## File: bn_mp_reduce.c

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libtommath 1.1.0-3
 `12345678910111213141516171819202122232425262728293031323334353637383940414243444546474849505152535455565758596061626364656667686970717273747576777879808182838485868788899091929394959697` ``````#include "tommath_private.h" #ifdef BN_MP_REDUCE_C /* LibTomMath, multiple-precision integer library -- Tom St Denis * * LibTomMath is a library that provides multiple-precision * integer arithmetic as well as number theoretic functionality. * * The library was designed directly after the MPI library by * Michael Fromberger but has been written from scratch with * additional optimizations in place. * * SPDX-License-Identifier: Unlicense */ /* reduces x mod m, assumes 0 < x < m**2, mu is * precomputed via mp_reduce_setup. * From HAC pp.604 Algorithm 14.42 */ int mp_reduce(mp_int *x, const mp_int *m, const mp_int *mu) { mp_int q; int res, um = m->used; /* q = x */ if ((res = mp_init_copy(&q, x)) != MP_OKAY) { return res; } /* q1 = x / b**(k-1) */ mp_rshd(&q, um - 1); /* according to HAC this optimization is ok */ if ((mp_digit)um > ((mp_digit)1 << (DIGIT_BIT - 1))) { if ((res = mp_mul(&q, mu, &q)) != MP_OKAY) { goto CLEANUP; } } else { #ifdef BN_S_MP_MUL_HIGH_DIGS_C if ((res = s_mp_mul_high_digs(&q, mu, &q, um)) != MP_OKAY) { goto CLEANUP; } #elif defined(BN_FAST_S_MP_MUL_HIGH_DIGS_C) if ((res = fast_s_mp_mul_high_digs(&q, mu, &q, um)) != MP_OKAY) { goto CLEANUP; } #else { res = MP_VAL; goto CLEANUP; } #endif } /* q3 = q2 / b**(k+1) */ mp_rshd(&q, um + 1); /* x = x mod b**(k+1), quick (no division) */ if ((res = mp_mod_2d(x, DIGIT_BIT * (um + 1), x)) != MP_OKAY) { goto CLEANUP; } /* q = q * m mod b**(k+1), quick (no division) */ if ((res = s_mp_mul_digs(&q, m, &q, um + 1)) != MP_OKAY) { goto CLEANUP; } /* x = x - q */ if ((res = mp_sub(x, &q, x)) != MP_OKAY) { goto CLEANUP; } /* If x < 0, add b**(k+1) to it */ if (mp_cmp_d(x, 0uL) == MP_LT) { mp_set(&q, 1uL); if ((res = mp_lshd(&q, um + 1)) != MP_OKAY) goto CLEANUP; if ((res = mp_add(x, &q, x)) != MP_OKAY) goto CLEANUP; } /* Back off if it's too big */ while (mp_cmp(x, m) != MP_LT) { if ((res = s_mp_sub(x, m, x)) != MP_OKAY) { goto CLEANUP; } } CLEANUP: mp_clear(&q); return res; } #endif /* ref: HEAD -> master, tag: v1.1.0 */ /* git commit: 08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */ /* commit time: 2019-01-28 20:32:32 +0100 */ ``````