89 lines
2.2 KiB
C
89 lines
2.2 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://math.libtomcrypt.org
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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 * x, mp_int * n, mp_digit rho)
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{
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int ix, res, digs;
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mp_digit mu;
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/* can the fast reduction [comba] method be used?
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*
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* Note that unlike in mp_mul you're safely allowed *less*
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* than the available columns [255 per default] since carries
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* are fixed up in the inner loop.
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*/
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digs = n->used * 2 + 1;
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if ((digs < MP_WARRAY) &&
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n->used <
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(1 << ((CHAR_BIT * sizeof (mp_word)) - (2 * DIGIT_BIT)))) {
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return fast_mp_montgomery_reduce (x, n, rho);
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}
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/* grow the input as required */
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if (x->alloc < digs) {
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if ((res = mp_grow (x, digs)) != MP_OKAY) {
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return res;
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}
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}
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x->used = digs;
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for (ix = 0; ix < n->used; ix++) {
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/* mu = ai * m' mod b */
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mu = (x->dp[ix] * rho) & MP_MASK;
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/* a = a + mu * m * b**i */
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{
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register int iy;
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register mp_digit *tmpn, *tmpx, u;
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register mp_word r;
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/* aliases */
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tmpn = n->dp;
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tmpx = x->dp + ix;
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/* set the carry to zero */
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u = 0;
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/* Multiply and add in place */
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for (iy = 0; iy < n->used; iy++) {
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r = ((mp_word) mu) * ((mp_word) * tmpn++) +
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((mp_word) u) + ((mp_word) * tmpx);
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u = (r >> ((mp_word) DIGIT_BIT));
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*tmpx++ = (r & ((mp_word) MP_MASK));
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}
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/* propagate carries */
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while (u) {
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*tmpx += u;
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u = *tmpx >> DIGIT_BIT;
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*tmpx++ &= MP_MASK;
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}
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}
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}
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/* x = x/b**n.used */
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mp_clamp(x);
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mp_rshd (x, n->used);
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/* if A >= m then A = A - m */
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if (mp_cmp_mag (x, n) != MP_LT) {
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return s_mp_sub (x, n, x);
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}
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return MP_OKAY;
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}
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