1 This is nettle.info, produced by makeinfo version 4.6 from
4 This manual is for the Nettle library (version 2.1), a low-level
7 Originally written 2001 by Niels Möller, updated 2010.
9 This manual is placed in the public domain. You may freely copy
10 it, in whole or in part, with or without modification. Attribution
11 is appreciated, but not required.
13 INFO-DIR-SECTION Encryption
15 * Nettle: (nettle). A low-level cryptographic library.
19 File: nettle.info, Node: Top, Next: Introduction, Prev: (dir), Up: (dir)
24 This document describes the Nettle low-level cryptographic library. You
25 can use the library directly from your C programs, or write or use an
26 object-oriented wrapper for your favorite language or application.
28 This manual is for the Nettle library (version 2.1), a low-level
29 cryptographic library.
31 Originally written 2001 by Niels Möller, updated 2010.
33 This manual is placed in the public domain. You may freely copy
34 it, in whole or in part, with or without modification. Attribution
35 is appreciated, but not required.
39 * Introduction:: What is Nettle?
40 * Copyright:: Your rights.
41 * Conventions:: General interface conventions.
42 * Example:: An example program.
43 * Linking:: Linking with the libnettle and libhogweed.
44 * Reference:: All Nettle functions and features.
45 * Nettle soup:: For the serious nettle hacker.
46 * Installation:: How to install Nettle.
47 * Index:: Function and concept index.
50 File: nettle.info, Node: Introduction, Next: Copyright, Prev: Top, Up: Top
55 Nettle is a cryptographic library that is designed to fit easily in more
56 or less any context: In crypto toolkits for object-oriented languages
57 (C++, Python, Pike, ...), in applications like LSH or GNUPG, or even in
58 kernel space. In most contexts, you need more than the basic
59 cryptographic algorithms, you also need some way to keep track of
60 available algorithms, their properties and variants. You often have
61 some algorithm selection process, often dictated by a protocol you want
64 And as the requirements of applications differ in subtle and not so
65 subtle ways, an API that fits one application well can be a pain to use
66 in a different context. And that is why there are so many different
67 cryptographic libraries around.
69 Nettle tries to avoid this problem by doing one thing, the low-level
70 crypto stuff, and providing a _simple_ but general interface to it. In
71 particular, Nettle doesn't do algorithm selection. It doesn't do memory
72 allocation. It doesn't do any I/O.
74 The idea is that one can build several application and context
75 specific interfaces on top of Nettle, and share the code, test cases,
76 benchmarks, documentation, etc. Examples are the Nettle module for the
77 Pike language, and LSH, which both use an object-oriented abstraction
78 on top of the library.
80 This manual explains how to use the Nettle library. It also tries to
81 provide some background on the cryptography, and advice on how to best
85 File: nettle.info, Node: Copyright, Next: Conventions, Prev: Introduction, Up: Top
90 Nettle is distributed under the GNU General Public License (GPL) (see
91 the file COPYING for details). However, most of the individual files
92 are dual licensed under less restrictive licenses like the GNU Lesser
93 General Public License (LGPL), or are in the public domain. This means
94 that if you don't use the parts of nettle that are GPL-only, you have
95 the option to use the Nettle library just as if it were licensed under
96 the LGPL. To find the current status of particular files, you have to
97 read the copyright notices at the top of the files.
99 This manual is in the public domain. You may freely copy it in whole
100 or in part, e.g., into documentation of programs that build on Nettle.
101 Attribution, as well as contribution of improvements to the text, is of
102 course appreciated, but it is not required.
104 A list of the supported algorithms, their origins and licenses:
107 The implementation of the AES cipher (also known as rijndael) is
108 written by Rafael Sevilla. Assembler for x86 by Rafael Sevilla and
109 Niels Möller, Sparc assembler by Niels Möller. Released under the
113 The implementation of the ARCFOUR (also known as RC4) cipher is
114 written by Niels Möller. Released under the LGPL.
117 The implementation of the ARCTWO (also known as RC2) cipher is
118 written by Nikos Mavroyanopoulos and modified by Werner Koch and
119 Simon Josefsson. Released under the LGPL.
122 The implementation of the BLOWFISH cipher is written by Werner
123 Koch, copyright owned by the Free Software Foundation. Also hacked
124 by Ray Dassen and Niels Möller. Released under the GPL.
127 The C implementation is by Nippon Telegraph and Telephone
128 Corporation (NTT), heavily modified by Niels Möller. Assembler for
129 x86 by Niels Möller. Released under the LGPL.
132 The implementation of the CAST128 cipher is written by Steve Reid.
133 Released into the public domain.
136 The implementation of the DES cipher is written by Dana L. How, and
137 released under the LGPL.
140 The implementation of MD2 is written by Andrew Kuchling, and hacked
141 some by Andreas Sigfridsson and Niels Möller. Python Cryptography
142 Toolkit license (essentially public domain).
145 This is almost the same code as for MD5 below, with modifications
146 by Marcus Comstedt. Released into the public domain.
149 The implementation of the MD5 message digest is written by Colin
150 Plumb. It has been hacked some more by Andrew Kuchling and Niels
151 Möller. Released into the public domain.
154 The implementation of the SERPENT cipher is written by Ross
155 Anderson, Eli Biham, and Lars Knudsen, adapted to LSH by Rafael
156 Sevilla, and to Nettle by Niels Möller. Released under the GPL.
159 The C implementation of the SHA1 message digest is written by Peter
160 Gutmann, and hacked some more by Andrew Kuchling and Niels Möller.
161 Released into the public domain. Assembler for x86 by Niels Möller,
162 released under the LGPL.
164 _SHA224, SHA256, SHA384, and SHA512_
165 Written by Niels Möller, using Peter Gutmann's SHA1 code as a
166 model. Released under the LGPL.
169 The implementation of the TWOFISH cipher is written by Ruud de
170 Rooij. Released under the LGPL.
173 Written by Niels Möller, released under the LGPL. Uses the GMP
174 library for bignum operations.
177 Written by Niels Möller, released under the LGPL. Uses the GMP
178 library for bignum operations.
181 File: nettle.info, Node: Conventions, Next: Example, Prev: Copyright, Up: Top
186 For each supported algorithm, there is an include file that defines a
187 _context struct_, a few constants, and declares functions for operating
188 on the context. The context struct encapsulates all information needed
189 by the algorithm, and it can be copied or moved in memory with no
192 For consistency, functions for different algorithms are very similar,
193 but there are some differences, for instance reflecting if the key setup
194 or encryption function differ for encryption and decryption, and whether
195 or not key setup can fail. There are also differences between algorithms
196 that don't show in function prototypes, but which the application must
197 nevertheless be aware of. There is no big difference between the
198 functions for stream ciphers and for block ciphers, although they should
199 be used quite differently by the application.
201 If your application uses more than one algorithm of the same type,
202 you should probably create an interface that is tailor-made for your
203 needs, and then write a few lines of glue code on top of Nettle.
205 By convention, for an algorithm named `foo', the struct tag for the
206 context struct is `foo_ctx', constants and functions uses prefixes like
207 `FOO_BLOCK_SIZE' (a constant) and `foo_set_key' (a function).
209 In all functions, strings are represented with an explicit length, of
210 type `unsigned', and a pointer of type `uint8_t *' or `const uint8_t
211 *'. For functions that transform one string to another, the argument
212 order is length, destination pointer and source pointer. Source and
213 destination areas are of the same length. Source and destination may be
214 the same, so that you can process strings in place, but they _must not_
215 overlap in any other way.
217 Many of the functions lack return value and can never fail. Those
218 functions which can fail, return one on success and zero on failure.
221 File: nettle.info, Node: Example, Next: Linking, Prev: Conventions, Up: Top
226 A simple example program that reads a file from standard input and
227 writes its SHA1 checksum on standard output should give the flavor of
233 #include <nettle/sha.h>
235 #define BUF_SIZE 1000
238 display_hex(unsigned length, uint8_t *data)
242 for (i = 0; i<length; i++)
243 printf("%02x ", data[i]);
249 main(int argc, char **argv)
252 uint8_t buffer[BUF_SIZE];
253 uint8_t digest[SHA1_DIGEST_SIZE];
258 int done = fread(buffer, 1, sizeof(buffer), stdin);
259 sha1_update(&ctx, done, buffer);
260 if (done < sizeof(buffer))
266 sha1_digest(&ctx, SHA1_DIGEST_SIZE, digest);
268 display_hex(SHA1_DIGEST_SIZE, digest);
272 On a typical Unix system, this program can be compiled and linked
273 with the command line
274 cc sha-example.c -o sha-example -lnettle
277 File: nettle.info, Node: Linking, Next: Reference, Prev: Example, Up: Top
282 Nettle actually consists of two libraries, `libnettle' and
283 `libhogweed'. The `libhogweed' library contains those functions of
284 Nettle that uses bignum operations, and depends on the GMP library.
285 With this division, linking works the same for both static and dynamic
288 If an application uses only the symmetric crypto algorithms of Nettle
289 (i.e., block ciphers, hash functions, and the like), it's sufficient to
290 link with `-lnettle'. If an application also uses public-key
291 algorithms, the recommended linker flags are `-lhogweed -lnettle
292 -lgmp'. If the involved libraries are installed as dynamic libraries, it
293 may be sufficient to link with just `-lhogweed', and the loader will
294 resolve the dependencies automatically.
297 File: nettle.info, Node: Reference, Next: Nettle soup, Prev: Linking, Up: Top
302 This chapter describes all the Nettle functions, grouped by family.
309 * Keyed hash functions::
310 * Public-key algorithms::
312 * Miscellaneous functions::
313 * Compatibility functions::
316 File: nettle.info, Node: Hash functions, Next: Cipher functions, Prev: Reference, Up: Reference
321 A cryptographic "hash function" is a function that takes variable size
322 strings, and maps them to strings of fixed, short, length. There are
323 naturally lots of collisions, as there are more possible 1MB files than
324 20 byte strings. But the function is constructed such that is hard to
325 find the collisions. More precisely, a cryptographic hash function `H'
326 should have the following properties:
329 Given a hash value `H(x)' it is hard to find a string `x' that
330 hashes to that value.
332 _Collision-resistant_
333 It is hard to find two different strings, `x' and `y', such that
337 Hash functions are useful as building blocks for digital signatures,
338 message authentication codes, pseudo random generators, association of
339 unique ids to documents, and many other things.
341 The most commonly used hash functions are MD5 and SHA1.
342 Unfortunately, both these fail the collision-resistance requirement;
343 cryptologists have found ways to construct colliding inputs. The
344 recommended hash function for new applications is SHA256, even though
345 it uses a structure similar to MD5 and SHA1. Constructing better hash
346 functions is an urgent research problem.
351 MD5 is a message digest function constructed by Ronald Rivest, and
352 described in `RFC 1321'. It outputs message digests of 128 bits, or 16
353 octets. Nettle defines MD5 in `<nettle/md5.h>'.
355 - Context struct: struct md5_ctx
357 - Constant: MD5_DIGEST_SIZE
358 The size of an MD5 digest, i.e. 16.
360 - Constant: MD5_DATA_SIZE
361 The internal block size of MD5. Useful for some special
362 constructions, in particular HMAC-MD5.
364 - Function: void md5_init (struct md5_ctx *CTX)
365 Initialize the MD5 state.
367 - Function: void md5_update (struct md5_ctx *CTX, unsigned LENGTH,
371 - Function: void md5_digest (struct md5_ctx *CTX, unsigned LENGTH,
373 Performs final processing and extracts the message digest, writing
374 it to DIGEST. LENGTH may be smaller than `MD5_DIGEST_SIZE', in
375 which case only the first LENGTH octets of the digest are written.
377 This function also resets the context in the same way as
380 The normal way to use MD5 is to call the functions in order: First
381 `md5_init', then `md5_update' zero or more times, and finally
382 `md5_digest'. After `md5_digest', the context is reset to its initial
383 state, so you can start over calling `md5_update' to hash new data.
385 To start over, you can call `md5_init' at any time.
390 MD2 is another hash function of Ronald Rivest's, described in `RFC
391 1319'. It outputs message digests of 128 bits, or 16 octets. Nettle
392 defines MD2 in `<nettle/md2.h>'.
394 - Context struct: struct md2_ctx
396 - Constant: MD2_DIGEST_SIZE
397 The size of an MD2 digest, i.e. 16.
399 - Constant: MD2_DATA_SIZE
400 The internal block size of MD2.
402 - Function: void md2_init (struct md2_ctx *CTX)
403 Initialize the MD2 state.
405 - Function: void md2_update (struct md2_ctx *CTX, unsigned LENGTH,
409 - Function: void md2_digest (struct md2_ctx *CTX, unsigned LENGTH,
411 Performs final processing and extracts the message digest, writing
412 it to DIGEST. LENGTH may be smaller than `MD2_DIGEST_SIZE', in
413 which case only the first LENGTH octets of the digest are written.
415 This function also resets the context in the same way as
421 MD4 is a predecessor of MD5, described in `RFC 1320'. Like MD5, it is
422 constructed by Ronald Rivest. It outputs message digests of 128 bits,
423 or 16 octets. Nettle defines MD4 in `<nettle/md4.h>'. Use of MD4 is not
424 recommended, but it is sometimes needed for compatibility with existing
425 applications and protocols.
427 - Context struct: struct md4_ctx
429 - Constant: MD4_DIGEST_SIZE
430 The size of an MD4 digest, i.e. 16.
432 - Constant: MD4_DATA_SIZE
433 The internal block size of MD4.
435 - Function: void md4_init (struct md4_ctx *CTX)
436 Initialize the MD4 state.
438 - Function: void md4_update (struct md4_ctx *CTX, unsigned LENGTH,
442 - Function: void md4_digest (struct md4_ctx *CTX, unsigned LENGTH,
444 Performs final processing and extracts the message digest, writing
445 it to DIGEST. LENGTH may be smaller than `MD4_DIGEST_SIZE', in
446 which case only the first LENGTH octets of the digest are written.
448 This function also resets the context in the same way as
454 SHA1 is a hash function specified by "NIST" (The U.S. National Institute
455 for Standards and Technology). It outputs hash values of 160 bits, or 20
456 octets. Nettle defines SHA1 in `<nettle/sha.h>'.
