2 * Copyright 1995-2019 The OpenSSL Project Authors. All Rights Reserved.
4 * Licensed under the Apache License 2.0 (the "License"). You may not use
5 * this file except in compliance with the License. You can obtain a copy
6 * in the file LICENSE in the source distribution or at
7 * https://www.openssl.org/source/license.html
11 * NB: These functions have been upgraded - the previous prototypes are in
12 * dh_depr.c as wrappers to these ones. - Geoff
16 #include "internal/cryptlib.h"
17 #include <openssl/bn.h>
20 static int dh_builtin_genparams(DH *ret, int prime_len, int generator,
23 int DH_generate_parameters_ex(DH *ret, int prime_len, int generator,
26 if (ret->meth->generate_params)
27 return ret->meth->generate_params(ret, prime_len, generator, cb);
28 return dh_builtin_genparams(ret, prime_len, generator, cb);
32 * We generate DH parameters as follows
33 * find a prime p which is prime_len bits long,
34 * where q=(p-1)/2 is also prime.
35 * In the following we assume that g is not 0, 1 or p-1, since it
36 * would generate only trivial subgroups.
37 * For this case, g is a generator of the order-q subgroup if
39 * Or in terms of the Legendre symbol: (g/p) == 1.
41 * Having said all that,
42 * there is another special case method for the generators 2, 3 and 5.
43 * Using the quadratic reciprocity law it is possible to solve
44 * (g/p) == 1 for the special values 2, 3, 5:
45 * (2/p) == 1 if p mod 8 == 1 or 7.
46 * (3/p) == 1 if p mod 12 == 1 or 11.
47 * (5/p) == 1 if p mod 5 == 1 or 4.
48 * See for instance: https://en.wikipedia.org/wiki/Legendre_symbol
50 * Since all safe primes > 7 must satisfy p mod 12 == 11
51 * and all safe primes > 11 must satisfy p mod 5 != 1
52 * we can further improve the condition for g = 2, 3 and 5:
53 * for 2, p mod 24 == 23
54 * for 3, p mod 12 == 11
55 * for 5, p mod 60 == 59
57 static int dh_builtin_genparams(DH *ret, int prime_len, int generator,
73 /* Make sure 'ret' has the necessary elements */
74 if (!ret->p && ((ret->p = BN_new()) == NULL))
76 if (!ret->g && ((ret->g = BN_new()) == NULL))
80 DHerr(DH_F_DH_BUILTIN_GENPARAMS, DH_R_BAD_GENERATOR);
83 if (generator == DH_GENERATOR_2) {
84 if (!BN_set_word(t1, 24))
86 if (!BN_set_word(t2, 23))
89 } else if (generator == DH_GENERATOR_5) {
90 if (!BN_set_word(t1, 60))
92 if (!BN_set_word(t2, 59))
97 * in the general case, don't worry if 'generator' is a generator or
98 * not: since we are using safe primes, it will generate either an
99 * order-q or an order-2q group, which both is OK
101 if (!BN_set_word(t1, 12))
103 if (!BN_set_word(t2, 11))
108 if (!BN_generate_prime_ex(ret->p, prime_len, 1, t1, t2, cb))
110 if (!BN_GENCB_call(cb, 3, 0))
112 if (!BN_set_word(ret->g, g))
118 DHerr(DH_F_DH_BUILTIN_GENPARAMS, ERR_R_BN_LIB);