text: /* $OpenBSD: dh_check.c,v 1.17 2019/01/20 01:56:59 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.]
   */
  
  #include <stdio.h>
  
  #include <openssl/bn.h>
  #include <openssl/dh.h>
  
  /*
   * Check that p is a safe prime and
   * if g is 2, 3 or 5, check that it is a suitable generator
   * where
   * for 2, p mod 24 == 11
   * for 3, p mod 12 == 5
   * for 5, p mod 10 == 3 or 7
   * should hold.
   */
  
  int
  DH_check(const DH *dh, int *ret)
  {
  	int is_prime, ok = 0;
  	BN_CTX *ctx = NULL;
  	BN_ULONG l;
  	BIGNUM *q = NULL;
  
  	*ret = 0;
  	ctx = BN_CTX_new();
  	if (ctx == NULL)
  		goto err;
  	q = BN_new();
  	if (q == NULL)
  		goto err;
  
  	if (BN_is_word(dh->g, DH_GENERATOR_2)) {
  		l = BN_mod_word(dh->p, 24);
  		if (l == (BN_ULONG)-1)
  			goto err;
  		if (l != 11)
  			*ret |= DH_NOT_SUITABLE_GENERATOR;
  	} else if (BN_is_word(dh->g, DH_GENERATOR_5)) {
  		l = BN_mod_word(dh->p, 10);
  		if (l == (BN_ULONG)-1)
  			goto err;
  		if (l != 3 && l != 7)
  			*ret |= DH_NOT_SUITABLE_GENERATOR;
  	} else
  		*ret |= DH_UNABLE_TO_CHECK_GENERATOR;
  
  	is_prime = BN_is_prime_ex(dh->p, BN_prime_checks, ctx, NULL);
  	if (is_prime < 0)
  		goto err;
  	if (is_prime == 0)
  		*ret |= DH_CHECK_P_NOT_PRIME;
  	else {
  		if (!BN_rshift1(q, dh->p))
  			goto err;
  		is_prime = BN_is_prime_ex(q, BN_prime_checks, ctx, NULL);
  		if (is_prime < 0)
  			goto err;
  		if (is_prime == 0)
  			*ret |= DH_CHECK_P_NOT_SAFE_PRIME;
  	}
  	ok = 1;
  
   err:
  	BN_CTX_free(ctx);
  	BN_free(q);
  	return ok;
  }
  
  int
  DH_check_pub_key(const DH *dh, const BIGNUM *pub_key, int *ret)
  {
  	BIGNUM *q = NULL;
  
  	*ret = 0;
  	q = BN_new();
  	if (q == NULL)
  		return 0;
  	BN_set_word(q, 1);
  	if (BN_cmp(pub_key, q) <= 0)
  		*ret |= DH_CHECK_PUBKEY_TOO_SMALL;
  	BN_copy(q, dh->p);
  	BN_sub_word(q, 1);
  	if (BN_cmp(pub_key, q) >= 0)
  		*ret |= DH_CHECK_PUBKEY_TOO_LARGE;
  
  	BN_free(q);
  	return 1;
  }
  