458 The functions are analogous to the MD5 ones.
460 - Context struct: struct sha1_ctx
462 - Constant: SHA1_DIGEST_SIZE
463 The size of an SHA1 digest, i.e. 20.
465 - Constant: SHA1_DATA_SIZE
466 The internal block size of SHA1. Useful for some special
467 constructions, in particular HMAC-SHA1.
469 - Function: void sha1_init (struct sha1_ctx *CTX)
470 Initialize the SHA1 state.
472 - Function: void sha1_update (struct sha1_ctx *CTX, unsigned LENGTH,
476 - Function: void sha1_digest (struct sha1_ctx *CTX, unsigned LENGTH,
478 Performs final processing and extracts the message digest, writing
479 it to DIGEST. LENGTH may be smaller than `SHA1_DIGEST_SIZE', in
480 which case only the first LENGTH octets of the digest are written.
482 This function also resets the context in the same way as
488 SHA256 is another hash function specified by "NIST", intended as a
489 replacement for SHA1, generating larger digests. It outputs hash values
490 of 256 bits, or 32 octets. Nettle defines SHA256 in `<nettle/sha.h>'.
492 The functions are analogous to the MD5 ones.
494 - Context struct: struct sha256_ctx
496 - Constant: SHA256_DIGEST_SIZE
497 The size of an SHA256 digest, i.e. 32.
499 - Constant: SHA256_DATA_SIZE
500 The internal block size of SHA256. Useful for some special
501 constructions, in particular HMAC-SHA256.
503 - Function: void sha256_init (struct sha256_ctx *CTX)
504 Initialize the SHA256 state.
506 - Function: void sha256_update (struct sha256_ctx *CTX, unsigned
507 LENGTH, const uint8_t *DATA)
510 - Function: void sha256_digest (struct sha256_ctx *CTX, unsigned
511 LENGTH, uint8_t *DIGEST)
512 Performs final processing and extracts the message digest, writing
513 it to DIGEST. LENGTH may be smaller than `SHA256_DIGEST_SIZE', in
514 which case only the first LENGTH octets of the digest are written.
516 This function also resets the context in the same way as
522 SHA224 is a variant of SHA256, with a different initial state, and with
523 the output truncated to 224 bits, or 28 octets. Nettle defines SHA224 in
526 The functions are analogous to the MD5 ones.
528 - Context struct: struct sha224_ctx
530 - Constant: SHA224_DIGEST_SIZE
531 The size of an SHA224 digest, i.e. 28.
533 - Constant: SHA224_DATA_SIZE
534 The internal block size of SHA224. Useful for some special
535 constructions, in particular HMAC-SHA224.
537 - Function: void sha224_init (struct sha224_ctx *CTX)
538 Initialize the SHA224 state.
540 - Function: void sha224_update (struct sha224_ctx *CTX, unsigned
541 LENGTH, const uint8_t *DATA)
544 - Function: void sha224_digest (struct sha224_ctx *CTX, unsigned
545 LENGTH, uint8_t *DIGEST)
546 Performs final processing and extracts the message digest, writing
547 it to DIGEST. LENGTH may be smaller than `SHA224_DIGEST_SIZE', in
548 which case only the first LENGTH octets of the digest are written.
550 This function also resets the context in the same way as
556 SHA512 is a larger sibling to SHA256, with a very similar structure but
557 with both the output and the internal variables of twice the size. The
558 internal variables are 64 bits rather than 32, making it significantly
559 slower on 32-bit computers. It outputs hash values of 512 bits, or 64
560 octets. Nettle defines SHA512 in `<nettle/sha.h>'.
562 The functions are analogous to the MD5 ones.
564 - Context struct: struct sha512_ctx
566 - Constant: SHA512_DIGEST_SIZE
567 The size of an SHA512 digest, i.e. 64.
569 - Constant: SHA512_DATA_SIZE
570 The internal block size of SHA512. Useful for some special
571 constructions, in particular HMAC-SHA512.
573 - Function: void sha512_init (struct sha512_ctx *CTX)
574 Initialize the SHA512 state.
576 - Function: void sha512_update (struct sha512_ctx *CTX, unsigned
577 LENGTH, const uint8_t *DATA)
580 - Function: void sha512_digest (struct sha512_ctx *CTX, unsigned
581 LENGTH, uint8_t *DIGEST)
582 Performs final processing and extracts the message digest, writing
583 it to DIGEST. LENGTH may be smaller than `SHA512_DIGEST_SIZE', in
584 which case only the first LENGTH octets of the digest are written.
586 This function also resets the context in the same way as
592 SHA384 is a variant of SHA512, with a different initial state, and with
593 the output truncated to 384 bits, or 48 octets. Nettle defines SHA384 in
596 The functions are analogous to the MD5 ones.
598 - Context struct: struct sha384_ctx
600 - Constant: SHA384_DIGEST_SIZE
601 The size of an SHA384 digest, i.e. 48.
603 - Constant: SHA384_DATA_SIZE
604 The internal block size of SHA384. Useful for some special
605 constructions, in particular HMAC-SHA384.
607 - Function: void sha384_init (struct sha384_ctx *CTX)
608 Initialize the SHA384 state.
610 - Function: void sha384_update (struct sha384_ctx *CTX, unsigned
611 LENGTH, const uint8_t *DATA)
614 - Function: void sha384_digest (struct sha384_ctx *CTX, unsigned
615 LENGTH, uint8_t *DIGEST)
616 Performs final processing and extracts the message digest, writing
617 it to DIGEST. LENGTH may be smaller than `SHA384_DIGEST_SIZE', in
618 which case only the first LENGTH octets of the digest are written.
620 This function also resets the context in the same way as
626 Nettle includes a struct including information about the supported hash
627 functions. It is defined in `<nettle/nettle-meta.h>', and is used by
628 Nettle's implementation of HMAC *note Keyed hash functions::.
630 - Meta struct: `struct nettle_hash' name context_size digest_size
631 block_size init update digest
632 The last three attributes are function pointers, of types
633 `nettle_hash_init_func', `nettle_hash_update_func', and
634 `nettle_hash_digest_func'. The first argument to these functions is
635 `void *' pointer to a context struct, which is of size
638 - Constant Struct: struct nettle_hash nettle_md2
639 - Constant Struct: struct nettle_hash nettle_md4
640 - Constant Struct: struct nettle_hash nettle_md5
641 - Constant Struct: struct nettle_hash nettle_sha1
642 - Constant Struct: struct nettle_hash nettle_sha224
643 - Constant Struct: struct nettle_hash nettle_sha256
644 - Constant Struct: struct nettle_hash nettle_sha384
645 - Constant Struct: struct nettle_hash nettle_sha512
646 These are all the hash functions that Nettle implements.
649 File: nettle.info, Node: Cipher functions, Next: Cipher modes, Prev: Hash functions, Up: Reference
654 A "cipher" is a function that takes a message or "plaintext" and a
655 secret "key" and transforms it to a "ciphertext". Given only the
656 ciphertext, but not the key, it should be hard to find the plaintext.
657 Given matching pairs of plaintext and ciphertext, it should be hard to
660 There are two main classes of ciphers: Block ciphers and stream
663 A block cipher can process data only in fixed size chunks, called
664 "blocks". Typical block sizes are 8 or 16 octets. To encrypt arbitrary
665 messages, you usually have to pad it to an integral number of blocks,
666 split it into blocks, and then process each block. The simplest way is
667 to process one block at a time, independent of each other. That mode of
668 operation is called "ECB", Electronic Code Book mode. However, using
669 ECB is usually a bad idea. For a start, plaintext blocks that are equal
670 are transformed to ciphertext blocks that are equal; that leaks
671 information about the plaintext. Usually you should apply the cipher is
672 some "feedback mode", "CBC" (Cipher Block Chaining) and "CTR" (Counter
673 mode) being two of of the most popular. See *Note Cipher modes::, for
674 information on how to apply CBC and CTR with Nettle.
676 A stream cipher can be used for messages of arbitrary length. A
677 typical stream cipher is a keyed pseudo-random generator. To encrypt a
678 plaintext message of N octets, you key the generator, generate N octets
679 of pseudo-random data, and XOR it with the plaintext. To decrypt,
680 regenerate the same stream using the key, XOR it to the ciphertext, and
681 the plaintext is recovered.
683 *Caution:* The first rule for this kind of cipher is the same as for
684 a One Time Pad: _never_ ever use the same key twice.
686 A common misconception is that encryption, by itself, implies
687 authentication. Say that you and a friend share a secret key, and you
688 receive an encrypted message. You apply the key, and get a plaintext
689 message that makes sense to you. Can you then be sure that it really was
690 your friend that wrote the message you're reading? The answer is no. For
691 example, if you were using a block cipher in ECB mode, an attacker may
692 pick up the message on its way, and reorder, delete or repeat some of
693 the blocks. Even if the attacker can't decrypt the message, he can
694 change it so that you are not reading the same message as your friend
695 wrote. If you are using a block cipher in CBC mode rather than ECB, or
696 are using a stream cipher, the possibilities for this sort of attack
697 are different, but the attacker can still make predictable changes to
700 It is recommended to _always_ use an authentication mechanism in
701 addition to encrypting the messages. Popular choices are Message
702 Authentication Codes like HMAC-SHA1 *note Keyed hash functions::, or
703 digital signatures like RSA.
705 Some ciphers have so called "weak keys", keys that results in
706 undesirable structure after the key setup processing, and should be
707 avoided. In Nettle, most key setup functions have no return value, but
708 for ciphers with weak keys, the return value indicates whether or not
709 the given key is weak. For good keys, key setup returns 1, and for weak
710 keys, it returns 0. When possible, avoid algorithms that have weak
711 keys. There are several good ciphers that don't have any weak keys.
713 To encrypt a message, you first initialize a cipher context for
714 encryption or decryption with a particular key. You then use the context
715 to process plaintext or ciphertext messages. The initialization is known
716 as "key setup". With Nettle, it is recommended to use each context
717 struct for only one direction, even if some of the ciphers use a single
718 key setup function that can be used for both encryption and decryption.
723 AES is a block cipher, specified by NIST as a replacement for the older
724 DES standard. The standard is the result of a competition between
725 cipher designers. The winning design, also known as RIJNDAEL, was
726 constructed by Joan Daemen and Vincent Rijnmen.
728 Like all the AES candidates, the winning design uses a block size of
729 128 bits, or 16 octets, and variable key-size, 128, 192 and 256 bits
730 (16, 24 and 32 octets) being the allowed key sizes. It does not have
731 any weak keys. Nettle defines AES in `<nettle/aes.h>'.
733 - Context struct: struct aes_ctx
735 - Constant: AES_BLOCK_SIZE
736 The AES block-size, 16
738 - Constant: AES_MIN_KEY_SIZE
740 - Constant: AES_MAX_KEY_SIZE
742 - Constant: AES_KEY_SIZE
743 Default AES key size, 32
745 - Function: void aes_set_encrypt_key (struct aes_ctx *CTX, unsigned
746 LENGTH, const uint8_t *KEY)
747 - Function: void aes_set_decrypt_key (struct aes_ctx *CTX, unsigned
748 LENGTH, const uint8_t *KEY)
749 Initialize the cipher, for encryption or decryption, respectively.
751 - Function: void aes_invert_key (struct aes_ctx *DST, const struct
753 Given a context SRC initialized for encryption, initializes the
754 context struct DST for decryption, using the same key. If the same
755 context struct is passed for both `src' and `dst', it is converted
756 in place. Calling `aes_set_encrypt_key' and `aes_invert_key' is
757 more efficient than calling `aes_set_encrypt_key' and
758 `aes_set_decrypt_key'. This function is mainly useful for
759 applications which needs to both encrypt and decrypt using the
762 - Function: void aes_encrypt (struct aes_ctx *CTX, unsigned LENGTH,
763 const uint8_t *DST, uint8_t *SRC)
764 Encryption function. LENGTH must be an integral multiple of the
765 block size. If it is more than one block, the data is processed in
766 ECB mode. `src' and `dst' may be equal, but they must not overlap
769 - Function: void aes_decrypt (struct aes_ctx *CTX, unsigned LENGTH,
770 const uint8_t *DST, uint8_t *SRC)
771 Analogous to `aes_encrypt'
776 ARCFOUR is a stream cipher, also known under the trade marked name RC4,
777 and it is one of the fastest ciphers around. A problem is that the key
778 setup of ARCFOUR is quite weak, you should never use keys with
779 structure, keys that are ordinary passwords, or sequences of keys like
780 "secret:1", "secret:2", ..... If you have keys that don't look like
781 random bit strings, and you want to use ARCFOUR, always hash the key
782 before feeding it to ARCFOUR. Furthermore, the initial bytes of the
783 generated key stream leak information about the key; for this reason, it
784 is recommended to discard the first 512 bytes of the key stream.
786 /* A more robust key setup function for ARCFOUR */
788 arcfour_set_key_hashed(struct arcfour_ctx *ctx,
789 unsigned length, const uint8_t *key)
791 struct sha256_ctx hash;
792 uint8_t digest[SHA256_DIGEST_SIZE];
793 uint8_t buffer[0x200];
796 sha256_update(&hash, length, key);
797 sha256_digest(&hash, SHA256_DIGEST_SIZE, digest);
799 arcfour_set_key(ctx, SHA256_DIGEST_SIZE, digest);
800 arcfour_crypt(ctx, sizeof(buffer), buffer, buffer);
803 Nettle defines ARCFOUR in `<nettle/arcfour.h>'.
805 - Context struct: struct arcfour_ctx
807 - Constant: ARCFOUR_MIN_KEY_SIZE
810 - Constant: ARCFOUR_MAX_KEY_SIZE
811 Maximum key size, 256
813 - Constant: ARCFOUR_KEY_SIZE
814 Default ARCFOUR key size, 16
816 - Function: void arcfour_set_key (struct arcfour_ctx *CTX, unsigned
817 LENGTH, const uint8_t *KEY)
818 Initialize the cipher. The same function is used for both
819 encryption and decryption.
821 - Function: void arcfour_crypt (struct arcfour_ctx *CTX, unsigned
822 LENGTH, const uint8_t *DST, uint8_t *SRC)
823 Encrypt some data. The same function is used for both encryption
824 and decryption. Unlike the block ciphers, this function modifies
825 the context, so you can split the data into arbitrary chunks and
826 encrypt them one after another. The result is the same as if you
827 had called `arcfour_crypt' only once with all the data.
