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