/* 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.]
 */

#include <stdio.h>
#include "cryptlib.h"
#include <openssl/bn.h>
#include <openssl/dh.h>

/* 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.
 */
DH *DH_generate_parameters(int prime_len, int generator,
	     void (*callback)(int,int,void *), void *cb_arg)
	{
	BIGNUM *p=NULL,*t1,*t2;
	DH *ret=NULL;
	int g,ok= -1;
	BN_CTX *ctx=NULL;

	ret=DH_new();
	if (ret == NULL) goto err;
	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;
	
	if (generator <= 1)
		{
		DHerr(DH_F_DH_GENERATE_PARAMETERS, 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;
		}
	
	p=BN_generate_prime(NULL,prime_len,1,t1,t2,callback,cb_arg);
	if (p == NULL) goto err;
	if (callback != NULL) callback(3,0,cb_arg);
	ret->p=p;
	ret->g=BN_new();
	if (!BN_set_word(ret->g,g)) goto err;
	ok=1;
err:
	if (ok == -1)
		{
		DHerr(DH_F_DH_GENERATE_PARAMETERS,ERR_R_BN_LIB);
		ok=0;
		}

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