832 ARCTWO (also known as the trade marked name RC2) is a block cipher
833 specified in RFC 2268. Nettle also include a variation of the ARCTWO
834 set key operation that lack one step, to be compatible with the reverse
835 engineered RC2 cipher description, as described in a Usenet post to
836 `sci.crypt' by Peter Gutmann.
838 ARCTWO uses a block size of 64 bits, and variable key-size ranging
839 from 1 to 128 octets. Besides the key, ARCTWO also has a second
840 parameter to key setup, the number of effective key bits, `ekb'. This
841 parameter can be used to artificially reduce the key size. In practice,
842 `ekb' is usually set equal to the input key size. Nettle defines
843 ARCTWO in `<nettle/arctwo.h>'.
845 We do not recommend the use of ARCTWO; the Nettle implementation is
846 provided primarily for interoperability with existing applications and
849 - Context struct: struct arctwo_ctx
851 - Constant: ARCTWO_BLOCK_SIZE
852 The AES block-size, 8
854 - Constant: ARCTWO_MIN_KEY_SIZE
856 - Constant: ARCTWO_MAX_KEY_SIZE
858 - Constant: ARCTWO_KEY_SIZE
859 Default ARCTWO key size, 8
861 - Function: void arctwo_set_key_ekb (struct arctwo_ctx *CTX, unsigned
862 LENGTH, const uint8_t *KEY, unsigned EKB)
863 - Function: void arctwo_set_key (struct arctwo_ctx *CTX, unsigned
864 LENGTH, const uint8_t *KEY)
865 - Function: void arctwo_set_key_gutmann (struct arctwo_ctx *CTX,
866 unsigned LENGTH, const uint8_t *KEY)
867 Initialize the cipher. The same function is used for both
868 encryption and decryption. The first function is the most general
869 one, which lets you provide both the variable size key, and the
870 desired effective key size (in bits). The maximum value for EKB is
871 1024, and for convenience, `ekb = 0' has the same effect as `ekb =
874 `arctwo_set_key(ctx, length, key)' is equivalent to
875 `arctwo_set_key_ekb(ctx, length, key, 8*length)', and
876 `arctwo_set_key_gutmann(ctx, length, key)' is equivalent to
877 `arctwo_set_key_ekb(ctx, length, key, 1024)'
879 - Function: void arctwo_encrypt (struct arctwo_ctx *CTX, unsigned
880 LENGTH, const uint8_t *DST, uint8_t *SRC)
881 Encryption function. LENGTH must be an integral multiple of the
882 block size. If it is more than one block, the data is processed in
883 ECB mode. `src' and `dst' may be equal, but they must not overlap
886 - Function: void arctwo_decrypt (struct arctwo_ctx *CTX, unsigned
887 LENGTH, const uint8_t *DST, uint8_t *SRC)
888 Analogous to `arctwo_encrypt'
893 BLOWFISH is a block cipher designed by Bruce Schneier. It uses a block
894 size of 64 bits (8 octets), and a variable key size, up to 448 bits. It
895 has some weak keys. Nettle defines BLOWFISH in `<nettle/blowfish.h>'.
897 - Context struct: struct blowfish_ctx
899 - Constant: BLOWFISH_BLOCK_SIZE
900 The BLOWFISH block-size, 8
902 - Constant: BLOWFISH_MIN_KEY_SIZE
903 Minimum BLOWFISH key size, 8
905 - Constant: BLOWFISH_MAX_KEY_SIZE
906 Maximum BLOWFISH key size, 56
908 - Constant: BLOWFISH_KEY_SIZE
909 Default BLOWFISH key size, 16
911 - Function: int blowfish_set_key (struct blowfish_ctx *CTX, unsigned
912 LENGTH, const uint8_t *KEY)
913 Initialize the cipher. The same function is used for both
914 encryption and decryption. Checks for weak keys, returning 1 for
915 good keys and 0 for weak keys. Applications that don't care about
916 weak keys can ignore the return value.
918 `blowfish_encrypt' or `blowfish_decrypt' with a weak key will
919 crash with an assert violation.
921 - Function: void blowfish_encrypt (struct blowfish_ctx *CTX, unsigned
922 LENGTH, const uint8_t *DST, uint8_t *SRC)
923 Encryption function. LENGTH must be an integral multiple of the
924 block size. If it is more than one block, the data is processed in
925 ECB mode. `src' and `dst' may be equal, but they must not overlap
928 - Function: void blowfish_decrypt (struct blowfish_ctx *CTX, unsigned
929 LENGTH, const uint8_t *DST, uint8_t *SRC)
930 Analogous to `blowfish_encrypt'
935 Camellia is a block cipher developed by Mitsubishi and Nippon Telegraph
936 and Telephone Corporation, described in `RFC3713', and recommended by
937 some Japanese and European authorities as an alternative to AES. The
938 algorithm is patented. The implementation in Nettle is derived from the
939 implementation released by NTT under the GNU LGPL (v2.1 or later), and
940 relies on the implicit patent license of the LGPL. There is also a
941 statement of royalty-free licensing for Camellia at
942 <http://www.ntt.co.jp/news/news01e/0104/010417.html>, but this
943 statement has some limitations which seem problematic for free software.
945 Camellia uses a the same block size and key sizes as AES: The block
946 size is 128 bits (16 octets), and the supported key sizes are 128, 192,
947 and 256 bits. Nettle defines Camellia in `<nettle/camellia.h>'.
949 - Context struct: struct camellia_ctx
951 - Constant: CAMELLIA_BLOCK_SIZE
952 The CAMELLIA block-size, 16
954 - Constant: CAMELLIA_MIN_KEY_SIZE
956 - Constant: CAMELLIA_MAX_KEY_SIZE
958 - Constant: CAMELLIA_KEY_SIZE
959 Default CAMELLIA key size, 32
961 - Function: void camellia_set_encrypt_key (struct camellia_ctx *CTX,
962 unsigned LENGTH, const uint8_t *KEY)
963 - Function: void camellia_set_decrypt_key (struct camellia_ctx *CTX,
964 unsigned LENGTH, const uint8_t *KEY)
965 Initialize the cipher, for encryption or decryption, respectively.
967 - Function: void camellia_invert_key (struct camellia_ctx *DST, const
968 struct camellia_ctx *SRC)
969 Given a context SRC initialized for encryption, initializes the
970 context struct DST for decryption, using the same key. If the same
971 context struct is passed for both `src' and `dst', it is converted
972 in place. Calling `camellia_set_encrypt_key' and
973 `camellia_invert_key' is more efficient than calling
974 `camellia_set_encrypt_key' and `camellia_set_decrypt_key'. This
975 function is mainly useful for applications which needs to both
976 encrypt and decrypt using the _same_ key.
978 - Function: void camellia_crypt (struct camellia_ctx *CTX, unsigned
979 LENGTH, const uint8_t *DST, uint8_t *SRC)
980 The same function is used for both encryption and decryption.
981 LENGTH must be an integral multiple of the block size. If it is
982 more than one block, the data is processed in ECB mode. `src' and
983 `dst' may be equal, but they must not overlap in any other way.
988 CAST-128 is a block cipher, specified in `RFC 2144'. It uses a 64 bit
989 (8 octets) block size, and a variable key size of up to 128 bits.
990 Nettle defines cast128 in `<nettle/cast128.h>'.
992 - Context struct: struct cast128_ctx
994 - Constant: CAST128_BLOCK_SIZE
995 The CAST128 block-size, 8
997 - Constant: CAST128_MIN_KEY_SIZE
998 Minimum CAST128 key size, 5
1000 - Constant: CAST128_MAX_KEY_SIZE
1001 Maximum CAST128 key size, 16
1003 - Constant: CAST128_KEY_SIZE
1004 Default CAST128 key size, 16
1006 - Function: void cast128_set_key (struct cast128_ctx *CTX, unsigned
1007 LENGTH, const uint8_t *KEY)
1008 Initialize the cipher. The same function is used for both
1009 encryption and decryption.
1011 - Function: void cast128_encrypt (struct cast128_ctx *CTX, unsigned
1012 LENGTH, const uint8_t *DST, uint8_t *SRC)
1013 Encryption function. LENGTH must be an integral multiple of the
1014 block size. If it is more than one block, the data is processed in
1015 ECB mode. `src' and `dst' may be equal, but they must not overlap
1018 - Function: void cast128_decrypt (struct cast128_ctx *CTX, unsigned
1019 LENGTH, const uint8_t *DST, uint8_t *SRC)
1020 Analogous to `cast128_encrypt'
1025 DES is the old Data Encryption Standard, specified by NIST. It uses a
1026 block size of 64 bits (8 octets), and a key size of 56 bits. However,
1027 the key bits are distributed over 8 octets, where the least significant
1028 bit of each octet may be used for parity. A common way to use DES is to
1029 generate 8 random octets in some way, then set the least significant bit
1030 of each octet to get odd parity, and initialize DES with the resulting
1033 The key size of DES is so small that keys can be found by brute
1034 force, using specialized hardware or lots of ordinary work stations in
1035 parallel. One shouldn't be using plain DES at all today, if one uses
1036 DES at all one should be using "triple DES", see DES3 below.
1038 DES also has some weak keys. Nettle defines DES in `<nettle/des.h>'.
1040 - Context struct: struct des_ctx
1042 - Constant: DES_BLOCK_SIZE
1043 The DES block-size, 8
1045 - Constant: DES_KEY_SIZE
1048 - Function: int des_set_key (struct des_ctx *CTX, const uint8_t *KEY)
1049 Initialize the cipher. The same function is used for both
1050 encryption and decryption. Parity bits are ignored. Checks for
1051 weak keys, returning 1 for good keys and 0 for weak keys.
1052 Applications that don't care about weak keys can ignore the return
1055 - Function: void des_encrypt (struct des_ctx *CTX, unsigned LENGTH,
1056 const uint8_t *DST, uint8_t *SRC)
1057 Encryption function. LENGTH must be an integral multiple of the
1058 block size. If it is more than one block, the data is processed in
1059 ECB mode. `src' and `dst' may be equal, but they must not overlap
1062 - Function: void des_decrypt (struct des_ctx *CTX, unsigned LENGTH,
1063 const uint8_t *DST, uint8_t *SRC)
1064 Analogous to `des_encrypt'
1066 - Function: int des_check_parity (unsigned LENGTH, const uint8_t *KEY);
1067 Checks that the given key has correct, odd, parity. Returns 1 for
1068 correct parity, and 0 for bad parity.
1070 - Function: void des_fix_parity (unsigned LENGTH, uint8_t *DST, const
1072 Adjusts the parity bits to match DES's requirements. You need this
1073 function if you have created a random-looking string by a key
1074 agreement protocol, and want to use it as a DES key. DST and SRC
1080 The inadequate key size of DES has already been mentioned. One way to
1081 increase the key size is to pipe together several DES boxes with
1082 independent keys. It turns out that using two DES ciphers is not as
1083 secure as one might think, even if the key size of the combination is a
1084 respectable 112 bits.
1086 The standard way to increase DES's key size is to use three DES
1087 boxes. The mode of operation is a little peculiar: the middle DES box
1088 is wired in the reverse direction. To encrypt a block with DES3, you
1089 encrypt it using the first 56 bits of the key, then _decrypt_ it using
1090 the middle 56 bits of the key, and finally encrypt it again using the
1091 last 56 bits of the key. This is known as "ede" triple-DES, for
1092 "encrypt-decrypt-encrypt".
1094 The "ede" construction provides some backward compatibility, as you
1095 get plain single DES simply by feeding the same key to all three boxes.
1096 That should help keeping down the gate count, and the price, of hardware
1097 circuits implementing both plain DES and DES3.
1099 DES3 has a key size of 168 bits, but just like plain DES, useless
1100 parity bits are inserted, so that keys are represented as 24 octets
1101 (192 bits). As a 112 bit key is large enough to make brute force
1102 attacks impractical, some applications uses a "two-key" variant of
1103 triple-DES. In this mode, the same key bits are used for the first and
1104 the last DES box in the pipe, while the middle box is keyed
1105 independently. The two-key variant is believed to be secure, i.e. there
1106 are no known attacks significantly better than brute force.
1108 Naturally, it's simple to implement triple-DES on top of Nettle's DES
1109 functions. Nettle includes an implementation of three-key "ede"
1110 triple-DES, it is defined in the same place as plain DES,
1113 - Context struct: struct des3_ctx
1115 - Constant: DES3_BLOCK_SIZE
1116 The DES3 block-size is the same as DES_BLOCK_SIZE, 8
1118 - Constant: DES3_KEY_SIZE
1121 - Function: int des3_set_key (struct des3_ctx *CTX, const uint8_t *KEY)
1122 Initialize the cipher. The same function is used for both
1123 encryption and decryption. Parity bits are ignored. Checks for
1124 weak keys, returning 1 if all three keys are good keys, and 0 if
1125 one or more key is weak. Applications that don't care about weak
1126 keys can ignore the return value.
1128 For random-looking strings, you can use `des_fix_parity' to adjust
1129 the parity bits before calling `des3_set_key'.
1131 - Function: void des3_encrypt (struct des3_ctx *CTX, unsigned LENGTH,
1132 const uint8_t *DST, uint8_t *SRC)
1133 Encryption function. LENGTH must be an integral multiple of the
1134 block size. If it is more than one block, the data is processed in
1135 ECB mode. `src' and `dst' may be equal, but they must not overlap
1138 - Function: void des3_decrypt (struct des3_ctx *CTX, unsigned LENGTH,
1139 const uint8_t *DST, uint8_t *SRC)
1140 Analogous to `des_encrypt'
1145 SERPENT is one of the AES finalists, designed by Ross Anderson, Eli
1146 Biham and Lars Knudsen. Thus, the interface and properties are similar
1147 to AES'. One peculiarity is that it is quite pointless to use it with
1148 anything but the maximum key size, smaller keys are just padded to
1149 larger ones. Nettle defines SERPENT in `<nettle/serpent.h>'.
