Mercurial > dropbear

annotate libtommath/bn_mp_reduce_2k.c @ 470:271887c81c07

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merge of '9879f7138f243bb2077a06e4c9ea925fa34abe2b' and 'e28650f207028a45182fc2de545b7bd218d13077'
author Matt Johnston <matt@ucc.asn.au>
date Thu, 27 Mar 2008 10:19:36 +0000
parents 5ff8218bcee9
children 60fc6476e044
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1 #include <tommath.h>
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2 #ifdef BN_MP_REDUCE_2K_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://math.libtomcrypt.com
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16 */
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17
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18 /* reduces a modulo n where n is of the form 2**p - d */
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19 int mp_reduce_2k(mp_int *a, mp_int *n, mp_digit d)
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20 {
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21 mp_int q;
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22 int p, res;
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23
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24 if ((res = mp_init(&q)) != MP_OKAY) {
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25 return res;
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26 }
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27
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28 p = mp_count_bits(n);
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29 top:
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30 /* q = a/2**p, a = a mod 2**p */
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31 if ((res = mp_div_2d(a, p, &q, a)) != MP_OKAY) {
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32 goto ERR;
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33 }
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34
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35 if (d != 1) {
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36 /* q = q * d */
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37 if ((res = mp_mul_d(&q, d, &q)) != MP_OKAY) {
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38 goto ERR;
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39 }
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40 }
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41
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42 /* a = a + q */
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43 if ((res = s_mp_add(a, &q, a)) != MP_OKAY) {
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44 goto ERR;
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45 }
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46
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47 if (mp_cmp_mag(a, n) != MP_LT) {
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48 s_mp_sub(a, n, a);
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49 goto top;
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50 }
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51
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52 ERR:
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53 mp_clear(&q);
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54 return res;
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55 }
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56
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57 #endif
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58
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59 /* $Source: /cvs/libtom/libtommath/bn_mp_reduce_2k.c,v $ */
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60 /* $Revision: 1.3 $ */
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61 /* $Date: 2006/03/31 14:18:44 $ */