Mercurial > dropbear

annotate libtommath/bn_mp_mod_2d.c @ 284:eed26cff980b

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propagate from branch 'au.asn.ucc.matt.ltm.dropbear' (head 6c790cad5a7fa866ad062cb3a0c279f7ba788583) to branch 'au.asn.ucc.matt.dropbear' (head fff0894a0399405a9410ea1c6d118f342cf2aa64)
author Matt Johnston <matt@ucc.asn.au>
date Wed, 08 Mar 2006 13:23:49 +0000
parents
children 5ff8218bcee9
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284
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1 #include <tommath.h>
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2 #ifdef BN_MP_MOD_2D_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.org
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16 */
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17
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18 /* calc a value mod 2**b */
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19 int
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20 mp_mod_2d (mp_int * a, int b, mp_int * c)
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21 {
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22 int x, res;
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23
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24 /* if b is <= 0 then zero the int */
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25 if (b <= 0) {
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26 mp_zero (c);
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27 return MP_OKAY;
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28 }
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29
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30 /* if the modulus is larger than the value than return */
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31 if (b >= (int) (a->used * DIGIT_BIT)) {
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32 res = mp_copy (a, c);
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33 return res;
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34 }
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35
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36 /* copy */
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37 if ((res = mp_copy (a, c)) != MP_OKAY) {
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38 return res;
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39 }
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40
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41 /* zero digits above the last digit of the modulus */
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42 for (x = (b / DIGIT_BIT) + ((b % DIGIT_BIT) == 0 ? 0 : 1); x < c->used; x++) {
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43 c->dp[x] = 0;
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44 }
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45 /* clear the digit that is not completely outside/inside the modulus */
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46 c->dp[b / DIGIT_BIT] &=
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47 (mp_digit) ((((mp_digit) 1) << (((mp_digit) b) % DIGIT_BIT)) - ((mp_digit) 1));
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48 mp_clamp (c);
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49 return MP_OKAY;
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50 }
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51 #endif