Mercurial > dropbear
annotate bn_mp_prime_next_prime.c @ 190:d8254fc979e9 libtommath-orig LTM_0.35
Initial import of libtommath 0.35
| author | Matt Johnston <matt@ucc.asn.au> |
|---|---|
| date | Fri, 06 May 2005 08:59:30 +0000 |
| parents | d29b64170cf0 |
| children |
| rev | line source |
|---|---|
| 142 | 1 #include <tommath.h> |
| 2 #ifdef BN_MP_PRIME_NEXT_PRIME_C | |
| 2 | 3 /* LibTomMath, multiple-precision integer library -- Tom St Denis |
| 4 * | |
| 5 * LibTomMath is a library that provides multiple-precision | |
| 6 * integer arithmetic as well as number theoretic functionality. | |
| 7 * | |
| 8 * The library was designed directly after the MPI library by | |
| 9 * Michael Fromberger but has been written from scratch with | |
| 10 * additional optimizations in place. | |
| 11 * | |
| 12 * The library is free for all purposes without any express | |
| 13 * guarantee it works. | |
| 14 * | |
| 15 * Tom St Denis, [email protected], http://math.libtomcrypt.org | |
| 16 */ | |
| 17 | |
| 18 /* finds the next prime after the number "a" using "t" trials | |
| 19 * of Miller-Rabin. | |
| 20 * | |
| 21 * bbs_style = 1 means the prime must be congruent to 3 mod 4 | |
| 22 */ | |
| 23 int mp_prime_next_prime(mp_int *a, int t, int bbs_style) | |
| 24 { | |
| 25 int err, res, x, y; | |
| 26 mp_digit res_tab[PRIME_SIZE], step, kstep; | |
| 27 mp_int b; | |
| 28 | |
| 29 /* ensure t is valid */ | |
| 30 if (t <= 0 || t > PRIME_SIZE) { | |
| 31 return MP_VAL; | |
| 32 } | |
| 33 | |
| 34 /* force positive */ | |
| 35 a->sign = MP_ZPOS; | |
| 36 | |
| 37 /* simple algo if a is less than the largest prime in the table */ | |
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38 if (mp_cmp_d(a, ltm_prime_tab[PRIME_SIZE-1]) == MP_LT) { |
| 2 | 39 /* find which prime it is bigger than */ |
| 40 for (x = PRIME_SIZE - 2; x >= 0; x--) { | |
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41 if (mp_cmp_d(a, ltm_prime_tab[x]) != MP_LT) { |
| 2 | 42 if (bbs_style == 1) { |
| 43 /* ok we found a prime smaller or | |
| 44 * equal [so the next is larger] | |
| 45 * | |
| 46 * however, the prime must be | |
| 47 * congruent to 3 mod 4 | |
| 48 */ | |
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49 if ((ltm_prime_tab[x + 1] & 3) != 3) { |
| 2 | 50 /* scan upwards for a prime congruent to 3 mod 4 */ |
| 51 for (y = x + 1; y < PRIME_SIZE; y++) { | |
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52 if ((ltm_prime_tab[y] & 3) == 3) { |
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53 mp_set(a, ltm_prime_tab[y]); |
| 2 | 54 return MP_OKAY; |
| 55 } | |
| 56 } | |
| 57 } | |
| 58 } else { | |
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59 mp_set(a, ltm_prime_tab[x + 1]); |
| 2 | 60 return MP_OKAY; |
| 61 } | |
| 62 } | |
| 63 } | |
| 64 /* at this point a maybe 1 */ | |
| 65 if (mp_cmp_d(a, 1) == MP_EQ) { | |
| 66 mp_set(a, 2); | |
| 67 return MP_OKAY; | |
| 68 } | |
| 69 /* fall through to the sieve */ | |
| 70 } | |
| 71 | |
| 72 /* generate a prime congruent to 3 mod 4 or 1/3 mod 4? */ | |
