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128 lines
3.8 KiB
C
128 lines
3.8 KiB
C
/*
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* Copyright 1995-2020 The OpenSSL Project Authors. All Rights Reserved.
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*
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* Licensed under the OpenSSL license (the "License"). You may not use
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* this file except in compliance with the License. You can obtain a copy
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* in the file LICENSE in the source distribution or at
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* https://www.openssl.org/source/license.html
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*/
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/*
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* NB: These functions have been upgraded - the previous prototypes are in
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* dh_depr.c as wrappers to these ones. - Geoff
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*/
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#include <stdio.h>
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#include "internal/cryptlib.h"
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#include <openssl/bn.h>
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#include "dh_local.h"
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static int dh_builtin_genparams(DH *ret, int prime_len, int generator,
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BN_GENCB *cb);
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int DH_generate_parameters_ex(DH *ret, int prime_len, int generator,
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BN_GENCB *cb)
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{
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if (ret->meth->generate_params)
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return ret->meth->generate_params(ret, prime_len, generator, cb);
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return dh_builtin_genparams(ret, prime_len, generator, cb);
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}
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/*-
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* We generate DH parameters as follows
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* find a prime p which is prime_len bits long,
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* where q=(p-1)/2 is also prime.
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* In the following we assume that g is not 0, 1 or p-1, since it
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* would generate only trivial subgroups.
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* For this case, g is a generator of the order-q subgroup if
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* g^q mod p == 1.
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* Or in terms of the Legendre symbol: (g/p) == 1.
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*
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* Having said all that,
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* there is another special case method for the generators 2, 3 and 5.
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* Using the quadratic reciprocity law it is possible to solve
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* (g/p) == 1 for the special values 2, 3, 5:
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* (2/p) == 1 if p mod 8 == 1 or 7.
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* (3/p) == 1 if p mod 12 == 1 or 11.
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* (5/p) == 1 if p mod 5 == 1 or 4.
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* See for instance: https://en.wikipedia.org/wiki/Legendre_symbol
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*
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* Since all safe primes > 7 must satisfy p mod 12 == 11
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* and all safe primes > 11 must satisfy p mod 5 != 1
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* we can further improve the condition for g = 2, 3 and 5:
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* for 2, p mod 24 == 23
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* for 3, p mod 12 == 11
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* for 5, p mod 60 == 59
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*
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* However for compatibility with previous versions we use:
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* for 2, p mod 24 == 11
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* for 5, p mod 60 == 23
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*/
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static int dh_builtin_genparams(DH *ret, int prime_len, int generator,
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BN_GENCB *cb)
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{
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BIGNUM *t1, *t2;
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int g, ok = -1;
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BN_CTX *ctx = NULL;
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ctx = BN_CTX_new();
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if (ctx == NULL)
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goto err;
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BN_CTX_start(ctx);
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t1 = BN_CTX_get(ctx);
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t2 = BN_CTX_get(ctx);
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if (t2 == NULL)
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goto err;
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/* Make sure 'ret' has the necessary elements */
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if (!ret->p && ((ret->p = BN_new()) == NULL))
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goto err;
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if (!ret->g && ((ret->g = BN_new()) == NULL))
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goto err;
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if (generator <= 1) {
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DHerr(DH_F_DH_BUILTIN_GENPARAMS, DH_R_BAD_GENERATOR);
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goto err;
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}
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if (generator == DH_GENERATOR_2) {
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if (!BN_set_word(t1, 24))
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goto err;
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if (!BN_set_word(t2, 11))
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goto err;
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g = 2;
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} else if (generator == DH_GENERATOR_5) {
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if (!BN_set_word(t1, 60))
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goto err;
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if (!BN_set_word(t2, 23))
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goto err;
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g = 5;
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} else {
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/*
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* in the general case, don't worry if 'generator' is a generator or
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* not: since we are using safe primes, it will generate either an
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* order-q or an order-2q group, which both is OK
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*/
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if (!BN_set_word(t1, 12))
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goto err;
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if (!BN_set_word(t2, 11))
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goto err;
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g = generator;
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}
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if (!BN_generate_prime_ex(ret->p, prime_len, 1, t1, t2, cb))
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goto err;
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if (!BN_GENCB_call(cb, 3, 0))
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goto err;
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if (!BN_set_word(ret->g, g))
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goto err;
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ok = 1;
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err:
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if (ok == -1) {
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DHerr(DH_F_DH_BUILTIN_GENPARAMS, ERR_R_BN_LIB);
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ok = 0;
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}
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BN_CTX_end(ctx);
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BN_CTX_free(ctx);
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return ok;
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}
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