annotate bn_mp_montgomery_calc_normalization.c @ 142:d29b64170cf0 libtommath-orig

import of libtommath 0.32
author Matt Johnston <matt@ucc.asn.au>
date Sun, 19 Dec 2004 11:33:56 +0000
parents 86e0b50a9b58
children d8254fc979e9
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1 #include <tommath.h>
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2 #ifdef BN_MP_MONTGOMERY_CALC_NORMALIZATION_C
2
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3 /* LibTomMath, multiple-precision integer library -- Tom St Denis
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4 *
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5 * LibTomMath is a library that provides multiple-precision
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6 * integer arithmetic as well as number theoretic functionality.
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7 *
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8 * The library was designed directly after the MPI library by
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9 * Michael Fromberger but has been written from scratch with
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10 * additional optimizations in place.
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11 *
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12 * The library is free for all purposes without any express
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13 * guarantee it works.
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14 *
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15 * Tom St Denis, [email protected], http://math.libtomcrypt.org
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16 */
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17
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18 /*
2
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19 * shifts with subtractions when the result is greater than b.
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20 *
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21 * The method is slightly modified to shift B unconditionally upto just under
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22 * the leading bit of b. This saves alot of multiple precision shifting.
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23 */
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24 int mp_montgomery_calc_normalization (mp_int * a, mp_int * b)
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25 {
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26 int x, bits, res;
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27
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28 /* how many bits of last digit does b use */
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29 bits = mp_count_bits (b) % DIGIT_BIT;
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30
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31
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32 if (b->used > 1) {
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33 if ((res = mp_2expt (a, (b->used - 1) * DIGIT_BIT + bits - 1)) != MP_OKAY) {
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34 return res;
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35 }
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36 } else {
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37 mp_set(a, 1);
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38 bits = 1;
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39 }
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41
2
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42 /* now compute C = A * B mod b */
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43 for (x = bits - 1; x < (int)DIGIT_BIT; x++) {
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44 if ((res = mp_mul_2 (a, a)) != MP_OKAY) {
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45 return res;
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46 }
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47 if (mp_cmp_mag (a, b) != MP_LT) {
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48 if ((res = s_mp_sub (a, b, a)) != MP_OKAY) {
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49 return res;
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50 }
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51 }
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52 }
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53
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54 return MP_OKAY;
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55 }
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56 #endif