Mercurial > dropbear
view libtommath/bn_mp_prime_next_prime.c @ 1712:d885a77b98e0
avoid zero length array in base64_decode
author | Matt Johnston <matt@ucc.asn.au> |
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date | Wed, 10 Jun 2020 23:26:05 +0800 |
parents | 1051e4eea25a |
children |
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#include "tommath_private.h" #ifdef BN_MP_PRIME_NEXT_PRIME_C /* LibTomMath, multiple-precision integer library -- Tom St Denis */ /* SPDX-License-Identifier: Unlicense */ /* finds the next prime after the number "a" using "t" trials * of Miller-Rabin. * * bbs_style = 1 means the prime must be congruent to 3 mod 4 */ mp_err mp_prime_next_prime(mp_int *a, int t, int bbs_style) { int x, y; mp_ord cmp; mp_err err; mp_bool res = MP_NO; mp_digit res_tab[PRIVATE_MP_PRIME_TAB_SIZE], step, kstep; mp_int b; /* force positive */ a->sign = MP_ZPOS; /* simple algo if a is less than the largest prime in the table */ if (mp_cmp_d(a, s_mp_prime_tab[PRIVATE_MP_PRIME_TAB_SIZE-1]) == MP_LT) { /* find which prime it is bigger than "a" */ for (x = 0; x < PRIVATE_MP_PRIME_TAB_SIZE; x++) { cmp = mp_cmp_d(a, s_mp_prime_tab[x]); if (cmp == MP_EQ) { continue; } if (cmp != MP_GT) { if ((bbs_style == 1) && ((s_mp_prime_tab[x] & 3u) != 3u)) { /* try again until we get a prime congruent to 3 mod 4 */ continue; } else { mp_set(a, s_mp_prime_tab[x]); return MP_OKAY; } } } /* fall through to the sieve */ } /* generate a prime congruent to 3 mod 4 or 1/3 mod 4? */ if (bbs_style == 1) { kstep = 4; } else { kstep = 2; } /* at this point we will use a combination of a sieve and Miller-Rabin */ if (bbs_style == 1) { /* if a mod 4 != 3 subtract the correct value to make it so */ if ((a->dp[0] & 3u) != 3u) { if ((err = mp_sub_d(a, (a->dp[0] & 3u) + 1u, a)) != MP_OKAY) { return err; } } } else { if (MP_IS_EVEN(a)) { /* force odd */ if ((err = mp_sub_d(a, 1uL, a)) != MP_OKAY) { return err; } } } /* generate the restable */ for (x = 1; x < PRIVATE_MP_PRIME_TAB_SIZE; x++) { if ((err = mp_mod_d(a, s_mp_prime_tab[x], res_tab + x)) != MP_OKAY) { return err; } } /* init temp used for Miller-Rabin Testing */ if ((err = mp_init(&b)) != MP_OKAY) { return err; } for (;;) { /* skip to the next non-trivially divisible candidate */ step = 0; do { /* y == 1 if any residue was zero [e.g. cannot be prime] */ y = 0; /* increase step to next candidate */ step += kstep; /* compute the new residue without using division */ for (x = 1; x < PRIVATE_MP_PRIME_TAB_SIZE; x++) { /* add the step to each residue */ res_tab[x] += kstep; /* subtract the modulus [instead of using division] */ if (res_tab[x] >= s_mp_prime_tab[x]) { res_tab[x] -= s_mp_prime_tab[x]; } /* set flag if zero */ if (res_tab[x] == 0u) { y = 1; } } } while ((y == 1) && (step < (((mp_digit)1 << MP_DIGIT_BIT) - kstep))); /* add the step */ if ((err = mp_add_d(a, step, a)) != MP_OKAY) { goto LBL_ERR; } /* if didn't pass sieve and step == MP_MAX then skip test */ if ((y == 1) && (step >= (((mp_digit)1 << MP_DIGIT_BIT) - kstep))) { continue; } if ((err = mp_prime_is_prime(a, t, &res)) != MP_OKAY) { goto LBL_ERR; } if (res == MP_YES) { break; } } err = MP_OKAY; LBL_ERR: mp_clear(&b); return err; } #endif