/*

 * 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;

}