81 lines
1.9 KiB
C
81 lines
1.9 KiB
C
/* LibTomMath, multiple-precision integer library -- Tom St Denis
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*
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* LibTomMath is library that provides for multiple-precision
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* integer arithmetic as well as number theoretic functionality.
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*
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* The library is designed directly after the MPI library by
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* Michael Fromberger but has been written from scratch with
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* additional optimizations in place.
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*
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* The library is free for all purposes without any express
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* guarantee it works.
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*
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* Tom St Denis, tomstdenis@iahu.ca, http://libtommath.iahu.ca
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*/
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#include <tommath.h>
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/* computes xR^-1 == x (mod N) via Montgomery Reduction */
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int
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mp_montgomery_reduce (mp_int * a, mp_int * m, mp_digit mp)
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{
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int ix, res, digs;
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mp_digit ui;
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digs = m->used * 2 + 1;
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if ((digs < 512)
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&& digs < (1 << ((CHAR_BIT * sizeof (mp_word)) - (2 * DIGIT_BIT)))) {
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res = fast_mp_montgomery_reduce (a, m, mp);
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return res;
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}
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if (a->alloc < m->used * 2 + 1) {
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if ((res = mp_grow (a, m->used * 2 + 1)) != MP_OKAY) {
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return res;
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}
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}
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a->used = m->used * 2 + 1;
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for (ix = 0; ix < m->used; ix++) {
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/* ui = ai * m' mod b */
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ui = (a->dp[ix] * mp) & MP_MASK;
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/* a = a + ui * m * b^i */
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{
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register int iy;
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register mp_digit *tmpx, *tmpy, mu;
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register mp_word r;
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/* aliases */
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tmpx = m->dp;
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tmpy = a->dp + ix;
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mu = 0;
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for (iy = 0; iy < m->used; iy++) {
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r =
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((mp_word) ui) * ((mp_word) * tmpx++) + ((mp_word) mu) +
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((mp_word) * tmpy);
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mu = (r >> ((mp_word) DIGIT_BIT));
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*tmpy++ = (r & ((mp_word) MP_MASK));
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}
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/* propagate carries */
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while (mu) {
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*tmpy += mu;
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mu = (*tmpy >> DIGIT_BIT) & 1;
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*tmpy++ &= MP_MASK;
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}
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}
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}
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/* A = A/b^n */
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mp_rshd (a, m->used);
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/* if A >= m then A = A - m */
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if (mp_cmp_mag (a, m) != MP_LT) {
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if ((res = s_mp_sub (a, m, a)) != MP_OKAY) {
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return res;
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}
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}
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return MP_OKAY;
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}
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