text: /* 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;
  	}
  