1151 - Context struct: struct serpent_ctx
1153 - Constant: SERPENT_BLOCK_SIZE
1154 The SERPENT block-size, 16
1156 - Constant: SERPENT_MIN_KEY_SIZE
1157 Minimum SERPENT key size, 16
1159 - Constant: SERPENT_MAX_KEY_SIZE
1160 Maximum SERPENT key size, 32
1162 - Constant: SERPENT_KEY_SIZE
1163 Default SERPENT key size, 32
1165 - Function: void serpent_set_key (struct serpent_ctx *CTX, unsigned
1166 LENGTH, const uint8_t *KEY)
1167 Initialize the cipher. The same function is used for both
1168 encryption and decryption.
1170 - Function: void serpent_encrypt (struct serpent_ctx *CTX, unsigned
1171 LENGTH, const uint8_t *DST, uint8_t *SRC)
1172 Encryption function. LENGTH must be an integral multiple of the
1173 block size. If it is more than one block, the data is processed in
1174 ECB mode. `src' and `dst' may be equal, but they must not overlap
1177 - Function: void serpent_decrypt (struct serpent_ctx *CTX, unsigned
1178 LENGTH, const uint8_t *DST, uint8_t *SRC)
1179 Analogous to `serpent_encrypt'
1184 Another AES finalist, this one designed by Bruce Schneier and others.
1185 Nettle defines it in `<nettle/twofish.h>'.
1187 - Context struct: struct twofish_ctx
1189 - Constant: TWOFISH_BLOCK_SIZE
1190 The TWOFISH block-size, 16
1192 - Constant: TWOFISH_MIN_KEY_SIZE
1193 Minimum TWOFISH key size, 16
1195 - Constant: TWOFISH_MAX_KEY_SIZE
1196 Maximum TWOFISH key size, 32
1198 - Constant: TWOFISH_KEY_SIZE
1199 Default TWOFISH key size, 32
1201 - Function: void twofish_set_key (struct twofish_ctx *CTX, unsigned
1202 LENGTH, const uint8_t *KEY)
1203 Initialize the cipher. The same function is used for both
1204 encryption and decryption.
1206 - Function: void twofish_encrypt (struct twofish_ctx *CTX, unsigned
1207 LENGTH, const uint8_t *DST, uint8_t *SRC)
1208 Encryption function. LENGTH must be an integral multiple of the
1209 block size. If it is more than one block, the data is processed in
1210 ECB mode. `src' and `dst' may be equal, but they must not overlap
1213 - Function: void twofish_decrypt (struct twofish_ctx *CTX, unsigned
1214 LENGTH, const uint8_t *DST, uint8_t *SRC)
1215 Analogous to `twofish_encrypt'
1217 `struct nettle_cipher'
1218 ----------------------
1220 Nettle includes a struct including information about some of the more
1221 regular cipher functions. It should be considered a little experimental,
1222 but can be useful for applications that need a simple way to handle
1223 various algorithms. Nettle defines these structs in
1224 `<nettle/nettle-meta.h>'.
1226 - Meta struct: `struct nettle_cipher' name context_size block_size
1227 key_size set_encrypt_key set_decrypt_key encrypt decrypt
1228 The last four attributes are function pointers, of types
1229 `nettle_set_key_func' and `nettle_crypt_func'. The first argument
1230 to these functions is a `void *' pointer to a context struct,
1231 which is of size `context_size'.
1233 - Constant Struct: struct nettle_cipher nettle_aes128
1234 - Constant Struct: struct nettle_cipher nettle_aes192
1235 - Constant Struct: struct nettle_cipher nettle_aes256
1236 - Constant Struct: struct nettle_cipher nettle_arctwo40;
1237 - Constant Struct: struct nettle_cipher nettle_arctwo64;
1238 - Constant Struct: struct nettle_cipher nettle_arctwo128;
1239 - Constant Struct: struct nettle_cipher nettle_arctwo_gutmann128;
1240 - Constant Struct: struct nettle_cipher nettle_arcfour128
1241 - Constant Struct: struct nettle_cipher nettle_camellia128
1242 - Constant Struct: struct nettle_cipher nettle_camellia192
1243 - Constant Struct: struct nettle_cipher nettle_camellia256
1244 - Constant Struct: struct nettle_cipher nettle_cast128
1245 - Constant Struct: struct nettle_cipher nettle_serpent128
1246 - Constant Struct: struct nettle_cipher nettle_serpent192
1247 - Constant Struct: struct nettle_cipher nettle_serpent256
1248 - Constant Struct: struct nettle_cipher nettle_twofish128
1249 - Constant Struct: struct nettle_cipher nettle_twofish192
1250 - Constant Struct: struct nettle_cipher nettle_twofish256
1251 - Constant Struct: struct nettle_cipher nettle_arctwo40;
1252 - Constant Struct: struct nettle_cipher nettle_arctwo64;
1253 - Constant Struct: struct nettle_cipher nettle_arctwo128;
1254 - Constant Struct: struct nettle_cipher nettle_arctwo_gutmann128;
1255 Nettle includes such structs for all the _regular_ ciphers, i.e.
1256 ones without weak keys or other oddities.
1259 File: nettle.info, Node: Cipher modes, Next: Keyed hash functions, Prev: Cipher functions, Up: Reference
1264 Cipher modes of operation specifies the procedure to use when
1265 encrypting a message that is larger than the cipher's block size. As
1266 explained in *Note Cipher functions::, splitting the message into blocks
1267 and processing them independently with the block cipher (Electronic Code
1268 Book mode, ECB) leaks information. Besides ECB, Nettle provides two
1269 other modes of operation: Cipher Block Chaining (CBC) and Counter mode
1270 (CTR). CBC is widely used, but there are a few subtle issues of
1271 information leakage. CTR was standardized more recently, and is
1272 believed to be more secure.
1274 Cipher Block Chaining
1275 ---------------------
1277 When using CBC mode, plaintext blocks are not encrypted independently
1278 of each other, like in Electronic Cook Book mode. Instead, when
1279 encrypting a block in CBC mode, the previous ciphertext block is XORed
1280 with the plaintext before it is fed to the block cipher. When
1281 encrypting the first block, a random block called an "IV", or
1282 Initialization Vector, is used as the "previous ciphertext block". The
1283 IV should be chosen randomly, but it need not be kept secret, and can
1284 even be transmitted in the clear together with the encrypted data.
1286 In symbols, if `E_k' is the encryption function of a block cipher,
1287 and `IV' is the initialization vector, then `n' plaintext blocks
1288 `M_1',... `M_n' are transformed into `n' ciphertext blocks `C_1',...
1291 C_1 = E_k(IV XOR M_1)
1292 C_2 = E_k(C_1 XOR M_2)
1296 C_n = E_k(C_(n-1) XOR M_n)
1298 Nettle's includes two functions for applying a block cipher in Cipher
1299 Block Chaining (CBC) mode, one for encryption and one for decryption.
1300 These functions uses `void *' to pass cipher contexts around.
1302 - Function: void cbc_encrypt (void *CTX, nettle_crypt_func F, unsigned
1303 BLOCK_SIZE, uint8_t *IV, unsigned LENGTH, uint8_t *DST, const
1305 - Function: void cbc_decrypt (void *CTX, void (*F)(), unsigned
1306 BLOCK_SIZE, uint8_t *IV, unsigned LENGTH, uint8_t *DST, const
1308 Applies the encryption or decryption function F in CBC mode. The
1309 final ciphertext block processed is copied into IV before
1310 returning, so that large message be processed be a sequence of
1311 calls to `cbc_encrypt'. The function F is of type
1313 `void f (void *CTX, unsigned LENGTH, uint8_t DST, const uint8_t
1316 and the `cbc_encrypt' and `cbc_decrypt' functions pass their
1317 argument CTX on to F.
1319 There are also some macros to help use these functions correctly.
1321 - Macro: CBC_CTX (CONTEXT_TYPE, BLOCK_SIZE)
1325 uint8_t iv[block_size];
1328 It can be used to define a CBC context struct, either directly,
1330 struct CBC_CTX(struct aes_ctx, AES_BLOCK_SIZE) ctx;
1332 or to give it a struct tag,
1334 struct aes_cbc_ctx CBC_CTX (struct aes_ctx, AES_BLOCK_SIZE);
1336 - Macro: CBC_SET_IV (CTX, IV)
1337 First argument is a pointer to a context struct as defined by
1338 `CBC_CTX', and the second is a pointer to an Initialization Vector
1339 (IV) that is copied into that context.
1341 - Macro: CBC_ENCRYPT (CTX, F, LENGTH, DST, SRC)
1342 - Macro: CBC_DECRYPT (CTX, F, LENGTH, DST, SRC)
1343 A simpler way to invoke `cbc_encrypt' and `cbc_decrypt'. The first
1344 argument is a pointer to a context struct as defined by `CBC_CTX',
1345 and the second argument is an encryption or decryption function
1346 following Nettle's conventions. The last three arguments define
1347 the source and destination area for the operation.
1349 These macros use some tricks to make the compiler display a warning
1350 if the types of F and CTX don't match, e.g. if you try to use an
1351 `struct aes_ctx' context with the `des_encrypt' function.
1356 Counter mode (CTR) uses the block cipher as a keyed pseudo-random
1357 generator. The output of the generator is XORed with the data to be
1358 encrypted. It can be understood as a way to transform a block cipher to
1361 The message is divided into `n' blocks `M_1',... `M_n', where `M_n'
1362 is of size `m' which may be smaller than the block size. Except for the
1363 last block, all the message blocks must be of size equal to the
1364 cipher's block size.
1366 If `E_k' is the encryption function of a block cipher, `IC' is the
1367 initial counter, then the `n' plaintext blocks are transformed into `n'
1368 ciphertext blocks `C_1',... `C_n' as follows:
1370 C_1 = E_k(IC) XOR M_1
1371 C_2 = E_k(IC + 1) XOR M_2
1375 C_(n-1) = E_k(IC + n - 2) XOR M_(n-1)
1376 C_n = E_k(IC + n - 1) [1..m] XOR M_n
1378 The IC is the initial value for the counter, it plays a similar role
1379 as the IV for CBC. When adding, `IC + x', IC is interpreted as an
1380 integer, in network byte order. For the last block, `E_k(IC + n - 1)
1381 [1..m]' means that the cipher output is truncated to `m' bytes.
1383 - Function: void ctr_crypt (void *CTX, nettle_crypt_func F, unsigned
1384 BLOCK_SIZE, uint8_t *CTR, unsigned LENGTH, uint8_t *DST,
1386 Applies the encryption function F in CTR mode. Note that for CTR
1387 mode, encryption and decryption is the same operation, and hence F
1388 should always be the encryption function for the underlying block
1391 When a message is encrypted using a sequence of calls to
1392 `ctr_crypt', all but the last call _must_ use a length that is a
1393 multiple of the block size.
1395 Like for CBC, there are also a couple of helper macros.
1397 - Macro: CTR_CTX (CONTEXT_TYPE, BLOCK_SIZE)
1401 uint8_t ctr[block_size];
1404 - Macro: CTR_SET_COUNTER (CTX, IV)
1405 First argument is a pointer to a context struct as defined by
1406 `CTR_CTX', and the second is a pointer to an initial counter that
1407 is copied into that context.
1409 - Macro: CTR_CRYPT (CTX, F, LENGTH, DST, SRC)
1410 A simpler way to invoke `ctr_crypt'. The first argument is a
1411 pointer to a context struct as defined by `CTR_CTX', and the second
1412 argument is an encryption function following Nettle's conventions.
1413 The last three arguments define the source and destination area
1417 File: nettle.info, Node: Keyed hash functions, Next: Public-key algorithms, Prev: Cipher modes, Up: Reference
1419 Keyed Hash Functions
1420 ====================
1422 A "keyed hash function", or "Message Authentication Code" (MAC) is a
1423 function that takes a key and a message, and produces fixed size MAC.
1424 It should be hard to compute a message and a matching MAC without
1425 knowledge of the key. It should also be hard to compute the key given
1426 only messages and corresponding MACs.
1428 Keyed hash functions are useful primarily for message authentication,
1429 when Alice and Bob shares a secret: The sender, Alice, computes the MAC
1430 and attaches it to the message. The receiver, Bob, also computes the
1431 MAC of the message, using the same key, and compares that to Alice's
1432 value. If they match, Bob can be assured that the message has not been
1433 modified on its way from Alice.
1435 However, unlike digital signatures, this assurance is not
1436 transferable. Bob can't show the message and the MAC to a third party
1437 and prove that Alice sent that message. Not even if he gives away the
1438 key to the third party. The reason is that the _same_ key is used on
1439 both sides, and anyone knowing the key can create a correct MAC for any
1440 message. If Bob believes that only he and Alice knows the key, and he
1441 knows that he didn't attach a MAC to a particular message, he knows it
1442 must be Alice who did it. However, the third party can't distinguish
1443 between a MAC created by Alice and one created by Bob.
1445 Keyed hash functions are typically a lot faster than digital
1451 One can build keyed hash functions from ordinary hash functions. Older
1452 constructions simply concatenate secret key and message and hashes
1453 that, but such constructions have weaknesses. A better construction is
1454 HMAC, described in `RFC 2104'.
1456 For an underlying hash function `H', with digest size `l' and
1457 internal block size `b', HMAC-H is constructed as follows: From a given
1458 key `k', two distinct subkeys `k_i' and `k_o' are constructed, both of
1459 length `b'. The HMAC-H of a message `m' is then computed as `H(k_o |
1460 H(k_i | m))', where `|' denotes string concatenation.
1462 HMAC keys can be of any length, but it is recommended to use keys of
1463 length `l', the digest size of the underlying hash function `H'. Keys
1464 that are longer than `b' are shortened to length `l' by hashing with
1465 `H', so arbitrarily long keys aren't very useful.
1467 Nettle's HMAC functions are defined in `<nettle/hmac.h>'. There are
1468 abstract functions that use a pointer to a `struct nettle_hash' to
1469 represent the underlying hash function and `void *' pointers that point
1470 to three different context structs for that hash function. There are
1471 also concrete functions for HMAC-MD5, HMAC-SHA1, HMAC-SHA256, and
1472 HMAC-SHA512. First, the abstract functions:
1474 - Function: void hmac_set_key (void *OUTER, void *INNER, void *STATE,
1475 const struct nettle_hash *H, unsigned LENGTH, const uint8_t
1477 Initializes the three context structs from the key. The OUTER and
1478 INNER contexts corresponds to the subkeys `k_o' and `k_i'. STATE
1479 is used for hashing the message, and is initialized as a copy of
1482 - Function: void hmac_update (void *STATE, const struct nettle_hash
1483 *H, unsigned LENGTH, const uint8_t *DATA)
1484 This function is called zero or more times to process the message.
1485 Actually, `hmac_update(state, H, length, data)' is equivalent to
1486 `H->update(state, length, data)', so if you wish you can use the
1487 ordinary update function of the underlying hash function instead.
