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1 | /* |
2 | * RSA key generation. |
3 | */ |
4 | |
5 | #include "ssh.h" |
6 | |
7 | #define RSA_EXPONENT 37 /* we like this prime */ |
8 | |
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9 | #if 0 /* bignum diagnostic function */ |
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10 | static void diagbn(char *prefix, Bignum md) { |
11 | int i, nibbles, morenibbles; |
12 | static const char hex[] = "0123456789ABCDEF"; |
13 | |
14 | printf("%s0x", prefix ? prefix : ""); |
15 | |
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16 | nibbles = (3 + bignum_bitcount(md))/4; if (nibbles<1) nibbles=1; |
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17 | morenibbles = 4*md[0] - nibbles; |
18 | for (i=0; i<morenibbles; i++) putchar('-'); |
19 | for (i=nibbles; i-- ;) |
20 | putchar(hex[(bignum_byte(md, i/2) >> (4*(i%2))) & 0xF]); |
21 | |
22 | if (prefix) putchar('\n'); |
23 | } |
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24 | #endif |
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25 | |
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26 | int rsa_generate(struct RSAKey *key, int bits, progfn_t pfn, void *pfnparam) { |
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27 | Bignum pm1, qm1, phi_n; |
28 | |
29 | /* |
30 | * Set up the phase limits for the progress report. We do this |
31 | * by passing minus the phase number. |
32 | * |
33 | * For prime generation: our initial filter finds things |
34 | * coprime to everything below 2^16. Computing the product of |
35 | * (p-1)/p for all prime p below 2^16 gives about 20.33; so |
36 | * among B-bit integers, one in every 20.33 will get through |
37 | * the initial filter to be a candidate prime. |
38 | * |
39 | * Meanwhile, we are searching for primes in the region of 2^B; |
40 | * since pi(x) ~ x/log(x), when x is in the region of 2^B, the |
41 | * prime density will be d/dx pi(x) ~ 1/log(B), i.e. about |
42 | * 1/0.6931B. So the chance of any given candidate being prime |
43 | * is 20.33/0.6931B, which is roughly 29.34 divided by B. |
44 | * |
45 | * So now we have this probability P, we're looking at an |
46 | * exponential distribution with parameter P: we will manage in |
47 | * one attempt with probability P, in two with probability |
48 | * P(1-P), in three with probability P(1-P)^2, etc. The |
49 | * probability that we have still not managed to find a prime |
50 | * after N attempts is (1-P)^N. |
51 | * |
52 | * We therefore inform the progress indicator of the number B |
53 | * (29.34/B), so that it knows how much to increment by each |
54 | * time. We do this in 16-bit fixed point, so 29.34 becomes |
55 | * 0x1D.57C4. |
56 | */ |
57 | pfn(pfnparam, -1, -0x1D57C4/(bits/2)); |
58 | pfn(pfnparam, -2, -0x1D57C4/(bits-bits/2)); |
59 | pfn(pfnparam, -3, 5); |
60 | |
61 | /* |
62 | * We don't generate e; we just use a standard one always. |
63 | */ |
64 | key->exponent = bignum_from_short(RSA_EXPONENT); |
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65 | |
66 | /* |
67 | * Generate p and q: primes with combined length `bits', not |
68 | * congruent to 1 modulo e. (Strictly speaking, we wanted (p-1) |
69 | * and e to be coprime, and (q-1) and e to be coprime, but in |
70 | * general that's slightly more fiddly to arrange. By choosing |
71 | * a prime e, we can simplify the criterion.) |
72 | */ |
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73 | key->p = primegen(bits/2, RSA_EXPONENT, 1, 1, pfn, pfnparam); |
74 | key->q = primegen(bits - bits/2, RSA_EXPONENT, 1, 2, pfn, pfnparam); |
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75 | |
76 | /* |
77 | * Ensure p > q, by swapping them if not. |
78 | */ |
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79 | if (bignum_cmp(key->p, key->q) < 0) { |
80 | Bignum t = key->p; |
81 | key->p = key->q; |
82 | key->q = t; |
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83 | } |
84 | |
85 | /* |
86 | * Now we have p, q and e. All we need to do now is work out |
87 | * the other helpful quantities: n=pq, d=e^-1 mod (p-1)(q-1), |
88 | * and (q^-1 mod p). |
89 | */ |
90 | pfn(pfnparam, 3, 1); |
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91 | key->modulus = bigmul(key->p, key->q); |
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92 | pfn(pfnparam, 3, 2); |
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93 | pm1 = copybn(key->p); |
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94 | decbn(pm1); |
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95 | qm1 = copybn(key->q); |
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96 | decbn(qm1); |
97 | phi_n = bigmul(pm1, qm1); |
98 | pfn(pfnparam, 3, 3); |
99 | freebn(pm1); |
100 | freebn(qm1); |
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101 | key->private_exponent = modinv(key->exponent, phi_n); |
102 | pfn(pfnparam, 3, 4); |
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103 | key->iqmp = modinv(key->q, key->p); |
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104 | pfn(pfnparam, 3, 5); |
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105 | |
106 | /* |
107 | * Clean up temporary numbers. |
108 | */ |
109 | freebn(phi_n); |
110 | |
111 | return 1; |
112 | } |