1/* $OpenBSD: dh_gen.c,v 1.21 2023/07/08 15:29:03 beck Exp $ */
2/* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
3 * All rights reserved.
4 *
5 * This package is an SSL implementation written
6 * by Eric Young (eay@cryptsoft.com).
7 * The implementation was written so as to conform with Netscapes SSL.
8 *
9 * This library is free for commercial and non-commercial use as long as
10 * the following conditions are aheared to.  The following conditions
11 * apply to all code found in this distribution, be it the RC4, RSA,
12 * lhash, DES, etc., code; not just the SSL code.  The SSL documentation
13 * included with this distribution is covered by the same copyright terms
14 * except that the holder is Tim Hudson (tjh@cryptsoft.com).
15 *
16 * Copyright remains Eric Young's, and as such any Copyright notices in
17 * the code are not to be removed.
18 * If this package is used in a product, Eric Young should be given attribution
19 * as the author of the parts of the library used.
20 * This can be in the form of a textual message at program startup or
21 * in documentation (online or textual) provided with the package.
22 *
23 * Redistribution and use in source and binary forms, with or without
24 * modification, are permitted provided that the following conditions
25 * are met:
26 * 1. Redistributions of source code must retain the copyright
27 *    notice, this list of conditions and the following disclaimer.
28 * 2. Redistributions in binary form must reproduce the above copyright
29 *    notice, this list of conditions and the following disclaimer in the
30 *    documentation and/or other materials provided with the distribution.
31 * 3. All advertising materials mentioning features or use of this software
32 *    must display the following acknowledgement:
33 *    "This product includes cryptographic software written by
34 *     Eric Young (eay@cryptsoft.com)"
35 *    The word 'cryptographic' can be left out if the rouines from the library
36 *    being used are not cryptographic related :-).
37 * 4. If you include any Windows specific code (or a derivative thereof) from
38 *    the apps directory (application code) you must include an acknowledgement:
39 *    "This product includes software written by Tim Hudson (tjh@cryptsoft.com)"
40 *
41 * THIS SOFTWARE IS PROVIDED BY ERIC YOUNG ``AS IS'' AND
42 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
43 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
44 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
45 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
46 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
47 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
48 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
49 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
50 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
51 * SUCH DAMAGE.
52 *
53 * The licence and distribution terms for any publically available version or
54 * derivative of this code cannot be changed.  i.e. this code cannot simply be
55 * copied and put under another distribution licence
56 * [including the GNU Public Licence.]
57 */
58
59#include <stdio.h>
60
61#include <openssl/bn.h>
62#include <openssl/dh.h>
63#include <openssl/err.h>
64
65#include "bn_local.h"
66#include "dh_local.h"
67
68static int dh_builtin_genparams(DH *ret, int prime_len, int generator,
69	    BN_GENCB *cb);
70
71int
72DH_generate_parameters_ex(DH *ret, int prime_len, int generator, BN_GENCB *cb)
73{
74	if (ret->meth->generate_params)
75		return ret->meth->generate_params(ret, prime_len, generator, cb);
76	return dh_builtin_genparams(ret, prime_len, generator, cb);
77}
78LCRYPTO_ALIAS(DH_generate_parameters_ex);
79
80/*
81 * We generate DH parameters as follows:
82 * find a prime q which is prime_len/2 bits long.
83 * p=(2*q)+1 or (p-1)/2 = q
84 * For this case, g is a generator if
85 * g^((p-1)/q) mod p != 1 for values of q which are the factors of p-1.
86 * Since the factors of p-1 are q and 2, we just need to check
87 * g^2 mod p != 1 and g^q mod p != 1.
88 *
89 * Having said all that,
90 * there is another special case method for the generators 2, 3 and 5.
91 * for 2, p mod 24 == 11
92 * for 3, p mod 12 == 5  <<<<< does not work for safe primes.
93 * for 5, p mod 10 == 3 or 7
94 *
95 * Thanks to Phil Karn <karn@qualcomm.com> for the pointers about the
96 * special generators and for answering some of my questions.
97 *
98 * I've implemented the second simple method :-).
99 * Since DH should be using a safe prime (both p and q are prime),
100 * this generator function can take a very very long time to run.
101 */
102/* Actually there is no reason to insist that 'generator' be a generator.
103 * It's just as OK (and in some sense better) to use a generator of the
104 * order-q subgroup.
105 */
106static int
107dh_builtin_genparams(DH *ret, int prime_len, int generator, BN_GENCB *cb)
108{
109	BIGNUM *t1, *t2;
110	int g, ok = -1;
111	BN_CTX *ctx = NULL;
112
113	ctx = BN_CTX_new();
114	if (ctx == NULL)
115		goto err;
116	BN_CTX_start(ctx);
117	if ((t1 = BN_CTX_get(ctx)) == NULL)
118		goto err;
119	if ((t2 = BN_CTX_get(ctx)) == NULL)
120		goto err;
121
122	/* Make sure 'ret' has the necessary elements */
123	if (!ret->p && ((ret->p = BN_new()) == NULL))
124		goto err;
125	if (!ret->g && ((ret->g = BN_new()) == NULL))
126		goto err;
127
128	if (generator <= 1) {
129		DHerror(DH_R_BAD_GENERATOR);
130		goto err;
131	}
132	if (generator == DH_GENERATOR_2) {
133		if (!BN_set_word(t1, 24))
134			goto err;
135		if (!BN_set_word(t2, 11))
136			goto err;
137		g = 2;
138	} else if (generator == DH_GENERATOR_5) {
139		if (!BN_set_word(t1, 10))
140			goto err;
141		if (!BN_set_word(t2, 3))
142			goto err;
143		/* BN_set_word(t3,7); just have to miss
144		 * out on these ones :-( */
145		g = 5;
146	} else {
147		/*
148		 * in the general case, don't worry if 'generator' is a
149		 * generator or not: since we are using safe primes,
150		 * it will generate either an order-q or an order-2q group,
151		 * which both is OK
152		 */
153		if (!BN_set_word(t1, 2))
154			goto err;
155		if (!BN_set_word(t2, 1))
156			goto err;
157		g = generator;
158	}
159
160	if (!BN_generate_prime_ex(ret->p, prime_len, 1, t1, t2, cb))
161		goto err;
162	if (!BN_GENCB_call(cb, 3, 0))
163		goto err;
164	if (!BN_set_word(ret->g, g))
165		goto err;
166	ok = 1;
167err:
168	if (ok == -1) {
169		DHerror(ERR_R_BN_LIB);
170		ok = 0;
171	}
172
173	if (ctx != NULL) {
174		BN_CTX_end(ctx);
175		BN_CTX_free(ctx);
176	}
177	return ok;
178}
179
180DH *
181DH_generate_parameters(int prime_len, int generator,
182    void (*callback)(int, int, void *), void *cb_arg)
183{
184	BN_GENCB cb;
185	DH *ret = NULL;
186
187	if ((ret = DH_new()) == NULL)
188		return NULL;
189
190	BN_GENCB_set_old(&cb, callback, cb_arg);
191
192	if (DH_generate_parameters_ex(ret, prime_len, generator, &cb))
193		return ret;
194	DH_free(ret);
195	return NULL;
196}
197LCRYPTO_ALIAS(DH_generate_parameters);
198