xref: /openbsd/lib/libcrypto/dh/dh_gen.c (revision 4bdff4be)
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 
68 static int dh_builtin_genparams(DH *ret, int prime_len, int generator,
69 	    BN_GENCB *cb);
70 
71 int
72 DH_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 }
78 LCRYPTO_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  */
106 static int
107 dh_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;
167 err:
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 
180 DH *
181 DH_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 }
197 LCRYPTO_ALIAS(DH_generate_parameters);
198