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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 } |
