1 /* 2 * Copyright 1995-2020 The OpenSSL Project Authors. All Rights Reserved. 3 * 4 * Licensed under the OpenSSL license (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 20 static int dh_builtin_genparams(DH *ret, int prime_len, int generator, 21 BN_GENCB *cb); 22 23 int 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 * 57 * However for compatibility with previous versions we use: 58 * for 2, p mod 24 == 11 59 * for 5, p mod 60 == 23 60 */ 61 static int dh_builtin_genparams(DH *ret, int prime_len, int generator, 62 BN_GENCB *cb) 63 { 64 BIGNUM *t1, *t2; 65 int g, ok = -1; 66 BN_CTX *ctx = NULL; 67 68 ctx = BN_CTX_new(); 69 if (ctx == NULL) 70 goto err; 71 BN_CTX_start(ctx); 72 t1 = BN_CTX_get(ctx); 73 t2 = BN_CTX_get(ctx); 74 if (t2 == NULL) 75 goto err; 76 77 /* Make sure 'ret' has the necessary elements */ 78 if (!ret->p && ((ret->p = BN_new()) == NULL)) 79 goto err; 80 if (!ret->g && ((ret->g = BN_new()) == NULL)) 81 goto err; 82 83 if (generator <= 1) { 84 DHerr(DH_F_DH_BUILTIN_GENPARAMS, DH_R_BAD_GENERATOR); 85 goto err; 86 } 87 if (generator == DH_GENERATOR_2) { 88 if (!BN_set_word(t1, 24)) 89 goto err; 90 if (!BN_set_word(t2, 11)) 91 goto err; 92 g = 2; 93 } else if (generator == DH_GENERATOR_5) { 94 if (!BN_set_word(t1, 60)) 95 goto err; 96 if (!BN_set_word(t2, 23)) 97 goto err; 98 g = 5; 99 } else { 100 /* 101 * in the general case, don't worry if 'generator' is a generator or 102 * not: since we are using safe primes, it will generate either an 103 * order-q or an order-2q group, which both is OK 104 */ 105 if (!BN_set_word(t1, 12)) 106 goto err; 107 if (!BN_set_word(t2, 11)) 108 goto err; 109 g = generator; 110 } 111 112 if (!BN_generate_prime_ex(ret->p, prime_len, 1, t1, t2, cb)) 113 goto err; 114 if (!BN_GENCB_call(cb, 3, 0)) 115 goto err; 116 if (!BN_set_word(ret->g, g)) 117 goto err; 118 ok = 1; 119 err: 120 if (ok == -1) { 121 DHerr(DH_F_DH_BUILTIN_GENPARAMS, ERR_R_BN_LIB); 122 ok = 0; 123 } 124 125 BN_CTX_end(ctx); 126 BN_CTX_free(ctx); 127 return ok; 128 } 129