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