niels Möller committed
1174 1175 1176
mode. The final ciphertext block processed is copied into @var{iv}
before returning, so that large message be processed be a sequence of
calls to @code{cbc_encrypt}. The function @var{f} is of type
1177 1178 1179 1180 1181 1182 1183

@code{void f (void *@var{ctx}, unsigned @var{length}, uint8_t @var{dst},
const uint8_t *@var{src})},

@noindent and the @code{cbc_encrypt} and @code{cbc_decrypt} functions pass their
argument @var{ctx} on to @var{f}.
@end deftypefun
1184

1185
There are also some macros to help use these functions correctly.
1186

1187
@deffn Macro CBC_CTX (@var{context_type}, @var{block_size})
1188 1189 1190 1191 1192 1193 1194
Expands into
@example
@{
   context_type ctx;
   uint8_t iv[block_size];
@}
@end example
1195 1196
@end deffn

Niels Möller's avatar
Niels Möller committed
1197
It can be used to define a @acronym{CBC} context struct, either directly,
1198

1199 1200 1201 1202 1203 1204 1205 1206 1207 1208
@example
struct CBC_CTX(struct aes_ctx, AES_BLOCK_SIZE) ctx;
@end example

or to give it a struct tag,

@example
struct aes_cbc_ctx CBC_CTX (struct aes_ctx, AES_BLOCK_SIZE);
@end example

1209
@deffn Macro CBC_SET_IV (@var{ctx}, @var{iv})
1210
First argument is a pointer to a context struct as defined by @code{CBC_CTX},
1211 1212 1213
and the second is a pointer to an Initialization Vector (IV) that is
copied into that context.
@end deffn
1214 1215 1216

@deffn Macro CBC_ENCRYPT (@var{ctx}, @var{f}, @var{length}, @var{dst}, @var{src})
@deffnx Macro CBC_DECRYPT (@var{ctx}, @var{f}, @var{length}, @var{dst}, @var{src})
1217 1218 1219 1220 1221
A simpler way to invoke @code{cbc_encrypt} and @code{cbc_decrypt}. The
first argument is a pointer to a context struct as defined by
@code{CBC_CTX}, and the second argument is an encryption or decryption
function following Nettle's conventions. The last three arguments define
the source and destination area for the operation.
1222
@end deffn
1223

1224 1225 1226
These macros use some tricks to make the compiler display a warning if
the types of @var{f} and @var{ctx} don't match, e.g. if you try to use
an @code{struct aes_ctx} context with the @code{des_encrypt} function.
1227

1228 1229 1230 1231 1232 1233 1234 1235 1236 1237 1238 1239
@subsection Counter mode

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

The message is divided into @code{n} blocks @code{M_1},@dots{}
@code{M_n}, where @code{M_n} is of size @code{m} which may be smaller
than the block size. Except for the last block, all the message blocks
must be of size equal to the cipher's block size.

Niels Möller's avatar
Niels Möller committed
1240 1241
If @code{E_k} is the encryption function of a block cipher, @code{IC} is
the initial counter, then the @code{n} plaintext blocks are
1242 1243 1244 1245 1246 1247 1248 1249 1250 1251 1252 1253 1254 1255 1256 1257 1258 1259 1260 1261 1262 1263 1264 1265 1266 1267 1268 1269 1270 1271 1272 1273 1274 1275 1276 1277 1278 1279 1280 1281 1282 1283 1284 1285 1286 1287 1288 1289 1290 1291 1292 1293 1294 1295 1296 1297 1298
transformed into @code{n} ciphertext blocks @code{C_1},@dots{}
@code{C_n} as follows:

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

@dots{}

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

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

@deftypefun {void} ctr_crypt (void *@var{ctx}, nettle_crypt_func @var{f}, unsigned @var{block_size}, uint8_t *@var{ctr}, unsigned @var{length}, uint8_t *@var{dst}, const uint8_t *@var{src})

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

When a message is encrypted using a sequence of calls to
@code{ctr_crypt}, all but the last call @emph{must} use a length that is
a multiple of the block size.
@end deftypefun

Like for @acronym{CBC}, there are also a couple of helper macros.

