1/*
2 * Copyright 1995-2019 The OpenSSL Project Authors. All Rights Reserved.
3 *
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
8 */
9
10/*
11 * NB: These functions have been upgraded - the previous prototypes are in
12 * dh_depr.c as wrappers to these ones. - Geoff
13 */
14
15#include <stdio.h>
16#include "internal/cryptlib.h"
17#include <openssl/bn.h>
18#include "dh_local.h"
19
20static int dh_builtin_genparams(DH *ret, int prime_len, int generator,
21 BN_GENCB *cb);
22
23int DH_generate_parameters_ex(DH *ret, int prime_len, int generator,
24 BN_GENCB *cb)
25{
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);
29}
30
31/*-
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
38 * g^q mod p == 1.
39 * Or in terms of the Legendre symbol: (g/p) == 1.
40 *
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
49 *
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
56 */
57static int dh_builtin_genparams(DH *ret, int prime_len, int generator,
58 BN_GENCB *cb)
59{
60 BIGNUM *t1, *t2;
61 int g, ok = -1;
62 BN_CTX *ctx = NULL;
63
64 if (prime_len > OPENSSL_DH_MAX_MODULUS_BITS) {
65 DHerr(DH_F_DH_BUILTIN_GENPARAMS, DH_R_MODULUS_TOO_LARGE);
66 return 0;
67 }
68
69 if (prime_len < DH_MIN_MODULUS_BITS) {
70 DHerr(DH_F_DH_BUILTIN_GENPARAMS, DH_R_MODULUS_TOO_SMALL);
71 return 0;
72 }
73
74 ctx = BN_CTX_new();
75 if (ctx == NULL)
76 goto err;
77 BN_CTX_start(ctx);
78 t1 = BN_CTX_get(ctx);
79 t2 = BN_CTX_get(ctx);
80 if (t2 == NULL)
81 goto err;
82
83 /* Make sure 'ret' has the necessary elements */
84 if (!ret->p && ((ret->p = BN_new()) == NULL))
85 goto err;
86 if (!ret->g && ((ret->g = BN_new()) == NULL))
87 goto err;
88
89 if (generator <= 1) {
90 DHerr(DH_F_DH_BUILTIN_GENPARAMS, DH_R_BAD_GENERATOR);
91 goto err;
92 }
93 if (generator == DH_GENERATOR_2) {
94 if (!BN_set_word(t1, 24))
95 goto err;
96 if (!BN_set_word(t2, 23))
97 goto err;
98 g = 2;
99 } else if (generator == DH_GENERATOR_5) {
100 if (!BN_set_word(t1, 60))
101 goto err;
102 if (!BN_set_word(t2, 59))
103 goto err;
104 g = 5;
105 } else {
106 /*
107 * in the general case, don't worry if 'generator' is a generator or
108 * not: since we are using safe primes, it will generate either an
109 * order-q or an order-2q group, which both is OK
110 */
111 if (!BN_set_word(t1, 12))
112 goto err;
113 if (!BN_set_word(t2, 11))
114 goto err;
115 g = generator;
116 }
117
118 if (!BN_generate_prime_ex(ret->p, prime_len, 1, t1, t2, cb))
119 goto err;
120 if (!BN_GENCB_call(cb, 3, 0))
121 goto err;
122 if (!BN_set_word(ret->g, g))
123 goto err;
124 ret->dirty_cnt++;
125 ok = 1;
126 err:
127 if (ok == -1) {
128 DHerr(DH_F_DH_BUILTIN_GENPARAMS, ERR_R_BN_LIB);
129 ok = 0;
130 }
131
132 BN_CTX_end(ctx);
133 BN_CTX_free(ctx);
134 return ok;
135}
136