| 73 if (bbs_style == 1) { | |
| 74 kstep = 4; | |
| 75 } else { | |
| 76 kstep = 2; | |
| 77 } | |
| 78 | |
| 79 /* at this point we will use a combination of a sieve and Miller-Rabin */ | |
| 80 | |
| 81 if (bbs_style == 1) { | |
| 82 /* if a mod 4 != 3 subtract the correct value to make it so */ | |
| 83 if ((a->dp[0] & 3) != 3) { | |
| 84 if ((err = mp_sub_d(a, (a->dp[0] & 3) + 1, a)) != MP_OKAY) { return err; }; | |
| 85 } | |
| 86 } else { | |
| 87 if (mp_iseven(a) == 1) { | |
| 88 /* force odd */ | |
| 89 if ((err = mp_sub_d(a, 1, a)) != MP_OKAY) { | |
| 90 return err; | |
| 91 } | |
| 92 } | |
| 93 } | |
| 94 | |
| 95 /* generate the restable */ | |
| 96 for (x = 1; x < PRIME_SIZE; x++) { | |
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97 if ((err = mp_mod_d(a, ltm_prime_tab[x], res_tab + x)) != MP_OKAY) { |
| 2 | 98 return err; |
| 99 } | |
| 100 } | |
| 101 | |
| 102 /* init temp used for Miller-Rabin Testing */ | |
| 103 if ((err = mp_init(&b)) != MP_OKAY) { | |
| 104 return err; | |
| 105 } | |
| 106 | |
| 107 for (;;) { | |
| 108 /* skip to the next non-trivially divisible candidate */ | |
| 109 step = 0; | |
| 110 do { | |
| 111 /* y == 1 if any residue was zero [e.g. cannot be prime] */ | |
| 112 y = 0; | |
| 113 | |
| 114 /* increase step to next candidate */ | |
| 115 step += kstep; | |
| 116 | |
| 117 /* compute the new residue without using division */ | |
| 118 for (x = 1; x < PRIME_SIZE; x++) { | |
| 119 /* add the step to each residue */ | |
| 120 res_tab[x] += kstep; | |
| 121 | |
| 122 /* subtract the modulus [instead of using division] */ | |
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123 if (res_tab[x] >= ltm_prime_tab[x]) { |
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124 res_tab[x] -= ltm_prime_tab[x]; |
| 2 | 125 } |
| 126 | |
| 127 /* set flag if zero */ | |
| 128 if (res_tab[x] == 0) { | |
| 129 y = 1; | |
| 130 } | |
| 131 } | |
| 132 } while (y == 1 && step < ((((mp_digit)1)<<DIGIT_BIT) - kstep)); | |
| 133 | |
| 134 /* add the step */ | |
| 135 if ((err = mp_add_d(a, step, a)) != MP_OKAY) { | |
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136 goto LBL_ERR; |
| 2 | 137 } |
| 138 | |
| 139 /* if didn't pass sieve and step == MAX then skip test */ | |
| 140 if (y == 1 && step >= ((((mp_digit)1)<<DIGIT_BIT) - kstep)) { | |
| 141 continue; | |
| 142 } | |
| 143 | |
| 144 /* is this prime? */ | |
| 145 for (x = 0; x < t; x++) { | |
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146 mp_set(&b, ltm_prime_tab[t]); |
| 2 | 147 if ((err = mp_prime_miller_rabin(a, &b, &res)) != MP_OKAY) { |
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148 goto LBL_ERR; |
| 2 | 149 } |
| 150 if (res == MP_NO) { | |
| 151 break; | |
| 152 } | |
| 153 } | |
| 154 | |
| 155 if (res == MP_YES) { | |
| 156 break; | |
| 157 } | |
| 158 } | |
| 159 | |
| 160 err = MP_OKAY; | |
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161 LBL_ERR: |
| 2 | 162 mp_clear(&b); |
| 163 return err; | |
| 164 } | |
| 165 | |
| 142 | 166 #endif |