annotate bn_mp_expt_d.c @ 1:22d5cf7d4b1a libtommath

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author Matt Johnston <matt@ucc.asn.au>
date Mon, 31 May 2004 18:23:46 +0000
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1 /* LibTomMath, multiple-precision integer library -- Tom St Denis
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2 *
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3 * LibTomMath is a library that provides multiple-precision
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4 * integer arithmetic as well as number theoretic functionality.
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5 *
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6 * The library was designed directly after the MPI library by
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7 * Michael Fromberger but has been written from scratch with
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8 * additional optimizations in place.
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9 *
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10 * The library is free for all purposes without any express
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11 * guarantee it works.
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12 *
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13 * Tom St Denis, [email protected], http://math.libtomcrypt.org
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14 */
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15 #include <tommath.h>
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16
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17 /* calculate c = a**b using a square-multiply algorithm */
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18 int mp_expt_d (mp_int * a, mp_digit b, mp_int * c)
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19 {
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20 int res, x;
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21 mp_int g;
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22
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23 if ((res = mp_init_copy (&g, a)) != MP_OKAY) {
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24 return res;
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25 }
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26
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27 /* set initial result */
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28 mp_set (c, 1);
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29
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30 for (x = 0; x < (int) DIGIT_BIT; x++) {
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31 /* square */
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32 if ((res = mp_sqr (c, c)) != MP_OKAY) {
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33 mp_clear (&g);
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34 return res;
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35 }
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36
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37 /* if the bit is set multiply */
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38 if ((b & (mp_digit) (((mp_digit)1) << (DIGIT_BIT - 1))) != 0) {
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39 if ((res = mp_mul (c, &g, c)) != MP_OKAY) {
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40 mp_clear (&g);
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41 return res;
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42 }
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43 }
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44
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45 /* shift to next bit */
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46 b <<= 1;
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47 }
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48
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49 mp_clear (&g);
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50 return MP_OKAY;
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51 }