1489 - Function: void hmac_digest (const void *OUTER, const void *INNER,
1490 void *STATE, const struct nettle_hash *H, unsigned LENGTH,
1492 Extracts the MAC of the message, writing it to DIGEST. OUTER and
1493 INNER are not modified. LENGTH is usually equal to
1494 `H->digest_size', but if you provide a smaller value, only the
1495 first LENGTH octets of the MAC are written.
1497 This function also resets the STATE context so that you can start
1498 over processing a new message (with the same key).
1500 Like for CBC, there are some macros to help use these functions
1503 - Macro: HMAC_CTX (TYPE)
1511 It can be used to define a HMAC context struct, either directly,
1513 struct HMAC_CTX(struct md5_ctx) ctx;
1515 or to give it a struct tag,
1517 struct hmac_md5_ctx HMAC_CTX (struct md5_ctx);
1519 - Macro: HMAC_SET_KEY (CTX, H, LENGTH, KEY)
1520 CTX is a pointer to a context struct as defined by `HMAC_CTX', H
1521 is a pointer to a `const struct nettle_hash' describing the
1522 underlying hash function (so it must match the type of the
1523 components of CTX). The last two arguments specify the secret key.
1525 - Macro: HMAC_DIGEST (CTX, H, LENGTH, DIGEST)
1526 CTX is a pointer to a context struct as defined by `HMAC_CTX', H
1527 is a pointer to a `const struct nettle_hash' describing the
1528 underlying hash function. The last two arguments specify where the
1531 Note that there is no `HMAC_UPDATE' macro; simply call `hmac_update'
1532 function directly, or the update function of the underlying hash
1535 Concrete HMAC functions
1536 -----------------------
1538 Now we come to the specialized HMAC functions, which are easier to use
1539 than the general HMAC functions.
1544 - Context struct: struct hmac_md5_ctx
1546 - Function: void hmac_md5_set_key (struct hmac_md5_ctx *CTX, unsigned
1547 KEY_LENGTH, const uint8_t *KEY)
1548 Initializes the context with the key.
1550 - Function: void hmac_md5_update (struct hmac_md5_ctx *CTX, unsigned
1551 LENGTH, const uint8_t *DATA)
1552 Process some more data.
1554 - Function: void hmac_md5_digest (struct hmac_md5_ctx *CTX, unsigned
1555 LENGTH, uint8_t *DIGEST)
1556 Extracts the MAC, writing it to DIGEST. LENGTH may be smaller than
1557 `MD5_DIGEST_SIZE', in which case only the first LENGTH octets of
1558 the MAC are written.
1560 This function also resets the context for processing new messages,
1566 - Context struct: struct hmac_sha1_ctx
1568 - Function: void hmac_sha1_set_key (struct hmac_sha1_ctx *CTX,
1569 unsigned KEY_LENGTH, const uint8_t *KEY)
1570 Initializes the context with the key.
1572 - Function: void hmac_sha1_update (struct hmac_sha1_ctx *CTX, unsigned
1573 LENGTH, const uint8_t *DATA)
1574 Process some more data.
1576 - Function: void hmac_sha1_digest (struct hmac_sha1_ctx *CTX, unsigned
1577 LENGTH, uint8_t *DIGEST)
1578 Extracts the MAC, writing it to DIGEST. LENGTH may be smaller than
1579 `SHA1_DIGEST_SIZE', in which case only the first LENGTH octets of
1580 the MAC are written.
1582 This function also resets the context for processing new messages,
1588 - Context struct: struct hmac_sha256_ctx
1590 - Function: void hmac_sha256_set_key (struct hmac_sha256_ctx *CTX,
1591 unsigned KEY_LENGTH, const uint8_t *KEY)
1592 Initializes the context with the key.
1594 - Function: void hmac_sha256_update (struct hmac_sha256_ctx *CTX,
1595 unsigned LENGTH, const uint8_t *DATA)
1596 Process some more data.
1598 - Function: void hmac_sha256_digest (struct hmac_sha256_ctx *CTX,
1599 unsigned LENGTH, uint8_t *DIGEST)
1600 Extracts the MAC, writing it to DIGEST. LENGTH may be smaller than
1601 `SHA256_DIGEST_SIZE', in which case only the first LENGTH octets
1602 of the MAC are written.
1604 This function also resets the context for processing new messages,
1610 - Context struct: struct hmac_sha512_ctx
1612 - Function: void hmac_sha512_set_key (struct hmac_sha512_ctx *CTX,
1613 unsigned KEY_LENGTH, const uint8_t *KEY)
1614 Initializes the context with the key.
1616 - Function: void hmac_sha512_update (struct hmac_sha512_ctx *CTX,
1617 unsigned LENGTH, const uint8_t *DATA)
1618 Process some more data.
1620 - Function: void hmac_sha512_digest (struct hmac_sha512_ctx *CTX,
1621 unsigned LENGTH, uint8_t *DIGEST)
1622 Extracts the MAC, writing it to DIGEST. LENGTH may be smaller than
1623 `SHA512_DIGEST_SIZE', in which case only the first LENGTH octets
1624 of the MAC are written.
1626 This function also resets the context for processing new messages,
1630 File: nettle.info, Node: Public-key algorithms, Next: Randomness, Prev: Keyed hash functions, Up: Reference
1632 Public-key algorithms
1633 =====================
1635 Nettle uses GMP, the GNU bignum library, for all calculations with
1636 large numbers. In order to use the public-key features of Nettle, you
1637 must install GMP, at least version 3.0, before compiling Nettle, and
1638 you need to link your programs with `-lhogweed -lnettle -lgmp'.
1640 The concept of "Public-key" encryption and digital signatures was
1641 discovered by Whitfield Diffie and Martin E. Hellman and described in a
1642 paper 1976. In traditional, "symmetric", cryptography, sender and
1643 receiver share the same keys, and these keys must be distributed in a
1644 secure way. And if there are many users or entities that need to
1645 communicate, each _pair_ needs a shared secret key known by nobody else.
1647 Public-key cryptography uses trapdoor one-way functions. A "one-way
1648 function" is a function `F' such that it is easy to compute the value
1649 `F(x)' for any `x', but given a value `y', it is hard to compute a
1650 corresponding `x' such that `y = F(x)'. Two examples are cryptographic
1651 hash functions, and exponentiation in certain groups.
1653 A "trapdoor one-way function" is a function `F' that is one-way,
1654 unless one knows some secret information about `F'. If one knows the
1655 secret, it is easy to compute both `F' and it's inverse. If this
1656 sounds strange, look at the RSA example below.
1658 Two important uses for one-way functions with trapdoors are
1659 public-key encryption, and digital signatures. The public-key
1660 encryption functions in Nettle are not yet documented; the rest of this
1661 chapter is about digital signatures.
1663 To use a digital signature algorithm, one must first create a
1664 "key-pair": A public key and a corresponding private key. The private
1665 key is used to sign messages, while the public key is used for verifying
1666 that that signatures and messages match. Some care must be taken when
1667 distributing the public key; it need not be kept secret, but if a bad
1668 guy is able to replace it (in transit, or in some user's list of known
1669 public keys), bad things may happen.
1671 There are two operations one can do with the keys. The signature
1672 operation takes a message and a private key, and creates a signature for
1673 the message. A signature is some string of bits, usually at most a few
1674 thousand bits or a few hundred octets. Unlike paper-and-ink signatures,
1675 the digital signature depends on the message, so one can't cut it out of
1676 context and glue it to a different message.
1678 The verification operation takes a public key, a message, and a
1679 string that is claimed to be a signature on the message, and returns
1680 true or false. If it returns true, that means that the three input
1681 values matched, and the verifier can be sure that someone went through
1682 with the signature operation on that very message, and that the
1683 "someone" also knows the private key corresponding to the public key.
1685 The desired properties of a digital signature algorithm are as
1686 follows: Given the public key and pairs of messages and valid
1687 signatures on them, it should be hard to compute the private key, and
1688 it should also be hard to create a new message and signature that is
1689 accepted by the verification operation.
1691 Besides signing meaningful messages, digital signatures can be used
1692 for authorization. A server can be configured with a public key, such
1693 that any client that connects to the service is given a random nonce
1694 message. If the server gets a reply with a correct signature matching
1695 the nonce message and the configured public key, the client is granted
1696 access. So the configuration of the server can be understood as "grant
1697 access to whoever knows the private key corresponding to this
1698 particular public key, and to no others".
1702 * RSA:: The RSA public key algorithm.
1703 * DSA:: The DSA digital signature algorithm.
1706 File: nettle.info, Node: RSA, Next: DSA, Prev: Public-key algorithms, Up: Public-key algorithms
1711 The RSA algorithm was the first practical digital signature algorithm
1712 that was constructed. It was described 1978 in a paper by Ronald
1713 Rivest, Adi Shamir and L.M. Adleman, and the technique was also
1714 patented in the USA in 1983. The patent expired on September 20, 2000,
1715 and since that day, RSA can be used freely, even in the USA.
1717 It's remarkably simple to describe the trapdoor function behind RSA.
1718 The "one-way"-function used is
1722 I.e. raise x to the `e':th power, while discarding all multiples of
1723 `n'. The pair of numbers `n' and `e' is the public key. `e' can be
1724 quite small, even `e = 3' has been used, although slightly larger
1725 numbers are recommended. `n' should be about 1000 bits or larger.
1727 If `n' is large enough, and properly chosen, the inverse of F, the
1728 computation of `e':th roots modulo `n', is very difficult. But,
1729 where's the trapdoor?
1731 Let's first look at how RSA key-pairs are generated. First `n' is
1732 chosen as the product of two large prime numbers `p' and `q' of roughly
1733 the same size (so if `n' is 1000 bits, `p' and `q' are about 500 bits
1734 each). One also computes the number `phi = (p-1)(q-1)', in mathematical
1735 speak, `phi' is the order of the multiplicative group of integers
1738 Next, `e' is chosen. It must have no factors in common with `phi' (in
1739 particular, it must be odd), but can otherwise be chosen more or less
1740 randomly. `e = 65537' is a popular choice, because it makes raising to
1741 the `e''th power particularly efficient, and being prime, it usually
1742 has no factors common with `phi'.
1744 Finally, a number `d', `d < n' is computed such that `e d mod phi =
1745 1'. It can be shown that such a number exists (this is why `e' and
1746 `phi' must have no common factors), and that for all x,
1748 (x^e)^d mod n = x^(ed) mod n = (x^d)^e mod n = x
1750 Using Euclid's algorithm, `d' can be computed quite easily from
1751 `phi' and `e'. But it is still hard to get `d' without knowing `phi',
1752 which depends on the factorization of `n'.
1754 So `d' is the trapdoor, if we know `d' and `y = F(x)', we can
1755 recover x as `y^d mod n'. `d' is also the private half of the RSA
1758 The most common signature operation for RSA is defined in `PKCS#1',
1759 a specification by RSA Laboratories. The message to be signed is first
1760 hashed using a cryptographic hash function, e.g. MD5 or SHA1. Next,
1761 some padding, the ASN.1 "Algorithm Identifier" for the hash function,
1762 and the message digest itself, are concatenated and converted to a
1763 number `x'. The signature is computed from `x' and the private key as
1764 `s = x^d mod n'(1) (*note RSA-Footnote-1::). The signature, `s' is a
1765 number of about the same size of `n', and it usually encoded as a
1766 sequence of octets, most significant octet first.
1768 The verification operation is straight-forward, `x' is computed from
1769 the message in the same way as above. Then `s^e mod n' is computed, the
1770 operation returns true if and only if the result equals `x'.
1772 Nettle's RSA support
1773 --------------------
1775 Nettle represents RSA keys using two structures that contain large
1776 numbers (of type `mpz_t').
1778 - Context struct: rsa_public_key size n e
1779 `size' is the size, in octets, of the modulo, and is used
1780 internally. `n' and `e' is the public key.
1782 - Context struct: rsa_private_key size d p q a b c
1783 `size' is the size, in octets, of the modulo, and is used
1784 internally. `d' is the secret exponent, but it is not actually
1785 used when signing. Instead, the factors `p' and `q', and the
1786 parameters `a', `b' and `c' are used. They are computed from `p',
1787 `q' and `e' such that `a e mod (p - 1) = 1, b e mod (q - 1) = 1, c
1790 Before use, these structs must be initialized by calling one of
1792 - Function: void rsa_public_key_init (struct rsa_public_key *PUB)
1793 - Function: void rsa_private_key_init (struct rsa_private_key *KEY)
1794 Calls `mpz_init' on all numbers in the key struct.
1796 and when finished with them, the space for the numbers must be
1797 deallocated by calling one of
1799 - Function: void rsa_public_key_clear (struct rsa_public_key *PUB)
1800 - Function: void rsa_private_key_clear (struct rsa_private_key *KEY)
1801 Calls `mpz_clear' on all numbers in the key struct.
1803 In general, Nettle's RSA functions deviates from Nettle's "no memory
1804 allocation"-policy. Space for all the numbers, both in the key structs
1805 above, and temporaries, are allocated dynamically. For information on
1806 how to customize allocation, see *Note GMP Allocation: (gmp)Custom
1809 When you have assigned values to the attributes of a key, you must
1812 - Function: int rsa_public_key_prepare (struct rsa_public_key *PUB)
1813 - Function: int rsa_private_key_prepare (struct rsa_private_key *KEY)
1814 Computes the octet size of the key (stored in the `size' attribute,
1815 and may also do other basic sanity checks. Returns one if
1816 successful, or zero if the key can't be used, for instance if the
1817 modulo is smaller than the minimum size needed for RSA operations
1818 specified by PKCS#1.
1820 Before signing or verifying a message, you first hash it with the
1821 appropriate hash function. You pass the hash function's context struct
1822 to the RSA signature function, and it will extract the message digest
1823 and do the rest of the work. There are also alternative functions that
1824 take the hash digest as argument.