@deffn Macro CTR_CTX (@var{context_type}, @var{block_size})
Expands into
@example
@{
   context_type ctx;
   uint8_t ctr[block_size];
@}
@end example
@end deffn

@deffn Macro CTR_SET_COUNTER (@var{ctx}, @var{iv})
First argument is a pointer to a context struct as defined by
@code{CTR_CTX}, and the second is a pointer to an initial counter that
is copied into that context.
@end deffn

@deffn Macro CTR_CRYPT (@var{ctx}, @var{f}, @var{length}, @var{dst}, @var{src})
A simpler way to invoke @code{ctr_crypt}. The first argument is a
pointer to a context struct as defined by @code{CTR_CTX}, and the second
argument is an encryption function following Nettle's conventions. The
last three arguments define the source and destination area for the
operation.
@end deffn

Niels Möller's avatar
Niels Möller committed
1299

1300
@node Keyed hash functions, Public-key algorithms, Cipher modes, Reference
Niels Möller's avatar
Niels Möller committed
1301 1302 1303 1304 1305 1306 1307 1308 1309 1310 1311
@comment  node-name,  next,  previous,  up
@section Keyed Hash Functions

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

Keyed hash functions are useful primarily for message authentication,
1312
when Alice and Bob shares a secret: The sender, Alice, computes the
Niels Möller's avatar
Niels Möller committed
1313 1314 1315
@acronym{MAC} and attaches it to the message. The receiver, Bob, also computes
the @acronym{MAC} of the message, using the same key, and compares that
to Alice's value. If they match, Bob can be assured that
1316
the message has not been modified on its way from Alice.
Niels Möller's avatar
Niels Möller committed
1317 1318 1319 1320 1321 1322 1323 1324 1325

However, unlike digital signatures, this assurance is not transferable.
Bob can't show the message and the @acronym{MAC} to a third party and
prove that Alice sent that message. Not even if he gives away the key to
the third party. The reason is that the @emph{same} key is used on both
sides, and anyone knowing the key can create a correct @acronym{MAC} for
any message. If Bob believes that only he and Alice knows the key, and
he knows that he didn't attach a @acronym{MAC} to a particular message,
he knows it must be Alice who did it. However, the third party can't
1326
distinguish between a @acronym{MAC} created by Alice and one created by
Niels Möller's avatar
Niels Möller committed
1327 1328 1329 1330 1331 1332 1333 1334 1335 1336 1337 1338 1339 1340
Bob.

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

@subsection @acronym{HMAC}

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

For an underlying hash function @code{H}, with digest size @code{l} and
internal block size @code{b}, @acronym{HMAC-H} is constructed as
Niels Möller's avatar
Niels Möller committed
1341
follows: From a given key @code{k}, two distinct subkeys @code{k_i} and
Niels Möller's avatar
Niels Möller committed
1342 1343 1344 1345 1346 1347 1348 1349 1350 1351 1352 1353 1354 1355 1356 1357 1358 1359 1360 1361
@code{k_o} are constructed, both of length @code{b}. The
@acronym{HMAC-H} of a message @code{m} is then computed as @code{H(k_o |
H(k_i | m))}, where @code{|} denotes string concatenation.

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

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

@deftypefun void hmac_set_key (void *@var{outer}, void *@var{inner}, void *@var{state}, const struct nettle_hash *@var{H}, unsigned @var{length}, const uint8_t *@var{key})
Initializes the three context structs from the key. The @var{outer} and
Niels Möller's avatar
Niels Möller committed
1362
@var{inner} contexts corresponds to the subkeys @code{k_o} and
Niels Möller's avatar
Niels Möller committed
1363 1364 1365 1366
@code{k_i}. @var{state} is used for hashing the message, and is
initialized as a copy of the @var{inner} context.
@end deftypefun

1367
@deftypefun void hmac_update (void *@var{state}, const struct nettle_hash *@var{H}, unsigned @var{length}, const uint8_t *@var{data})
Niels Möller's avatar
Niels Möller committed
1368 1369 1370 1371 1372 1373 1374 1375 1376 1377 1378 1379 1380 1381 1382 1383 1384 1385 1386 1387 1388 1389 1390 1391 1392 1393 1394 1395 1396 1397
This function is called zero or more times to process the message.
Actually, @code{hmac_update(state, H, length, data)} is equivalent to
@code{H->update(state, length, data)}, so if you wish you can use the
ordinary update function of the underlying hash function instead.
@end deftypefun

@deftypefun void hmac_digest (const void *@var{outer}, const void *@var{inner}, void *@var{state}, const struct nettle_hash *@var{H}, unsigned @var{length}, uint8_t *@var{digest})
Extracts the @acronym{MAC} of the message, writing it to @var{digest}.
@var{outer} and @var{inner} are not modified. @var{length} is usually
equal to @code{H->digest_size}, but if you provide a smaller value,
only the first @var{length} octets of the @acronym{MAC} are written.

