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Diffstat (limited to 'libtommath/bn_s_mp_montgomery_reduce_fast.c')
| -rw-r--r-- | libtommath/bn_s_mp_montgomery_reduce_fast.c | 159 |
1 files changed, 159 insertions, 0 deletions
diff --git a/libtommath/bn_s_mp_montgomery_reduce_fast.c b/libtommath/bn_s_mp_montgomery_reduce_fast.c new file mode 100644 index 0000000..3f0c672 --- /dev/null +++ b/libtommath/bn_s_mp_montgomery_reduce_fast.c @@ -0,0 +1,159 @@ +#include "tommath_private.h" +#ifdef BN_S_MP_MONTGOMERY_REDUCE_FAST_C +/* LibTomMath, multiple-precision integer library -- Tom St Denis */ +/* SPDX-License-Identifier: Unlicense */ + +/* computes xR**-1 == x (mod N) via Montgomery Reduction + * + * This is an optimized implementation of montgomery_reduce + * which uses the comba method to quickly calculate the columns of the + * reduction. + * + * Based on Algorithm 14.32 on pp.601 of HAC. +*/ +mp_err s_mp_montgomery_reduce_fast(mp_int *x, const mp_int *n, mp_digit rho) +{ + int ix, olduse; + mp_err err; + mp_word W[MP_WARRAY]; + + if (x->used > MP_WARRAY) { + return MP_VAL; + } + + /* get old used count */ + olduse = x->used; + + /* grow a as required */ + if (x->alloc < (n->used + 1)) { + if ((err = mp_grow(x, n->used + 1)) != MP_OKAY) { + return err; + } + } + + /* first we have to get the digits of the input into + * an array of double precision words W[...] + */ + { + mp_word *_W; + mp_digit *tmpx; + + /* alias for the W[] array */ + _W = W; + + /* alias for the digits of x*/ + tmpx = x->dp; + + /* copy the digits of a into W[0..a->used-1] */ + for (ix = 0; ix < x->used; ix++) { + *_W++ = *tmpx++; + } + + /* zero the high words of W[a->used..m->used*2] */ + if (ix < ((n->used * 2) + 1)) { + MP_ZERO_BUFFER(_W, sizeof(mp_word) * (size_t)(((n->used * 2) + 1) - ix)); + } + } + + /* now we proceed to zero successive digits + * from the least significant upwards + */ + for (ix = 0; ix < n->used; ix++) { + /* mu = ai * m' mod b + * + * We avoid a double precision multiplication (which isn't required) + * by casting the value down to a mp_digit. Note this requires + * that W[ix-1] have the carry cleared (see after the inner loop) + */ + mp_digit mu; + mu = ((W[ix] & MP_MASK) * rho) & MP_MASK; + + /* a = a + mu * m * b**i + * + * This is computed in place and on the fly. The multiplication + * by b**i is handled by offseting which columns the results + * are added to. + * + * Note the comba method normally doesn't handle carries in the + * inner loop In this case we fix the carry from the previous + * column since the Montgomery reduction requires digits of the + * result (so far) [see above] to work. This is + * handled by fixing up one carry after the inner loop. The + * carry fixups are done in order so after these loops the + * first m->used words of W[] have the carries fixed + */ + { + int iy; + mp_digit *tmpn; + mp_word *_W; + + /* alias for the digits of the modulus */ + tmpn = n->dp; + + /* Alias for the columns set by an offset of ix */ + _W = W + ix; + + /* inner loop */ + for (iy = 0; iy < n->used; iy++) { + *_W++ += (mp_word)mu * (mp_word)*tmpn++; + } + } + + /* now fix carry for next digit, W[ix+1] */ + W[ix + 1] += W[ix] >> (mp_word)MP_DIGIT_BIT; + } + + /* now we have to propagate the carries and + * shift the words downward [all those least + * significant digits we zeroed]. + */ + { + mp_digit *tmpx; + mp_word *_W, *_W1; + + /* nox fix rest of carries */ + + /* alias for current word */ + _W1 = W + ix; + + /* alias for next word, where the carry goes */ + _W = W + ++ix; + + for (; ix < ((n->used * 2) + 1); ix++) { + *_W++ += *_W1++ >> (mp_word)MP_DIGIT_BIT; + } + + /* copy out, A = A/b**n + * + * The result is A/b**n but instead of converting from an + * array of mp_word to mp_digit than calling mp_rshd + * we just copy them in the right order + */ + + /* alias for destination word */ + tmpx = x->dp; + + /* alias for shifted double precision result */ + _W = W + n->used; + + for (ix = 0; ix < (n->used + 1); ix++) { + *tmpx++ = *_W++ & (mp_word)MP_MASK; + } + + /* zero oldused digits, if the input a was larger than + * m->used+1 we'll have to clear the digits + */ + MP_ZERO_DIGITS(tmpx, olduse - ix); + } + + /* set the max used and clamp */ + x->used = n->used + 1; + mp_clamp(x); + + /* if A >= m then A = A - m */ + if (mp_cmp_mag(x, n) != MP_LT) { + return s_mp_sub(x, n, x); + } + return MP_OKAY; +} +#endif |