1826 There is currently no support for using SHA224 or SHA384 with RSA
1827 signatures, since there's no gain in either computation time nor
1828 message size compared to using SHA256 and SHA512, respectively.
1830 Creation and verification of signatures is done with the following
1833 - Function: int rsa_md5_sign (const struct rsa_private_key *KEY,
1834 struct md5_ctx *HASH, mpz_t SIGNATURE)
1835 - Function: int rsa_sha1_sign (const struct rsa_private_key *KEY,
1836 struct sha1_ctx *HASH, mpz_t SIGNATURE)
1837 - Function: int rsa_sha256_sign (const struct rsa_private_key *KEY,
1838 struct sha256_ctx *HASH, mpz_t SIGNATURE)
1839 - Function: int rsa_sha512_sign (const struct rsa_private_key *KEY,
1840 struct sha512_ctx *HASH, mpz_t SIGNATURE)
1841 The signature is stored in SIGNATURE (which must have been
1842 `mpz_init''ed earlier). The hash context is reset so that it can be
1843 used for new messages. Returns one on success, or zero on failure.
1844 Signing fails if the key is too small for the given hash size,
1845 e.g., it's not possible to create a signature using SHA512 and a
1848 - Function: int rsa_md5_sign_digest (const struct rsa_private_key
1849 *KEY, const uint8_t *DIGEST, mpz_t SIGNATURE)
1850 - Function: int rsa_sha1_sign_digest (const struct rsa_private_key
1851 *KEY, const uint8_t *DIGEST, mpz_t SIGNATURE);
1852 - Function: int rsa_sha256_sign_digest (const struct rsa_private_key
1853 *KEY, const uint8_t *DIGEST, mpz_t SIGNATURE);
1854 - Function: int rsa_sha512_sign_digest (const struct rsa_private_key
1855 *KEY, const uint8_t *DIGEST, mpz_t SIGNATURE);
1856 Creates a signature from the given hash digest. DIGEST should
1857 point to a digest of size `MD5_DIGEST_SIZE', `SHA1_DIGEST_SIZE',
1858 or `SHA256_DIGEST_SIZE', respectively. The signature is stored in
1859 SIGNATURE (which must have been `mpz_init':ed earlier). Returns
1860 one on success, or zero on failure.
1862 - Function: int rsa_md5_verify (const struct rsa_public_key *KEY,
1863 struct md5_ctx *HASH, const mpz_t SIGNATURE)
1864 - Function: int rsa_sha1_verify (const struct rsa_public_key *KEY,
1865 struct sha1_ctx *HASH, const mpz_t SIGNATURE)
1866 - Function: int rsa_sha256_verify (const struct rsa_public_key *KEY,
1867 struct sha256_ctx *HASH, const mpz_t SIGNATURE)
1868 - Function: int rsa_sha512_verify (const struct rsa_public_key *KEY,
1869 struct sha512_ctx *HASH, const mpz_t SIGNATURE)
1870 Returns 1 if the signature is valid, or 0 if it isn't. In either
1871 case, the hash context is reset so that it can be used for new
1874 - Function: int rsa_md5_verify_digest (const struct rsa_public_key
1875 *KEY, const uint8_t *DIGEST, const mpz_t SIGNATURE)
1876 - Function: int rsa_sha1_verify_digest (const struct rsa_public_key
1877 *KEY, const uint8_t *DIGEST, const mpz_t SIGNATURE)
1878 - Function: int rsa_sha256_verify_digest (const struct rsa_public_key
1879 *KEY, const uint8_t *DIGEST, const mpz_t SIGNATURE)
1880 - Function: int rsa_sha512_verify_digest (const struct rsa_public_key
1881 *KEY, const uint8_t *DIGEST, const mpz_t SIGNATURE)
1882 Returns 1 if the signature is valid, or 0 if it isn't. DIGEST
1883 should point to a digest of size `MD5_DIGEST_SIZE',
1884 `SHA1_DIGEST_SIZE', or `SHA256_DIGEST_SIZE', respectively.
1886 If you need to use the RSA trapdoor, the private key, in a way that
1887 isn't supported by the above functions Nettle also includes a function
1888 that computes `x^d mod n' and nothing more, using the CRT optimization.
1890 - Function: void rsa_compute_root (struct rsa_private_key *KEY, mpz_t
1892 Computes `x = m^d', efficiently.
1894 At last, how do you create new keys?
1896 - Function: int rsa_generate_keypair (struct rsa_public_key *PUB,
1897 struct rsa_private_key *KEY, void *RANDOM_CTX,
1898 nettle_random_func RANDOM, void *PROGRESS_CTX,
1899 nettle_progress_func PROGRESS, unsigned N_SIZE, unsigned
1901 There are lots of parameters. PUB and KEY is where the resulting
1902 key pair is stored. The structs should be initialized, but you
1903 don't need to call `rsa_public_key_prepare' or
1904 `rsa_private_key_prepare' after key generation.
1906 RANDOM_CTX and RANDOM is a randomness generator.
1907 `random(random_ctx, length, dst)' should generate `length' random
1908 octets and store them at `dst'. For advice, see *Note Randomness::.
1910 PROGRESS and PROGRESS_CTX can be used to get callbacks during the
1911 key generation process, in order to uphold an illusion of
1912 progress. PROGRESS can be NULL, in that case there are no
1915 SIZE_N is the desired size of the modulo, in bits. If SIZE_E is
1916 non-zero, it is the desired size of the public exponent and a
1917 random exponent of that size is selected. But if E_SIZE is zero,
1918 it is assumed that the caller has already chosen a value for `e',
1919 and stored it in PUB. Returns one on success, and zero on
1920 failure. The function can fail for example if if N_SIZE is too
1921 small, or if E_SIZE is zero and `pub->e' is an even number.
1924 File: nettle.info, Node: RSA-Footnotes, Up: RSA
1926 (1) Actually, the computation is not done like this, it is done more
1927 efficiently using `p', `q' and the Chinese remainder theorem (CRT). But
1928 the result is the same.
1931 File: nettle.info, Node: DSA, Prev: RSA, Up: Public-key algorithms
1933 Nettle's DSA support
1934 --------------------
1936 The DSA digital signature algorithm is more complex than RSA. It was
1937 specified during the early 1990s, and in 1994 NIST published FIPS 186
1938 which is the authoritative specification. Sometimes DSA is referred to
1939 using the acronym DSS, for Digital Signature Standard. The most recent
1940 revision of the specification, FIPS186-3, was issueed in 2009, and it
1941 adds support for larger hash functions than sha1.
1943 For DSA, the underlying mathematical problem is the computation of
1944 discreet logarithms. The public key consists of a large prime `p', a
1945 small prime `q' which is a factor of `p-1', a number `g' which
1946 generates a subgroup of order `q' modulo `p', and an element `y' in
1949 In the original DSA, the size of `q' is fixed to 160 bits, to match
1950 with the SHA1 hash algorithm. The size of `p' is in principle
1951 unlimited, but the standard specifies only nine specific sizes: `512 +
1952 l*64', where `l' is between 0 and 8. Thus, the maximum size of `p' is
1953 1024 bits, and sizes less than 1024 bits are considered obsolete and not
1956 The subgroup requirement means that if you compute
1960 for all possible integers `t', you will get precisely `q' distinct
1963 The private key is a secret exponent `x', such that
1967 In mathematical speak, `x' is the "discrete logarithm" of `y' mod
1968 `p', with respect to the generator `g'. The size of `x' will also be
1969 about the same size as `q'. The security of the DSA algorithm relies on
1970 the difficulty of the discrete logarithm problem. Current algorithms to
1971 compute discrete logarithms in this setting, and hence crack DSA, are
1972 of two types. The first type works directly in the (multiplicative)
1973 group of integers mod `p'. The best known algorithm of this type is the
1974 Number Field Sieve, and it's complexity is similar to the complexity of
1975 factoring numbers of the same size as `p'. The other type works in the
1976 smaller `q'-sized subgroup generated by `g', which has a more difficult
1977 group structure. One good algorithm is Pollard-rho, which has
1978 complexity `sqrt(q)'.
1980 The important point is that security depends on the size of _both_
1981 `p' and `q', and they should be choosen so that the difficulty of both
1982 discrete logarithm methods are comparable. Today, the security margin
1983 of the original DSA may be uncomfortably small. Using a `p' of 1024
1984 bits implies that cracking using the number field sieve is expected to
1985 take about the same time as factoring a 1024-bit RSA modulo, and using
1986 a `q' of size 160 bits implies that cracking using Pollard-rho will
1987 take roughly `2^80' group operations. With the size of `q' fixed, tied
1988 to the SHA1 digest size, it may be tempting to increase the size of `p'
1989 to, say, 4096 bits. This will provide excellent resistance against
1990 attacks like the number field sieve which works in the large group. But
1991 it will do very little to defend against Pollard-rho attacking the small
1992 subgroup; the attacker is slowed down at most by a single factor of 10
1993 due to the more expensive group operation. And the attacker will surely
1994 choose the latter attack.
1996 The signature generation algorithm is randomized; in order to create
1997 a DSA signature, you need a good source for random numbers (*note
1998 Randomness::). Let us describe the common case of a 160-bit `q'.
2000 To create a signature, one starts with the hash digest of the
2001 message, `h', which is a 160 bit number, and a random number `k,
2002 0<k<q', also 160 bits. Next, one computes
2004 r = (g^k mod p) mod q
2005 s = k^-1 (h + x r) mod q
2007 The signature is the pair `(r, s)', two 160 bit numbers. Note the
2008 two different mod operations when computing `r', and the use of the
2009 secret exponent `x'.
2011 To verify a signature, one first checks that `0 < r,s < q', and then
2012 one computes backwards,
2015 v = (g^(w h) y^(w r) mod p) mod q
2017 The signature is valid if `v = r'. This works out because `w = s^-1
2018 mod q = k (h + x r)^-1 mod q', so that
2020 g^(w h) y^(w r) = g^(w h) (g^x)^(w r) = g^(w (h + x r)) = g^k
2022 When reducing mod `q' this yields `r'. Note that when verifying a
2023 signature, we don't know either `k' or `x': those numbers are secret.
2025 If you can choose between RSA and DSA, which one is best? Both are
2026 believed to be secure. DSA gained popularity in the late 1990s, as a
2027 patent free alternative to RSA. Now that the RSA patents have expired,
2028 there's no compelling reason to want to use DSA. Today, the original
2029 DSA key size does not provide a large security margin, and it should
2030 probably be phased out together with RSA keys of 1024 bits. Using the
2031 revised DSA algorithm with a larger hash function, in particular,
2032 SHA256, a 256-bit `q', and `p' of size 2048 bits or more, should
2033 provide for a more comfortable security margin, but these variants are
2034 not yet in wide use.
2036 DSA signatures are smaller than RSA signatures, which is important
2037 for some specialized applications.
2039 From a practical point of view, DSA's need for a good randomness
2040 source is a serious disadvantage. If you ever use the same `k' (and
2041 `r') for two different message, you leak your private key.
2043 Nettle's DSA support
2044 --------------------
2046 Like for RSA, Nettle represents DSA keys using two structures,
2047 containing values of type `mpz_t'. For information on how to customize
2048 allocation, see *Note GMP Allocation: (gmp)Custom Allocation.
2050 Most of the DSA functions are very similar to the corresponding RSA
2051 functions, but there are a few differences pointed out below. For a
2052 start, there are no functions corresponding to `rsa_public_key_prepare'
2053 and `rsa_private_key_prepare'.
2055 - Context struct: dsa_public_key p q g y
2056 The public parameters described above.
2058 - Context struct: dsa_private_key x
2059 The private key `x'.
2061 Before use, these structs must be initialized by calling one of
2063 - Function: void dsa_public_key_init (struct dsa_public_key *PUB)
2064 - Function: void dsa_private_key_init (struct dsa_private_key *KEY)
2065 Calls `mpz_init' on all numbers in the key struct.
2067 When finished with them, the space for the numbers must be
2068 deallocated by calling one of
2070 - Function: void dsa_public_key_clear (struct dsa_public_key *PUB)
2071 - Function: void dsa_private_key_clear (struct dsa_private_key *KEY)
2072 Calls `mpz_clear' on all numbers in the key struct.
2074 Signatures are represented using the structure below, and need to be
2075 initialized and cleared in the same way as the key structs.
2077 - Context struct: dsa_signature r s
2079 - Function: void dsa_signature_init (struct dsa_signature *SIGNATURE)
2080 - Function: void dsa_signature_clear (struct dsa_signature *SIGNATURE)
2081 You must call `dsa_signature_init' before creating or using a
2082 signature, and call `dsa_signature_clear' when you are finished
2085 For signing, you need to provide both the public and the private key
2086 (unlike RSA, where the private key struct includes all information
2087 needed for signing), and a source for random numbers. Signatures can
2088 use the SHA1 or the SHA256 hash function, although the implementation
2089 of DSA with SHA256 should be considered somewhat experimental due to
2090 lack of official test vectors and interoperability testing.
2092 - Function: int dsa_sha1_sign (const struct dsa_public_key *PUB, const
2093 struct dsa_private_key *KEY, void *RANDOM_CTX,
2094 nettle_random_func RANDOM, struct sha1_ctx *HASH, struct
2095 dsa_signature *SIGNATURE)
2096 - Function: int dsa_sha1_sign_digest (const struct dsa_public_key
2097 *PUB, const struct dsa_private_key *KEY, void *RANDOM_CTX,
2098 nettle_random_func RANDOM, const uint8_t *DIGEST, struct
2099 dsa_signature *SIGNATURE)
2100 - Function: int dsa_sha256_sign (const struct dsa_public_key *PUB,
2101 const struct dsa_private_key *KEY, void *RANDOM_CTX,
2102 nettle_random_func RANDOM, struct sha256_ctx *HASH, struct
2103 dsa_signature *SIGNATURE)
2104 - Function: int dsa_sha256_sign_digest (const struct dsa_public_key
2105 *PUB, const struct dsa_private_key *KEY, void *RANDOM_CTX,
2106 nettle_random_func RANDOM, const uint8_t *DIGEST, struct
2107 dsa_signature *SIGNATURE)
2108 Creates a signature from the given hash context or digest.
2109 RANDOM_CTX and RANDOM is a randomness generator.
2110 `random(random_ctx, length, dst)' should generate `length' random
2111 octets and store them at `dst'. For advice, see *Note
2112 Randomness::. Returns one on success, or zero on failure. Signing
2113 fails if the key size and the hash size don't match.
