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annotate tomsfastmath/src/mont/fp_montgomery_calc_normalization.c @ 643:a362b62d38b2 dropbear-tfm

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Add tomsfastmath from git rev bfa4582842bc3bab42e4be4aed5703437049502a with Makefile.in renamed
author Matt Johnston <matt@ucc.asn.au>
date Wed, 23 Nov 2011 18:10:20 +0700
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a362b62d38b2 Add tomsfastmath from git rev bfa4582842bc3bab42e4be4aed5703437049502a
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1 /* TomsFastMath, a fast ISO C bignum library.
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2 *
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3 * This project is meant to fill in where LibTomMath
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4 * falls short. That is speed ;-)
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5 *
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6 * This project is public domain and free for all purposes.
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7 *
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8 * Tom St Denis, [email protected]
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9 */
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10 #include <tfm.h>
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12 /* computes a = B**n mod b without division or multiplication useful for
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13 * normalizing numbers in a Montgomery system.
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14 */
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15 void fp_montgomery_calc_normalization(fp_int *a, fp_int *b)
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16 {
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17 int x, bits;
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19 /* how many bits of last digit does b use */
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20 bits = fp_count_bits (b) % DIGIT_BIT;
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21 if (!bits) bits = DIGIT_BIT;
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23 /* compute A = B^(n-1) * 2^(bits-1) */
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24 if (b->used > 1) {
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25 fp_2expt (a, (b->used - 1) * DIGIT_BIT + bits - 1);
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26 } else {
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27 fp_set(a, 1);
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28 bits = 1;
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29 }
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31 /* now compute C = A * B mod b */
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32 for (x = bits - 1; x < (int)DIGIT_BIT; x++) {
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33 fp_mul_2 (a, a);
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34 if (fp_cmp_mag (a, b) != FP_LT) {
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35 s_fp_sub (a, b, a);
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36 }
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37 }
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38 }
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41 /* $Source$ */
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42 /* $Revision$ */
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43 /* $Date$ */