59 lines
1.3 KiB
C
59 lines
1.3 KiB
C
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#include <tommath.h>
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#ifdef BN_MP_REDUCE_2K_L_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@iahu.ca, http://math.libtomcrypt.org
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*/
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/* reduces a modulo n where n is of the form 2**p - d
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This differs from reduce_2k since "d" can be larger
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than a single digit.
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*/
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int mp_reduce_2k_l(mp_int *a, mp_int *n, mp_int *d)
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{
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mp_int q;
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int p, res;
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if ((res = mp_init(&q)) != MP_OKAY) {
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return res;
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}
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p = mp_count_bits(n);
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top:
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/* q = a/2**p, a = a mod 2**p */
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if ((res = mp_div_2d(a, p, &q, a)) != MP_OKAY) {
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goto ERR;
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}
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/* q = q * d */
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if ((res = mp_mul(&q, d, &q)) != MP_OKAY) {
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goto ERR;
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}
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/* a = a + q */
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if ((res = s_mp_add(a, &q, a)) != MP_OKAY) {
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goto ERR;
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}
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if (mp_cmp_mag(a, n) != MP_LT) {
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s_mp_sub(a, n, a);
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goto top;
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}
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ERR:
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mp_clear(&q);
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return res;
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}
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#endif
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