2115 Verifying signatures is a little easier, since no randomness
2116 generator is needed. The functions are
2118 - Function: int dsa_sha1_verify (const struct dsa_public_key *KEY,
2119 struct sha1_ctx *HASH, const struct dsa_signature *SIGNATURE)
2120 - Function: int dsa_sha1_verify_digest (const struct dsa_public_key
2121 *KEY, const uint8_t *DIGEST, const struct dsa_signature
2123 - Function: int dsa_sha256_verify (const struct dsa_public_key *KEY,
2124 struct sha256_ctx *HASH, const struct dsa_signature
2126 - Function: int dsa_sha256_verify_digest (const struct dsa_public_key
2127 *KEY, const uint8_t *DIGEST, const struct dsa_signature
2129 Verifies a signature. Returns 1 if the signature is valid,
2132 Key generation uses mostly the same parameters as the corresponding
2135 - Function: int dsa_generate_keypair (struct dsa_public_key *PUB,
2136 struct dsa_private_key *KEY, void *RANDOM_CTX,
2137 nettle_random_func RANDOM, void *PROGRESS_CTX,
2138 nettle_progress_func PROGRESS, unsigned P_BITS, unsigned
2140 PUB and KEY is where the resulting key pair is stored. The structs
2141 should be initialized before you call this function.
2143 RANDOM_CTX and RANDOM is a randomness generator.
2144 `random(random_ctx, length, dst)' should generate `length' random
2145 octets and store them at `dst'. For advice, see *Note Randomness::.
2147 PROGRESS and PROGRESS_CTX can be used to get callbacks during the
2148 key generation process, in order to uphold an illusion of
2149 progress. PROGRESS can be NULL, in that case there are no
2152 P_BITS and Q_BITS are the desired sizes of `p' and `q'. To
2153 generate keys that conform to the original DSA standard, you must
2154 use `q_bits = 160' and select P_BITS of the form `p_bits = 512 +
2155 l*64', for `0 <= l <= 8', where the smaller sizes are no longer
2156 recommended, so you should most likely stick to `p_bits = 1024'.
2157 Non-standard sizes are possible, in particular `p_bits' larger
2158 than 1024, although DSA implementations can not in general be
2159 expected to support such keys. Also note that using very large
2160 P_BITS, with Q_BITS fixed at 160, doesn't make much sense, because
2161 the security is also limited by the size of the smaller prime.
2162 Using a larger `q_bits' requires switchign to a larger hash
2163 function. To generate DSA keys for use with SHA256, use `q_bits =
2164 256' and, e.g., `p_bits = 2048'.
2166 Returns one on success, and zero on failure. The function will
2167 fail if Q_BITS is neither 160 nor 256, or if P_BITS is unreasonably
2171 File: nettle.info, Node: Randomness, Next: Miscellaneous functions, Prev: Public-key algorithms, Up: Reference
2176 A crucial ingredient in many cryptographic contexts is randomness: Let
2177 `p' be a random prime, choose a random initialization vector `iv', a
2178 random key `k' and a random exponent `e', etc. In the theories, it is
2179 assumed that you have plenty of randomness around. If this assumption
2180 is not true in practice, systems that are otherwise perfectly secure,
2181 can be broken. Randomness has often turned out to be the weakest link
2184 In non-cryptographic applications, such as games as well as
2185 scientific simulation, a good randomness generator usually means a
2186 generator that has good statistical properties, and is seeded by some
2187 simple function of things like the current time, process id, and host
2190 However, such a generator is inadequate for cryptography, for at
2193 * It's too easy for an attacker to guess the initial seed. Even if
2194 it will take some 2^32 tries before he guesses right, that's far
2195 too easy. For example, if the process id is 16 bits, the
2196 resolution of "current time" is one second, and the attacker knows
2197 what day the generator was seeded, there are only about 2^32
2198 possibilities to try if all possible values for the process id and
2199 time-of-day are tried.
2201 * The generator output reveals too much. By observing only a small
2202 segment of the generator's output, its internal state can be
2203 recovered, and from there, all previous output and all future
2204 output can be computed by the attacker.
2206 A randomness generator that is used for cryptographic purposes must
2207 have better properties. Let's first look at the seeding, as the issues
2208 here are mostly independent of the rest of the generator. The initial
2209 state of the generator (its seed) must be unguessable by the attacker.
2210 So what's unguessable? It depends on what the attacker already knows.
2211 The concept used in information theory to reason about such things is
2212 called "entropy", or "conditional entropy" (not to be confused with the
2213 thermodynamic concept with the same name). A reasonable requirement is
2214 that the seed contains a conditional entropy of at least some 80-100
2215 bits. This property can be explained as follows: Allow the attacker to
2216 ask `n' yes-no-questions, of his own choice, about the seed. If the
2217 attacker, using this question-and-answer session, as well as any other
2218 information he knows about the seeding process, still can't guess the
2219 seed correctly, then the conditional entropy is more than `n' bits.
2221 Let's look at an example. Say information about timing of received
2222 network packets is used in the seeding process. If there is some random
2223 network traffic going on, this will contribute some bits of entropy or
2224 "unguessability" to the seed. However, if the attacker can listen in to
2225 the local network, or if all but a small number of the packets were
2226 transmitted by machines that the attacker can monitor, this additional
2227 information makes the seed easier for the attacker to figure out. Even
2228 if the information is exactly the same, the conditional entropy, or
2229 unguessability, is smaller for an attacker that knows some of it already
2230 before the hypothetical question-and-answer session.
2232 Seeding of good generators is usually based on several sources. The
2233 key point here is that the amount of unguessability that each source
2234 contributes, depends on who the attacker is. Some sources that have been
2237 High resolution timing of i/o activities
2238 Such as completed blocks from spinning hard disks, network
2239 packets, etc. Getting access to such information is quite system
2240 dependent, and not all systems include suitable hardware. If
2241 available, it's one of the better randomness source one can find
2242 in a digital, mostly predictable, computer.
2245 Timing and contents of user interaction events is another popular
2246 source that is available for interactive programs (even if I
2247 suspect that it is sometimes used in order to make the user feel
2248 good, not because the quality of the input is needed or used
2249 properly). Obviously, not available when a machine is unattended.
2250 Also beware of networks: User interaction that happens across a
2251 long serial cable, TELNET session, or even SSH session may be
2252 visible to an attacker, in full or partially.
2255 Any room, or even a microphone input that's left unconnected, is a
2256 source of some random background noise, which can be fed into the
2259 Specialized hardware
2260 Hardware devices with the sole purpose of generating random data
2261 have been designed. They range from radioactive samples with an
2262 attached Geiger counter, to amplification of the inherent noise in
2263 electronic components such as diodes and resistors, to
2264 low-frequency sampling of chaotic systems. Hashing successive
2265 images of a Lava lamp is a spectacular example of the latter type.
2268 Secret information, such as user passwords or keys, or private
2269 files stored on disk, can provide some unguessability. A problem
2270 is that if the information is revealed at a later time, the
2271 unguessability vanishes. Another problem is that this kind of
2272 information tends to be fairly constant, so if you rely on it and
2273 seed your generator regularly, you risk constructing almost
2274 similar seeds or even constructing the same seed more than once.
2276 For all practical sources, it's difficult but important to provide a
2277 reliable lower bound on the amount of unguessability that it provides.
2278 Two important points are to make sure that the attacker can't observe
2279 your sources (so if you like the Lava lamp idea, remember that you have
2280 to get your own lamp, and not put it by a window or anywhere else where
2281 strangers can see it), and that hardware failures are detected. What if
2282 the bulb in the Lava lamp, which you keep locked into a cupboard
2283 following the above advice, breaks after a few months?
2285 So let's assume that we have been able to find an unguessable seed,
2286 which contains at least 80 bits of conditional entropy, relative to all
2287 attackers that we care about (typically, we must at the very least
2288 assume that no attacker has root privileges on our machine).
2290 How do we generate output from this seed, and how much can we get?
2291 Some generators (notably the Linux `/dev/random' generator) tries to
2292 estimate available entropy and restrict the amount of output. The goal
2293 is that if you read 128 bits from `/dev/random', you should get 128
2294 "truly random" bits. This is a property that is useful in some
2295 specialized circumstances, for instance when generating key material for
2296 a one time pad, or when working with unconditional blinding, but in most
2297 cases, it doesn't matter much. For most application, there's no limit on
2298 the amount of useful "random" data that we can generate from a small
2299 seed; what matters is that the seed is unguessable and that the
2300 generator has good cryptographic properties.
2302 At the heart of all generators lies its internal state. Future output
2303 is determined by the internal state alone. Let's call it the generator's
2304 key. The key is initialized from the unguessable seed. Important
2305 properties of a generator are:
2308 An attacker observing the output should not be able to recover the
2311 "Independence of outputs"
2312 Observing some of the output should not help the attacker to guess
2313 previous or future output.
2316 Even if an attacker compromises the generator's key, he should not
2317 be able to guess the generator output _before_ the key compromise.
2319 "Recovery from key compromise"
2320 If an attacker compromises the generator's key, he can compute
2321 _all_ future output. This is inevitable if the generator is seeded
2322 only once, at startup. However, the generator can provide a
2323 reseeding mechanism, to achieve recovery from key compromise. More
2324 precisely: If the attacker compromises the key at a particular
2325 time `t_1', there is another later time `t_2', such that if the
2326 attacker observes all output generated between `t_1' and `t_2', he
2327 still can't guess what output is generated after `t_2'.
2330 Nettle includes one randomness generator that is believed to have all
2331 the above properties, and two simpler ones.
2333 ARCFOUR, like any stream cipher, can be used as a randomness
2334 generator. Its output should be of reasonable quality, if the seed is
2335 hashed properly before it is used with `arcfour_set_key'. There's no
2336 single natural way to reseed it, but if you need reseeding, you should
2337 be using Yarrow instead.
2339 The "lagged Fibonacci" generator in `<nettle/knuth-lfib.h>' is a
2340 fast generator with good statistical properties, but is *not* for
2341 cryptographic use, and therefore not documented here. It is included
2342 mostly because the Nettle test suite needs to generate some test data
2345 The recommended generator to use is Yarrow, described below.
2350 Yarrow is a family of pseudo-randomness generators, designed for
2351 cryptographic use, by John Kelsey, Bruce Schneier and Niels Ferguson.
2352 Yarrow-160 is described in a paper at
2353 <http://www.counterpane.com/yarrow.html>, and it uses SHA1 and
2354 triple-DES, and has a 160-bit internal state. Nettle implements
2355 Yarrow-256, which is similar, but uses SHA256 and AES to get an
2356 internal state of 256 bits.
2358 Yarrow was an almost finished project, the paper mentioned above is
2359 the closest thing to a specification for it, but some smaller details
2360 are left out. There is no official reference implementation or test
2361 cases. This section includes an overview of Yarrow, but for the
2362 details of Yarrow-256, as implemented by Nettle, you have to consult
2363 the source code. Maybe a complete specification can be written later.
2365 Yarrow can use many sources (at least two are needed for proper
2366 reseeding), and two randomness "pools", referred to as the "slow pool"
2367 and the "fast pool". Input from the sources is fed alternatingly into
2368 the two pools. When one of the sources has contributed 100 bits of
2369 entropy to the fast pool, a "fast reseed" happens and the fast pool is
2370 mixed into the internal state. When at least two of the sources have
2371 contributed at least 160 bits each to the slow pool, a "slow reseed"
2372 takes place. The contents of both pools are mixed into the internal
2373 state. These procedures should ensure that the generator will eventually
2374 recover after a key compromise.
2376 The output is generated by using AES to encrypt a counter, using the
2377 generator's current key. After each request for output, another 256
2378 bits are generated which replace the key. This ensures forward secrecy.
2380 Yarrow can also use a "seed file" to save state across restarts.
2381 Yarrow is seeded by either feeding it the contents of the previous seed
2382 file, or feeding it input from its sources until a slow reseed happens.
2384 Nettle defines Yarrow-256 in `<nettle/yarrow.h>'.
2386 - Context struct: struct yarrow256_ctx
2388 - Context struct: struct yarrow_source
2389 Information about a single source.
2391 - Constant: YARROW256_SEED_FILE_SIZE
2392 Recommanded size of the Yarrow-256 seed file.
2394 - Function: void yarrow256_init (struct yarrow256_ctx *CTX, unsigned
2395 NSOURCES, struct yarrow_source *SOURCES)
2396 Initializes the yarrow context, and its NSOURCES sources. It's
2397 possible to call it with NSOURCES=0 and SOURCES=NULL, if you don't
2398 need the update features.
2400 - Function: void yarrow256_seed (struct yarrow256_ctx *CTX, unsigned
2401 LENGTH, uint8_t *SEED_FILE)
2402 Seeds Yarrow-256 from a previous seed file. LENGTH should be at
2403 least `YARROW256_SEED_FILE_SIZE', but it can be larger.
2405 The generator will trust you that the SEED_FILE data really is
2406 unguessable. After calling this function, you _must_ overwrite the
2407 old seed file with newly generated data from `yarrow256_random'.
2408 If it's possible for several processes to read the seed file at
2409 about the same time, access must be coordinated using some locking
2412 - Function: int yarrow256_update (struct yarrow256_ctx *CTX, unsigned
2413 SOURCE, unsigned ENTROPY, unsigned LENGTH, const uint8_t
2415 Updates the generator with data from source SOURCE (an index that
2416 must be smaller than the number of sources). ENTROPY is your
2417 estimated lower bound for the entropy in the data, measured in
2418 bits. Calling update with zero ENTROPY is always safe, no matter
2419 if the data is random or not.
2421 Returns 1 if a reseed happened, in which case an application using
2422 a seed file may want to generate new seed data with
2423 `yarrow256_random' and overwrite the seed file. Otherwise, the
2426 - Function: void yarrow256_random (struct yarrow256_ctx *CTX, unsigned
2427 LENGTH, uint8_t *DST)
2428 Generates LENGTH octets of output. The generator must be seeded
2429 before you call this function.