This function also resets the @var{state} context so that you can start
over processing a new message (with the same key).
@end deftypefun

Like for @acronym{CBC}, there are some macros to help use these
functions correctly.

@deffn Macro HMAC_CTX (@var{type})
Expands into
@example
@{
   type outer;
   type inner;
   type state;
@}
@end example
@end deffn

Niels Möller's avatar
Niels Möller committed
1398
It can be used to define a @acronym{HMAC} context struct, either
Niels Möller's avatar
Niels Möller committed
1399 1400 1401 1402 1403 1404 1405 1406 1407 1408 1409 1410 1411 1412 1413 1414 1415 1416 1417 1418 1419 1420 1421 1422 1423 1424 1425
directly,

@example
struct HMAC_CTX(struct md5_ctx) ctx;
@end example

or to give it a struct tag,

@example
struct hmac_md5_ctx HMAC_CTX (struct md5_ctx);
@end example

@deffn Macro HMAC_SET_KEY (@var{ctx}, @var{H}, @var{length}, @var{key})
@var{ctx} is a pointer to a context struct as defined by
@code{HMAC_CTX}, @var{H} is a pointer to a @code{const struct
nettle_hash} describing the underlying hash function (so it must match
the type of the components of @var{ctx}). The last two arguments specify
the secret key.
@end deffn

@deffn Macro HMAC_DIGEST (@var{ctx}, @var{H}, @var{length}, @var{digest})
@var{ctx} is a pointer to a context struct as defined by
@code{HMAC_CTX}, @var{H} is a pointer to a @code{const struct
nettle_hash} describing the underlying hash function. The last two
arguments specify where the digest is written.
@end deffn

Niels Möller's avatar
Niels Möller committed
1426 1427 1428
Note that there is no @code{HMAC_UPDATE} macro; simply call
@code{hmac_update} function directly, or the update function of the
underlying hash function.
Niels Möller's avatar
Niels Möller committed
1429 1430 1431 1432 1433 1434 1435 1436 1437 1438 1439 1440 1441 1442 1443 1444 1445 1446 1447 1448 1449 1450 1451 1452 1453 1454 1455 1456 1457 1458 1459 1460 1461 1462 1463 1464 1465 1466 1467 1468 1469 1470 1471 1472 1473 1474 1475 1476 1477 1478 1479 1480 1481 1482 1483 1484 1485 1486 1487 1488 1489 1490 1491 1492 1493 1494 1495 1496 1497 1498 1499 1500

@subsection Concrete @acronym{HMAC} functions
Now we come to the specialized @acronym{HMAC} functions, which are
easier to use than the general @acronym{HMAC} functions.

@subsubsection @acronym{HMAC-MD5}

@deftp {Context struct} {struct hmac_md5_ctx}
@end deftp

@deftypefun void hmac_md5_set_key (struct hmac_md5_ctx *@var{ctx}, unsigned @var{key_length}, const uint8_t *@var{key})
Initializes the context with the key.
@end deftypefun

@deftypefun void hmac_md5_update (struct hmac_md5_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{data})
Process some more data.
@end deftypefun

@deftypefun void hmac_md5_digest (struct hmac_md5_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{digest})
Extracts the @acronym{MAC}, 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 @acronym{MAC} are written.

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

@subsubsection @acronym{HMAC-SHA1}

@deftp {Context struct} {struct hmac_sha1_ctx}
@end deftp

@deftypefun void hmac_sha1_set_key (struct hmac_sha1_ctx *@var{ctx}, unsigned @var{key_length}, const uint8_t *@var{key})
Initializes the context with the key.
@end deftypefun

@deftypefun void hmac_sha1_update (struct hmac_sha1_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{data})
Process some more data.
@end deftypefun

@deftypefun void hmac_sha1_digest (struct hmac_sha1_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{digest})
Extracts the @acronym{MAC}, 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 @acronym{MAC} are written.

