85 lines
2.1 KiB
C
85 lines
2.1 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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/* b = a*2 */
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int
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mp_mul_2 (mp_int * a, mp_int * b)
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{
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int x, res, oldused;
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/* Optimization: should copy and shift at the same time */
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if (b->alloc < a->used) {
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if ((res = mp_grow (b, a->used)) != MP_OKAY) {
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return res;
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}
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}
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oldused = b->used;
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b->used = a->used;
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/* shift any bit count < DIGIT_BIT */
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{
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register mp_digit r, rr, *tmpa, *tmpb;
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/* alias for source */
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tmpa = a->dp;
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/* alias for dest */
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tmpb = b->dp;
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/* carry */
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r = 0;
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for (x = 0; x < b->used; x++) {
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/* get what will be the *next* carry bit from the MSB of the current digit */
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rr = *tmpa >> (DIGIT_BIT - 1);
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/* now shift up this digit, add in the carry [from the previous] */
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*tmpb++ = ((*tmpa++ << 1) | r) & MP_MASK;
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/* copy the carry that would be from the source digit into the next iteration */
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r = rr;
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}
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/* new leading digit? */
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if (r != 0) {
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/* do we have to grow to accomodate the new digit? */
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if (b->alloc == b->used) {
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if ((res = mp_grow (b, b->used + 1)) != MP_OKAY) {
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return res;
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}
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/* after the grow *tmpb is no longer valid so we have to reset it!
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* (this bug took me about 17 minutes to find...!)
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*/
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tmpb = b->dp + b->used;
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}
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/* add a MSB which is always 1 at this point */
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*tmpb = 1;
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++b->used;
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}
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/* now zero any excess digits on the destination that we didn't write to */
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tmpb = b->dp + b->used;
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for (x = b->used; x < oldused; x++) {
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*tmpb++ = 0;
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}
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}
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b->sign = a->sign;
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return MP_OKAY;
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}
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