2431 If you don't need forward secrecy, e.g. if you need non-secret
2432 randomness for initialization vectors or padding, you can gain some
2433 efficiency by buffering, calling this function for reasonably large
2434 blocks of data, say 100-1000 octets at a time.
2436 - Function: int yarrow256_is_seeded (struct yarrow256_ctx *CTX)
2437 Returns 1 if the generator is seeded and ready to generate output,
2440 - Function: unsigned yarrow256_needed_sources (struct yarrow256_ctx
2442 Returns the number of sources that must reach the threshold before
2443 a slow reseed will happen. Useful primarily when the generator is
2446 - Function: void yarrow256_fast_reseed (struct yarrow256_ctx *CTX)
2447 - Function: void yarrow256_slow_reseed (struct yarrow256_ctx *CTX)
2448 Causes a fast or slow reseed to take place immediately, regardless
2449 of the current entropy estimates of the two pools. Use with care.
2451 Nettle includes an entropy estimator for one kind of input source:
2452 User keyboard input.
2454 - Context struct: struct yarrow_key_event_ctx
2455 Information about recent key events.
2457 - Function: void yarrow_key_event_init (struct yarrow_key_event_ctx
2459 Initializes the context.
2461 - Function: unsigned yarrow_key_event_estimate (struct
2462 yarrow_key_event_ctx *CTX, unsigned KEY, unsigned TIME)
2463 KEY is the id of the key (ASCII value, hardware key code, X
2464 keysym, ..., it doesn't matter), and TIME is the timestamp of the
2465 event. The time must be given in units matching the resolution by
2466 which you read the clock. If you read the clock with microsecond
2467 precision, TIME should be provided in units of microseconds. But
2468 if you use `gettimeofday' on a typical Unix system where the clock
2469 ticks 10 or so microseconds at a time, TIME should be given in
2470 units of 10 microseconds.
2472 Returns an entropy estimate, in bits, suitable for calling
2473 `yarrow256_update'. Usually, 0, 1 or 2 bits.
2476 File: nettle.info, Node: Miscellaneous functions, Next: Compatibility functions, Prev: Randomness, Up: Reference
2478 Miscellaneous functions
2479 =======================
2481 - Function: uint8_t * memxor (uint8_t *DST, const uint8_t *SRC, size_t
2483 XORs the source area on top of the destination area. The interface
2484 doesn't follow the Nettle conventions, because it is intended to be
2485 similar to the ANSI-C `memcpy' function.
2487 `memxor' is declared in `<nettle/memxor.h>'.
2490 File: nettle.info, Node: Compatibility functions, Prev: Miscellaneous functions, Up: Reference
2492 Compatibility functions
2493 =======================
2495 For convenience, Nettle includes alternative interfaces to some
2496 algorithms, for compatibility with some other popular crypto toolkits.
2497 These are not fully documented here; refer to the source or to the
2498 documentation for the original implementation.
2500 MD5 is defined in [RFC 1321], which includes a reference
2501 implementation. Nettle defines a compatible interface to MD5 in
2502 `<nettle/md5-compat.h>'. This file defines the typedef `MD5_CTX', and
2503 declares the functions `MD5Init', `MD5Update' and `MD5Final'.
2505 Eric Young's "libdes" (also part of OpenSSL) is a quite popular DES
2506 implementation. Nettle includes a subset if its interface in
2507 `<nettle/des-compat.h>'. This file defines the typedefs
2508 `des_key_schedule' and `des_cblock', two constants `DES_ENCRYPT' and
2509 `DES_DECRYPT', and declares one global variable `des_check_key', and
2510 the functions `des_cbc_cksum' `des_cbc_encrypt', `des_ecb2_encrypt',
2511 `des_ecb3_encrypt', `des_ecb_encrypt', `des_ede2_cbc_encrypt',
2512 `des_ede3_cbc_encrypt', `des_is_weak_key', `des_key_sched',
2513 `des_ncbc_encrypt' `des_set_key', and `des_set_odd_parity'.
2516 File: nettle.info, Node: Nettle soup, Next: Installation, Prev: Reference, Up: Top
2518 Traditional Nettle Soup
2519 ***********************
2521 For the serious nettle hacker, here is a recipe for nettle soup. 4
2524 1 liter fresh nettles (urtica dioica)
2526 2 tablespoons butter
2530 1 liter stock (meat or vegetable)
2538 Gather 1 liter fresh nettles. Use gloves! Small, tender shoots are
2539 preferable but the tops of larger nettles can also be used.
2541 Rinse the nettles very well. Boil them for 10 minutes in lightly
2542 salted water. Strain the nettles and save the water. Hack the nettles.
2543 Melt the butter and mix in the flour. Dilute with stock and the
2544 nettle-water you saved earlier. Add the hacked nettles. If you wish you
2545 can add some milk or cream at this stage. Bring to a boil and let boil
2546 for a few minutes. Season with salt and pepper.
2548 Serve with boiled egg-halves.
2551 File: nettle.info, Node: Installation, Next: Index, Prev: Nettle soup, Up: Top
2556 Nettle uses `autoconf'. To build it, unpack the source and run
2563 to install in the default location, `/usr/local'. The library files are
2564 installed in `/use/local/lib/libnettle.a' `/use/local/lib/libhogweed.a'
2565 and the include files are installed in `/use/local/include/nettle/'.
2567 To get a list of configure options, use `./configure --help'.
2569 By default, only static libraries are built and installed. To also
2570 build and install shared libraries, use the ` --enable-shared' option
2573 Using GNU make is recommended. For other make programs, in particular
2574 BSD make, you may have to use the `--disable-dependency-tracking'
2575 option to `./configure'.
2578 File: nettle.info, Node: Index, Prev: Installation, Up: Top
2580 Function and Concept Index
2581 **************************
2585 * aes_decrypt: Cipher functions.
2586 * aes_encrypt: Cipher functions.
2587 * aes_invert_key: Cipher functions.
2588 * aes_set_decrypt_key: Cipher functions.
2589 * aes_set_encrypt_key: Cipher functions.
2590 * arcfour_crypt: Cipher functions.
2591 * arcfour_set_key: Cipher functions.
2592 * arctwo_decrypt: Cipher functions.
2593 * arctwo_encrypt: Cipher functions.
2594 * arctwo_set_key: Cipher functions.
2595 * arctwo_set_key_ekb: Cipher functions.
2596 * arctwo_set_key_gutmann: Cipher functions.
2597 * Block Cipher: Cipher functions.
2598 * blowfish_decrypt: Cipher functions.
2599 * blowfish_encrypt: Cipher functions.
2600 * blowfish_set_key: Cipher functions.
2601 * camellia_crypt: Cipher functions.
2602 * camellia_invert_key: Cipher functions.
2603 * camellia_set_decrypt_key: Cipher functions.
2604 * camellia_set_encrypt_key: Cipher functions.
2605 * cast128_decrypt: Cipher functions.
2606 * cast128_encrypt: Cipher functions.
2607 * cast128_set_key: Cipher functions.
2608 * CBC Mode: Cipher modes.
2609 * CBC_CTX: Cipher modes.
2610 * CBC_DECRYPT: Cipher modes.
2611 * cbc_decrypt: Cipher modes.
2612 * CBC_ENCRYPT: Cipher modes.
2613 * cbc_encrypt: Cipher modes.
2614 * CBC_SET_IV: Cipher modes.
2615 * Cipher: Cipher functions.
2616 * Cipher Block Chaining: Cipher modes.
2617 * Collision-resistant: Hash functions.
2618 * Conditional entropy: Randomness.
2619 * Counter Mode: Cipher modes.
2620 * CTR Mode: Cipher modes.
2621 * CTR_CRYPT: Cipher modes.
2622 * ctr_crypt: Cipher modes.
2623 * CTR_CTX: Cipher modes.
2624 * CTR_SET_COUNTER: Cipher modes.
2625 * des3_decrypt: Cipher functions.
2626 * des3_encrypt: Cipher functions.
2627 * des3_set_key: Cipher functions.
2628 * des_check_parity: Cipher functions.
2629 * des_decrypt: Cipher functions.
2630 * des_encrypt: Cipher functions.
2631 * des_fix_parity: Cipher functions.
2632 * des_set_key: Cipher functions.
2633 * dsa_generate_keypair: DSA.
2634 * dsa_private_key_clear: DSA.
2635 * dsa_private_key_init: DSA.
2636 * dsa_public_key_clear: DSA.
2637 * dsa_public_key_init: DSA.
2638 * dsa_sha1_sign: DSA.
2639 * dsa_sha1_sign_digest: DSA.
2640 * dsa_sha1_verify: DSA.
2641 * dsa_sha1_verify_digest: DSA.
2642 * dsa_sha256_sign: DSA.
2643 * dsa_sha256_sign_digest: DSA.
2644 * dsa_sha256_verify: DSA.
2645 * dsa_sha256_verify_digest: DSA.
2646 * dsa_signature_clear: DSA.
2647 * dsa_signature_init: DSA.
2648 * Entropy: Randomness.
2649 * Hash function: Hash functions.
2650 * HMAC_CTX: Keyed hash functions.
2651 * HMAC_DIGEST: Keyed hash functions.
2652 * hmac_digest: Keyed hash functions.
2653 * hmac_md5_digest: Keyed hash functions.
2654 * hmac_md5_set_key: Keyed hash functions.
2655 * hmac_md5_update: Keyed hash functions.
2656 * HMAC_SET_KEY: Keyed hash functions.
2657 * hmac_set_key: Keyed hash functions.
2658 * hmac_sha1_digest: Keyed hash functions.
2659 * hmac_sha1_set_key: Keyed hash functions.
2660 * hmac_sha1_update: Keyed hash functions.
2661 * hmac_sha256_digest: Keyed hash functions.
2662 * hmac_sha256_set_key: Keyed hash functions.
2663 * hmac_sha256_update: Keyed hash functions.
2664 * hmac_sha512_digest: Keyed hash functions.
2665 * hmac_sha512_set_key: Keyed hash functions.
2666 * hmac_sha512_update: Keyed hash functions.
2667 * hmac_update: Keyed hash functions.
2668 * Keyed Hash Function: Keyed hash functions.
2669 * MAC: Keyed hash functions.
2670 * md2_digest: Hash functions.
2671 * md2_init: Hash functions.
2672 * md2_update: Hash functions.
2673 * md4_digest: Hash functions.
2674 * md4_init: Hash functions.
2675 * md4_update: Hash functions.
2676 * md5_digest: Hash functions.
2677 * md5_init: Hash functions.
2678 * md5_update: Hash functions.
2679 * memxor: Miscellaneous functions.
2680 * Message Authentication Code: Keyed hash functions.
2681 * One-way: Hash functions.
2682 * One-way function: Public-key algorithms.
2683 * Public Key Cryptography: Public-key algorithms.
2684 * Randomness: Randomness.
2685 * rsa_compute_root: RSA.
2686 * rsa_generate_keypair: RSA.
2687 * rsa_md5_sign: RSA.
2688 * rsa_md5_sign_digest: RSA.
2689 * rsa_md5_verify: RSA.
2690 * rsa_md5_verify_digest: RSA.
2691 * rsa_private_key_clear: RSA.
2692 * rsa_private_key_init: RSA.
2693 * rsa_private_key_prepare: RSA.
2694 * rsa_public_key_clear: RSA.
2695 * rsa_public_key_init: RSA.
2696 * rsa_public_key_prepare: RSA.
2697 * rsa_sha1_sign: RSA.
2698 * rsa_sha1_sign_digest: RSA.
2699 * rsa_sha1_verify: RSA.
2700 * rsa_sha1_verify_digest: RSA.
2701 * rsa_sha256_sign: RSA.
2702 * rsa_sha256_sign_digest: RSA.
2703 * rsa_sha256_verify: RSA.
2704 * rsa_sha256_verify_digest: RSA.
2705 * rsa_sha512_sign: RSA.
2706 * rsa_sha512_sign_digest: RSA.
2707 * rsa_sha512_verify: RSA.
2708 * rsa_sha512_verify_digest: RSA.
2709 * serpent_decrypt: Cipher functions.
2710 * serpent_encrypt: Cipher functions.
2711 * serpent_set_key: Cipher functions.
2712 * sha1_digest: Hash functions.
2713 * sha1_init: Hash functions.
2714 * sha1_update: Hash functions.
2715 * sha224_digest: Hash functions.
2716 * sha224_init: Hash functions.
2717 * sha224_update: Hash functions.
2718 * sha256_digest: Hash functions.
2719 * sha256_init: Hash functions.
2720 * sha256_update: Hash functions.
2721 * sha384_digest: Hash functions.
2722 * sha384_init: Hash functions.
2723 * sha384_update: Hash functions.
2724 * sha512_digest: Hash functions.
2725 * sha512_init: Hash functions.
2726 * sha512_update: Hash functions.
2727 * Stream Cipher: Cipher functions.
2728 * twofish_decrypt: Cipher functions.
2729 * twofish_encrypt: Cipher functions.
2730 * twofish_set_key: Cipher functions.
2731 * yarrow256_fast_reseed: Randomness.
2732 * yarrow256_init: Randomness.
2733 * yarrow256_is_seeded: Randomness.
2734 * yarrow256_needed_sources: Randomness.
2735 * yarrow256_random: Randomness.
2736 * yarrow256_seed: Randomness.
2737 * yarrow256_slow_reseed: Randomness.
2738 * yarrow256_update: Randomness.
2739 * yarrow_key_event_estimate: Randomness.
2740 * yarrow_key_event_init: Randomness.
2746 Node: Introduction
\7f1719
2747 Node: Copyright
\7f3281
2748 Node: Conventions
\7f6951
2749 Node: Example
\7f8909
2750 Node: Linking
\7f10201
2751 Node: Reference
\7f11030
2752 Node: Hash functions
\7f11394
2753 Node: Cipher functions
\7f22959
2754 Node: Cipher modes
\7f48587
2755 Node: Keyed hash functions
\7f54929
2756 Node: Public-key algorithms
\7f63350
2758 Node: RSA-Footnotes
\7f77826
2759 Ref: RSA-Footnote-1
\7f77879
2761 Node: Randomness
\7f89349
2762 Node: Miscellaneous functions
\7f104428
2763 Node: Compatibility functions
\7f104923
2764 Node: Nettle soup
\7f106160
2765 Node: Installation
\7f107149
2766 Node: Index
\7f107992
projects / external / nettle.git / blob