Mercurial > dropbear
view libtommath/bn_s_mp_invmod_fast.c @ 1723:5386011b740d
Disable toom and karatsuba for new libtommath
author | Matt Johnston <matt@ucc.asn.au> |
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date | Thu, 18 Jun 2020 19:12:07 +0800 |
parents | 1051e4eea25a |
children |
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#include "tommath_private.h" #ifdef BN_S_MP_INVMOD_FAST_C /* LibTomMath, multiple-precision integer library -- Tom St Denis */ /* SPDX-License-Identifier: Unlicense */ /* computes the modular inverse via binary extended euclidean algorithm, * that is c = 1/a mod b * * Based on slow invmod except this is optimized for the case where b is * odd as per HAC Note 14.64 on pp. 610 */ mp_err s_mp_invmod_fast(const mp_int *a, const mp_int *b, mp_int *c) { mp_int x, y, u, v, B, D; mp_sign neg; mp_err err; /* 2. [modified] b must be odd */ if (MP_IS_EVEN(b)) { return MP_VAL; } /* init all our temps */ if ((err = mp_init_multi(&x, &y, &u, &v, &B, &D, NULL)) != MP_OKAY) { return err; } /* x == modulus, y == value to invert */ if ((err = mp_copy(b, &x)) != MP_OKAY) goto LBL_ERR; /* we need y = |a| */ if ((err = mp_mod(a, b, &y)) != MP_OKAY) goto LBL_ERR; /* if one of x,y is zero return an error! */ if (MP_IS_ZERO(&x) || MP_IS_ZERO(&y)) { err = MP_VAL; goto LBL_ERR; } /* 3. u=x, v=y, A=1, B=0, C=0,D=1 */ if ((err = mp_copy(&x, &u)) != MP_OKAY) goto LBL_ERR; if ((err = mp_copy(&y, &v)) != MP_OKAY) goto LBL_ERR; mp_set(&D, 1uL); top: /* 4. while u is even do */ while (MP_IS_EVEN(&u)) { /* 4.1 u = u/2 */ if ((err = mp_div_2(&u, &u)) != MP_OKAY) goto LBL_ERR; /* 4.2 if B is odd then */ if (MP_IS_ODD(&B)) { if ((err = mp_sub(&B, &x, &B)) != MP_OKAY) goto LBL_ERR; } /* B = B/2 */ if ((err = mp_div_2(&B, &B)) != MP_OKAY) goto LBL_ERR; } /* 5. while v is even do */ while (MP_IS_EVEN(&v)) { /* 5.1 v = v/2 */ if ((err = mp_div_2(&v, &v)) != MP_OKAY) goto LBL_ERR; /* 5.2 if D is odd then */ if (MP_IS_ODD(&D)) { /* D = (D-x)/2 */ if ((err = mp_sub(&D, &x, &D)) != MP_OKAY) goto LBL_ERR; } /* D = D/2 */ if ((err = mp_div_2(&D, &D)) != MP_OKAY) goto LBL_ERR; } /* 6. if u >= v then */ if (mp_cmp(&u, &v) != MP_LT) { /* u = u - v, B = B - D */ if ((err = mp_sub(&u, &v, &u)) != MP_OKAY) goto LBL_ERR; if ((err = mp_sub(&B, &D, &B)) != MP_OKAY) goto LBL_ERR; } else { /* v - v - u, D = D - B */ if ((err = mp_sub(&v, &u, &v)) != MP_OKAY) goto LBL_ERR; if ((err = mp_sub(&D, &B, &D)) != MP_OKAY) goto LBL_ERR; } /* if not zero goto step 4 */ if (!MP_IS_ZERO(&u)) { goto top; } /* now a = C, b = D, gcd == g*v */ /* if v != 1 then there is no inverse */ if (mp_cmp_d(&v, 1uL) != MP_EQ) { err = MP_VAL; goto LBL_ERR; } /* b is now the inverse */ neg = a->sign; while (D.sign == MP_NEG) { if ((err = mp_add(&D, b, &D)) != MP_OKAY) goto LBL_ERR; } /* too big */ while (mp_cmp_mag(&D, b) != MP_LT) { if ((err = mp_sub(&D, b, &D)) != MP_OKAY) goto LBL_ERR; } mp_exch(&D, c); c->sign = neg; err = MP_OKAY; LBL_ERR: mp_clear_multi(&x, &y, &u, &v, &B, &D, NULL); return err; } #endif