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1 | /* -*-c-*- |
2 | * |
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3 | * $Id: dh-check.c,v 1.3 2004/04/08 01:36:15 mdw Exp $ |
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4 | * |
5 | * Checks Diffie-Hellman group parameters |
6 | * |
7 | * (c) 2001 Straylight/Edgeware |
8 | */ |
9 | |
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10 | /*----- Licensing notice --------------------------------------------------* |
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11 | * |
12 | * This file is part of Catacomb. |
13 | * |
14 | * Catacomb is free software; you can redistribute it and/or modify |
15 | * it under the terms of the GNU Library General Public License as |
16 | * published by the Free Software Foundation; either version 2 of the |
17 | * License, or (at your option) any later version. |
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18 | * |
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19 | * Catacomb is distributed in the hope that it will be useful, |
20 | * but WITHOUT ANY WARRANTY; without even the implied warranty of |
21 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
22 | * GNU Library General Public License for more details. |
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23 | * |
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24 | * You should have received a copy of the GNU Library General Public |
25 | * License along with Catacomb; if not, write to the Free |
26 | * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, |
27 | * MA 02111-1307, USA. |
28 | */ |
29 | |
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30 | /*----- Header files ------------------------------------------------------*/ |
31 | |
32 | #include <mLib/dstr.h> |
33 | |
34 | #include "dh.h" |
35 | #include "keycheck.h" |
36 | #include "mp.h" |
37 | #include "mpmont.h" |
38 | #include "mpmul.h" |
39 | |
40 | /*----- Main code ---------------------------------------------------------*/ |
41 | |
42 | /* --- @dh_checkparam@ --- * |
43 | * |
44 | * Arguments: @keycheck *kc@ = keycheck state |
45 | * @const dh_param *dp@ = pointer to the parameter set |
46 | * @mp **v@ = optional vector of factors |
47 | * @size_t n@ = size of vector |
48 | * |
49 | * Returns: Zero if all OK, or return status from function. |
50 | * |
51 | * Use: Checks a set of Diffie-Hellman parameters for consistency and |
52 | * security. |
53 | */ |
54 | |
55 | int dh_checkparam(keycheck *kc, const dh_param *dp, mp **v, size_t n) |
56 | { |
57 | int rc = 0; |
58 | mpmont mm; |
59 | mp *pm1 = MP_NEW; |
60 | mp *q = MP_NEW; |
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61 | mp *x; |
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62 | mpmul mu; |
63 | size_t i; |
64 | |
65 | /* --- Check that the numbers which are supposed to be prime are --- */ |
66 | |
67 | if ((!v && keycheck_prime(kc, KCSEV_WARN, dp->q, "q")) || |
68 | keycheck_prime(kc, KCSEV_ERR, dp->p, "p")) |
69 | goto fail; |
70 | |
71 | /* --- Ensure that %$q$% is a sensible choice of number --- */ |
72 | |
73 | pm1 = mp_sub(pm1, dp->p, MP_ONE); |
74 | mp_div(0, &q, pm1, dp->q); |
75 | if (!mp_eq(q, MP_ZERO) && |
76 | keycheck_report(kc, KCSEV_ERR, "q not a factor of p - 1")) |
77 | goto fail; |
78 | |
79 | /* --- Check that %$g$% is actually right --- * |
80 | * |
81 | * This isn't perfect. If %$q$% is composite and we don't have the factors |
82 | * of %$p - 1$% then the order of %$g$% may be some factor of %$q$% which |
83 | * we can't find. (If we do have the factors, we check them all lower |
84 | * down.) We do strip out powers of two from %$q$% before testing, though. |
85 | */ |
86 | |
87 | if ((mp_eq(dp->g, MP_ONE) || mp_eq(dp->g, pm1)) && |
88 | keycheck_report(kc, KCSEV_ERR, "g is degenerate (+/-1 mod p)")) |
89 | goto fail; |
90 | q = mp_odd(q, dp->q, &i); |
91 | mpmont_create(&mm, dp->p); |
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92 | x = mpmont_mul(&mm, MP_NEW, dp->g, mm.r2); |
93 | q = mpmont_expr(&mm, q, x, q); |
94 | mp_drop(x); |
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95 | do { |
96 | if (mp_eq(q, mm.r) != !i) { |
97 | if (keycheck_report(kc, KCSEV_ERR, "order of g != q")) { |
98 | mpmont_destroy(&mm); |
99 | goto fail; |
100 | } |
101 | break; |
102 | } |
103 | if (i) { |
104 | q = mp_sqr(q, q); |
105 | q = mpmont_reduce(&mm, q, q); |
106 | } |
107 | } while (i--); |
108 | |
109 | /* --- Check Lim-Lee primes more carefully --- * |
110 | * |
111 | * In this case, we really can be sure whether the order of %$g$% is |
112 | * actually %$q$% as advertised. Also ensure that the individual primes |
113 | * are really prime, and that their product is correct. |
114 | */ |
115 | |
116 | if (!v) |
117 | mpmont_destroy(&mm); |
118 | else { |
119 | dstr d = DSTR_INIT; |
120 | mp *r = MP_NEW; |
121 | |
122 | mpmul_init(&mu); |
123 | for (i = 0; i < n; i++) { |
124 | DRESET(&d); |
125 | dstr_putf(&d, "factor f_%lu of p", (unsigned long)i); |
126 | if ((rc = keycheck_prime(kc, KCSEV_ERR, v[i], d.buf)) != 0) |
127 | break; |
128 | mp_div(&q, &r, dp->q, v[i]); |
129 | if (mp_eq(r, MP_ZERO) && !mp_eq(q, MP_ONE)) { |
130 | q = mpmont_exp(&mm, q, dp->g, q); |
131 | if (mp_eq(q, MP_ONE) && |
132 | (rc = keycheck_report(kc, KCSEV_ERR, |
133 | "order of g is proper divisor of q")) != 0) |
134 | break; |
135 | } |
136 | mpmul_add(&mu, v[i]); |
137 | } |
138 | mp_drop(q); |
139 | mp_drop(r); |
140 | q = mpmul_done(&mu); |
141 | mpmont_destroy(&mm); |
142 | dstr_destroy(&d); |
143 | if (rc) |
144 | goto fail; |
145 | q = mp_lsl(q, q, 1); |
146 | if (!mp_eq(q, pm1) && |
147 | keycheck_report(kc, KCSEV_ERR, "product of f_i != (p - 1)/2")) |
148 | goto fail; |
149 | } |
150 | |
151 | /* --- Finally, check the key sizes --- */ |
152 | |
153 | if ((mp_bits(dp->p) < 1024 && |
154 | keycheck_report(kc, KCSEV_WARN, |
155 | "p too small to resist index calculus attacks")) || |
156 | (mp_bits(dp->q) < 160 && |
157 | keycheck_report(kc, KCSEV_WARN, |
158 | "q too small to resist collision-finding attacks"))) |
159 | goto fail; |
160 | |
161 | /* --- Done --- */ |
162 | |
163 | tidy: |
164 | mp_drop(q); |
165 | mp_drop(pm1); |
166 | return (rc); |
167 | fail: |
168 | rc = -1; |
169 | goto tidy; |
170 | } |
171 | |
172 | /*----- That's all, folks -------------------------------------------------*/ |