openssl/crypto/dh/dh_gen.c

/* crypto/dh/dh_gen.c */
/* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
 * All rights reserved.
 *
 * This package is an SSL implementation written
 * by Eric Young (eay@cryptsoft.com).
 * The implementation was written so as to conform with Netscapes SSL.
 *
 * This library is free for commercial and non-commercial use as long as
 * the following conditions are aheared to.  The following conditions
 * apply to all code found in this distribution, be it the RC4, RSA,
 * lhash, DES, etc., code; not just the SSL code.  The SSL documentation
 * included with this distribution is covered by the same copyright terms
 * except that the holder is Tim Hudson (tjh@cryptsoft.com).
 *
 * Copyright remains Eric Young's, and as such any Copyright notices in
 * the code are not to be removed.
 * If this package is used in a product, Eric Young should be given attribution
 * as the author of the parts of the library used.
 * This can be in the form of a textual message at program startup or
 * in documentation (online or textual) provided with the package.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *    "This product includes cryptographic software written by
 *     Eric Young (eay@cryptsoft.com)"
 *    The word 'cryptographic' can be left out if the rouines from the library
 *    being used are not cryptographic related :-).
 * 4. If you include any Windows specific code (or a derivative thereof) from
 *    the apps directory (application code) you must include an acknowledgement:
 *    "This product includes software written by Tim Hudson (tjh@cryptsoft.com)"
 *
 * THIS SOFTWARE IS PROVIDED BY ERIC YOUNG ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 * The licence and distribution terms for any publically available version or
 * derivative of this code cannot be changed.  i.e. this code cannot simply be
 * copied and put under another distribution licence
 * [including the GNU Public Licence.]
 */

/*
 * NB: These functions have been upgraded - the previous prototypes are in
 * dh_depr.c as wrappers to these ones.  - Geoff
 */

#include <stdio.h>
#include "cryptlib.h"
#include <openssl/bn.h>
#include <openssl/dh.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 q which is prime_len/2 bits long.
 * p=(2*q)+1 or (p-1)/2 = q
 * For this case, g is a generator if
 * g^((p-1)/q) mod p != 1 for values of q which are the factors of p-1.
 * Since the factors of p-1 are q and 2, we just need to check
 * g^2 mod p != 1 and g^q mod p != 1.
 *
 * Having said all that,
 * there is another special case method for the generators 2, 3 and 5.
 * for 2, p mod 24 == 11
 * for 3, p mod 12 == 5  <<<<< does not work for safe primes.
 * for 5, p mod 10 == 3 or 7
 *
 * Thanks to Phil Karn <karn@qualcomm.com> for the pointers about the
 * special generators and for answering some of my questions.
 *
 * I've implemented the second simple method :-).
 * Since DH should be using a safe prime (both p and q are prime),
 * this generator function can take a very very long time to run.
 */
/*
 * Actually there is no reason to insist that 'generator' be a generator.
 * It's just as OK (and in some sense better) to use a generator of the
 * order-q subgroup.
 */
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 (t1 == NULL || 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;
    }
#if 0                           /* does not work for safe primes */
    else if (generator == DH_GENERATOR_3) {
        if (!BN_set_word(t1, 12))
            goto err;
        if (!BN_set_word(t2, 5))
            goto err;
        g = 3;
    }
#endif
    else if (generator == DH_GENERATOR_5) {
        if (!BN_set_word(t1, 10))
            goto err;
        if (!BN_set_word(t2, 3))
            goto err;
        /*
         * BN_set_word(t3,7); just have to miss out on these ones :-(
         */
        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, 2))
            goto err;
        if (!BN_set_word(t2, 1))
            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;
    }

    if (ctx != NULL) {
        BN_CTX_end(ctx);
        BN_CTX_free(ctx);
    }
    return ok;
}