Mercurial > dropbear

annotate libtommath/bn_mp_montgomery_calc_normalization.c @ 1612:470aa609c7e4

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author Matt Johnston <matt@ucc.asn.au>
date Tue, 24 Jul 2018 20:19:05 +0800
parents 8bba51a55704
children f52919ffd3b1
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1 #include <tommath_private.h>
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2 #ifdef BN_MP_MONTGOMERY_CALC_NORMALIZATION_C
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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://libtom.org
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16 */
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17
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18 /*
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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 if (b->used > 1) {
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32 if ((res = mp_2expt (a, ((b->used - 1) * DIGIT_BIT) + bits - 1)) != MP_OKAY) {
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33 return res;
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34 }
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35 } else {
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36 mp_set(a, 1);
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37 bits = 1;
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38 }
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40
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41 /* now compute C = A * B mod b */
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42 for (x = bits - 1; x < (int)DIGIT_BIT; x++) {
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43 if ((res = mp_mul_2 (a, a)) != MP_OKAY) {
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44 return res;
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45 }
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46 if (mp_cmp_mag (a, b) != MP_LT) {
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47 if ((res = s_mp_sub (a, b, a)) != MP_OKAY) {
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48 return res;
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49 }
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50 }
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51 }
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52
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53 return MP_OKAY;
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54 }
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55 #endif
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56
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57 /* ref: $Format:%D$ */
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58 /* git commit: $Format:%H$ */
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59 /* commit time: $Format:%ai$ */