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Diffstat (limited to 'libtommath/bn_fast_mp_montgomery_reduce.c')
| -rw-r--r-- | libtommath/bn_fast_mp_montgomery_reduce.c | 172 |
1 files changed, 172 insertions, 0 deletions
diff --git a/libtommath/bn_fast_mp_montgomery_reduce.c b/libtommath/bn_fast_mp_montgomery_reduce.c new file mode 100644 index 0000000..13538c8 --- /dev/null +++ b/libtommath/bn_fast_mp_montgomery_reduce.c @@ -0,0 +1,172 @@ +#include <tommath.h> +#ifdef BN_FAST_MP_MONTGOMERY_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. + * + * The library is free for all purposes without any express + * guarantee it works. + * + * Tom St Denis, tomstdenis@gmail.com, http://math.libtomcrypt.com + */ + +/* 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. +*/ +int fast_mp_montgomery_reduce (mp_int * x, mp_int * n, mp_digit rho) +{ + int ix, res, olduse; + mp_word W[MP_WARRAY]; + + /* get old used count */ + olduse = x->used; + + /* grow a as required */ + if (x->alloc < n->used + 1) { + if ((res = mp_grow (x, n->used + 1)) != MP_OKAY) { + return res; + } + } + + /* first we have to get the digits of the input into + * an array of double precision words W[...] + */ + { + register mp_word *_W; + register 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] */ + for (; ix < n->used * 2 + 1; ix++) { + *_W++ = 0; + } + } + + /* 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) + */ + register mp_digit mu; + mu = (mp_digit) (((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 + */ + { + register int iy; + register mp_digit *tmpn; + register 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) DIGIT_BIT); + } + + /* now we have to propagate the carries and + * shift the words downward [all those least + * significant digits we zeroed]. + */ + { + register mp_digit *tmpx; + register 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) 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++ = (mp_digit)(*_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 + */ + for (; ix < olduse; ix++) { + *tmpx++ = 0; + } + } + + /* 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 + +/* $Source: /cvs/libtom/libtommath/bn_fast_mp_montgomery_reduce.c,v $ */ +/* $Revision: 1.3 $ */ +/* $Date: 2006/03/31 14:18:44 $ */ |