82 lines
1.8 KiB
C
82 lines
1.8 KiB
C
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#include <tommath.h>
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#ifdef BN_MP_EXPT_D_EX_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis
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*
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* LibTomMath is a library that provides multiple-precision
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* integer arithmetic as well as number theoretic functionality.
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*
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* The library was 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@gmail.com, http://libtom.org
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*/
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/* calculate c = a**b using a square-multiply algorithm */
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int mp_expt_d_ex (mp_int * a, mp_digit b, mp_int * c, int fast)
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{
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int res;
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unsigned int x;
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mp_int g;
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if ((res = mp_init_copy (&g, a)) != MP_OKAY) {
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return res;
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}
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/* set initial result */
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mp_set (c, 1);
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if (fast) {
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while (b > 0) {
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/* if the bit is set multiply */
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if (b & 1) {
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if ((res = mp_mul (c, &g, c)) != MP_OKAY) {
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mp_clear (&g);
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return res;
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}
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}
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/* square */
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if (b > 1 && (res = mp_sqr (&g, &g)) != MP_OKAY) {
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mp_clear (&g);
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return res;
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}
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/* shift to next bit */
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b >>= 1;
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}
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}
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else {
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for (x = 0; x < DIGIT_BIT; x++) {
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/* square */
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if ((res = mp_sqr (c, c)) != MP_OKAY) {
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mp_clear (&g);
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return res;
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}
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/* if the bit is set multiply */
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if ((b & (mp_digit) (((mp_digit)1) << (DIGIT_BIT - 1))) != 0) {
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if ((res = mp_mul (c, &g, c)) != MP_OKAY) {
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mp_clear (&g);
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return res;
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}
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}
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/* shift to next bit */
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b <<= 1;
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}
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} /* if ... else */
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mp_clear (&g);
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return MP_OKAY;
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}
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#endif
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/* $Source$ */
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/* $Revision$ */
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/* $Date$ */
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