/* $OpenBSD: dh_gen.c,v 1.17 2022/01/07 09:27:13 tb Exp $ */

/* 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 <openssl/bn.h>

#include <openssl/dh.h>

#include <openssl/err.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 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);

  if ((t1 = BN_CTX_get(ctx)) == NULL)

    goto err;

  if ((t2 = BN_CTX_get(ctx)) == 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) {

    DHerror(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) {

    DHerror(ERR_R_BN_LIB);

    ok = 0;

  }

  if (ctx != NULL) {

    BN_CTX_end(ctx);

    BN_CTX_free(ctx);

  }

  return ok;

}