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1 /*
2  * Copyright 1995-2020 The OpenSSL Project Authors. All Rights Reserved.
3  *
4  * Licensed under the OpenSSL license (the "License").  You may not use
5  * this file except in compliance with the License.  You can obtain a copy
6  * in the file LICENSE in the source distribution or at
7  * https://www.openssl.org/source/license.html
8  */
9 
10 /*
11  * NB: These functions have been upgraded - the previous prototypes are in
12  * dh_depr.c as wrappers to these ones.  - Geoff
13  */
14 
15 #include <stdio.h>
16 #include "internal/cryptlib.h"
17 #include <openssl/bn.h>
18 #include "dh_local.h"
19 
20 static int dh_builtin_genparams(DH *ret, int prime_len, int generator,
21                                 BN_GENCB *cb);
22 
DH_generate_parameters_ex(DH * ret,int prime_len,int generator,BN_GENCB * cb)23 int DH_generate_parameters_ex(DH *ret, int prime_len, int generator,
24                               BN_GENCB *cb)
25 {
26     if (ret->meth->generate_params)
27         return ret->meth->generate_params(ret, prime_len, generator, cb);
28     return dh_builtin_genparams(ret, prime_len, generator, cb);
29 }
30 
31 /*-
32  * We generate DH parameters as follows
33  * find a prime p which is prime_len bits long,
34  * where q=(p-1)/2 is also prime.
35  * In the following we assume that g is not 0, 1 or p-1, since it
36  * would generate only trivial subgroups.
37  * For this case, g is a generator of the order-q subgroup if
38  * g^q mod p == 1.
39  * Or in terms of the Legendre symbol: (g/p) == 1.
40  *
41  * Having said all that,
42  * there is another special case method for the generators 2, 3 and 5.
43  * Using the quadratic reciprocity law it is possible to solve
44  * (g/p) == 1 for the special values 2, 3, 5:
45  * (2/p) == 1 if p mod 8 == 1 or 7.
46  * (3/p) == 1 if p mod 12 == 1 or 11.
47  * (5/p) == 1 if p mod 5 == 1 or 4.
48  * See for instance: https://en.wikipedia.org/wiki/Legendre_symbol
49  *
50  * Since all safe primes > 7 must satisfy p mod 12 == 11
51  * and all safe primes > 11 must satisfy p mod 5 != 1
52  * we can further improve the condition for g = 2, 3 and 5:
53  * for 2, p mod 24 == 23
54  * for 3, p mod 12 == 11
55  * for 5, p mod 60 == 59
56  *
57  * However for compatibility with previous versions we use:
58  * for 2, p mod 24 == 11
59  * for 5, p mod 60 == 23
60  */
dh_builtin_genparams(DH * ret,int prime_len,int generator,BN_GENCB * cb)61 static int dh_builtin_genparams(DH *ret, int prime_len, int generator,
62                                 BN_GENCB *cb)
63 {
64     BIGNUM *t1, *t2;
65     int g, ok = -1;
66     BN_CTX *ctx = NULL;
67 
68     ctx = BN_CTX_new();
69     if (ctx == NULL)
70         goto err;
71     BN_CTX_start(ctx);
72     t1 = BN_CTX_get(ctx);
73     t2 = BN_CTX_get(ctx);
74     if (t2 == NULL)
75         goto err;
76 
77     /* Make sure 'ret' has the necessary elements */
78     if (!ret->p && ((ret->p = BN_new()) == NULL))
79         goto err;
80     if (!ret->g && ((ret->g = BN_new()) == NULL))
81         goto err;
82 
83     if (generator <= 1) {
84         DHerr(DH_F_DH_BUILTIN_GENPARAMS, DH_R_BAD_GENERATOR);
85         goto err;
86     }
87     if (generator == DH_GENERATOR_2) {
88         if (!BN_set_word(t1, 24))
89             goto err;
90         if (!BN_set_word(t2, 11))
91             goto err;
92         g = 2;
93     } else if (generator == DH_GENERATOR_5) {
94         if (!BN_set_word(t1, 60))
95             goto err;
96         if (!BN_set_word(t2, 23))
97             goto err;
98         g = 5;
99     } else {
100         /*
101          * in the general case, don't worry if 'generator' is a generator or
102          * not: since we are using safe primes, it will generate either an
103          * order-q or an order-2q group, which both is OK
104          */
105         if (!BN_set_word(t1, 12))
106             goto err;
107         if (!BN_set_word(t2, 11))
108             goto err;
109         g = generator;
110     }
111 
112     if (!BN_generate_prime_ex(ret->p, prime_len, 1, t1, t2, cb))
113         goto err;
114     if (!BN_GENCB_call(cb, 3, 0))
115         goto err;
116     if (!BN_set_word(ret->g, g))
117         goto err;
118     ok = 1;
119  err:
120     if (ok == -1) {
121         DHerr(DH_F_DH_BUILTIN_GENPARAMS, ERR_R_BN_LIB);
122         ok = 0;
123     }
124 
125     BN_CTX_end(ctx);
126     BN_CTX_free(ctx);
127     return ok;
128 }
129