44 lines
1.1 KiB
C
44 lines
1.1 KiB
C
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/* TomsFastMath, a fast ISO C bignum library.
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*
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* This project is meant to fill in where LibTomMath
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* falls short. That is speed ;-)
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*
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* This project is public domain and free for all purposes.
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*
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* Tom St Denis, tomstdenis@gmail.com
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*/
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#include "bignum_fast.h"
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/* computes a = B**n mod b without division or multiplication useful for
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* normalizing numbers in a Montgomery system.
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*/
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void fp_montgomery_calc_normalization(fp_int *a, fp_int *b)
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{
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int x, bits;
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/* how many bits of last digit does b use */
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bits = fp_count_bits (b) % DIGIT_BIT;
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if (!bits) bits = DIGIT_BIT;
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/* compute A = B^(n-1) * 2^(bits-1) */
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if (b->used > 1) {
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fp_2expt (a, (b->used - 1) * DIGIT_BIT + bits - 1);
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} else {
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fp_set(a, 1);
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bits = 1;
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}
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/* now compute C = A * B mod b */
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for (x = bits - 1; x < (int)DIGIT_BIT; x++) {
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fp_mul_2 (a, a);
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if (fp_cmp_mag (a, b) != FP_LT) {
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s_fp_sub (a, b, a);
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}
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}
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}
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/* $Source: /cvs/libtom/tomsfastmath/src/mont/fp_montgomery_calc_normalization.c,v $ */
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/* $Revision: 1.1 $ */
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/* $Date: 2006/12/31 21:25:53 $ */
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