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


@subsubsection @acronym{HMAC-SHA256}

@deftp {Context struct} {struct hmac_sha256_ctx}
@end deftp

@deftypefun void hmac_sha256_set_key (struct hmac_sha256_ctx *@var{ctx}, unsigned @var{key_length}, const uint8_t *@var{key})
Initializes the context with the key.
@end deftypefun

@deftypefun void hmac_sha256_update (struct hmac_sha256_ctx *@var{ctx}, unsigned @var{length}, const uint8_t *@var{data})
Process some more data.
@end deftypefun

@deftypefun void hmac_sha256_digest (struct hmac_sha256_ctx *@var{ctx}, unsigned @var{length}, uint8_t *@var{digest})
Extracts the @acronym{MAC}, 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 @acronym{MAC} are written.

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

1501 1502 1503 1504 1505 1506 1507 1508 1509 1510 1511
@node Public-key algorithms, Randomness, Keyed hash functions, Reference
@comment  node-name,  next,  previous,  up
@section Public-key algorithms

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

The concept of @dfn{Public-key} encryption and digital signatures was
discovered by Whitfield Diffie and Martin E. Hellman and described in a
1512
paper 1976. In traditional, ``symmetric'', cryptography, sender and
1513 1514 1515 1516 1517 1518 1519 1520 1521
receiver share the same keys, and these keys must be distributed in a
secure way. And if there are many users or entities that need to
communicate, each @emph{pair} needs a shared secret key known by nobody
else.

Public-key cryptography uses trapdoor one-way functions. A
@dfn{one-way function} is a function @code{F} such that it is easy to
compute the value @code{F(x)} for any @code{x}, but given a value
@code{y}, it is hard to compute a corresponding @code{x} such that
1522
@code{y = F(x)}. Two examples are cryptographic hash functions, and
1523 1524 1525 1526 1527 1528 1529 1530
exponentiation in certain groups.

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

Two important uses for one-way functions with trapdoors are public-key
1531 1532 1533
encryption, and digital signatures. The public-key encryption functions
in Nettle are not yet documented; the rest of this chapter is about
digital signatures.
1534 1535

To use a digital signature algorithm, one must first create a
Niels Möller's avatar
Niels Möller committed
1536
@dfn{key-pair}: A public key and a corresponding private key. The private
1537 1538 1539 1540 1541 1542 1543 1544 1545 1546 1547 1548 1549 1550 1551 1552 1553
key is used to sign messages, while the public key is used for verifying
that that signatures and messages match. Some care must be taken when
distributing the public key; it need not be kept secret, but if a bad
guy is able to replace it (in transit, or in some user's list of known
public keys), bad things may happen.

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

The verification operation takes a public key, a message, and a string
that is claimed to be a signature on the message, and returns true or
false. If it returns true, that means that the three input values
matched, and the verifier can be sure that someone went through with the
1554
signature operation on that very message, and that the ``someone'' also
1555 1556 1557 1558 1559 1560 1561 1562 1563 1564 1565 1566 1567
knows the private key corresponding to the public key.

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

Besides signing meaningful messages, digital signatures can be used for
authorization. A server can be configured with a public key, such that
any client that connects to the service is given a random nonce message.
If the server gets a reply with a correct signature matching the nonce
message and the configured public key, the client is granted access. So
1568
the configuration of the server can be understood as ``grant access to
1569
whoever knows the private key corresponding to this particular public
1570
key, and to no others''.
1571

Niels Möller's avatar
Niels Möller committed
1572 1573 1574 1575 1576 1577 1578 1579

@menu
* RSA::                         The RSA public key algorithm.
* DSA::                         The DSA digital signature algorithm.
@end menu

@node RSA, DSA, Public-key algorithms, Public-key algorithms
@comment  node-name,  next,  previous,  up
1580 1581
@subsection @acronym{RSA}

Niels Möller's avatar
Niels Möller committed
1582 1583 1584
The @acronym{RSA} algorithm was the first practical digital signature
algorithm that was constructed. It was described 1978 in a paper by
Ronald Rivest, Adi Shamir and L.M. Adleman, and the technique was also
1585 1586
patented in the @acronym{USA} in 1983. The patent expired on September 20, 2000, and since
that day, @acronym{RSA} can be used freely, even in the @acronym{USA}.
1587

1588
It's remarkably simple to describe the trapdoor function behind
1589
@acronym{RSA}. The ``one-way''-function used is
1590 1591 1592 1593 1594 1595 1596

@example
F(x) = x^e mod n
@end example

I.e. raise x to the @code{e}:th power, while discarding all multiples of
@code{n}. The pair of numbers @code{n} and @code{e} is the public key.
1597
@code{e} can be quite small, even @code{e = 3} has been used, although