/*

 * Copyright 1995-2017 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_locl.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 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 (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, 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;

}