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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 /* multiply by a digit */ |
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18 int |
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19 mp_mul_d (mp_int * a, mp_digit b, mp_int * c) |
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20 { |
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21 mp_digit u, *tmpa, *tmpc; |
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22 mp_word r; |
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23 int ix, res, olduse; |
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24 |
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25 /* make sure c is big enough to hold a*b */ |
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26 if (c->alloc < a->used + 1) { |
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27 if ((res = mp_grow (c, a->used + 1)) != MP_OKAY) { |
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28 return res; |
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29 } |
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30 } |
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31 |
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32 /* get the original destinations used count */ |
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33 olduse = c->used; |
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34 |
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35 /* set the sign */ |
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36 c->sign = a->sign; |
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37 |
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38 /* alias for a->dp [source] */ |
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39 tmpa = a->dp; |
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40 |
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41 /* alias for c->dp [dest] */ |
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42 tmpc = c->dp; |
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43 |
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44 /* zero carry */ |
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45 u = 0; |
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46 |
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47 /* compute columns */ |
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48 for (ix = 0; ix < a->used; ix++) { |
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49 /* compute product and carry sum for this term */ |
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50 r = ((mp_word) u) + ((mp_word)*tmpa++) * ((mp_word)b); |
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51 |
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52 /* mask off higher bits to get a single digit */ |
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53 *tmpc++ = (mp_digit) (r & ((mp_word) MP_MASK)); |
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54 |
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55 /* send carry into next iteration */ |
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56 u = (mp_digit) (r >> ((mp_word) DIGIT_BIT)); |
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57 } |
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58 |
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59 /* store final carry [if any] */ |
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60 *tmpc++ = u; |
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61 |
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62 /* now zero digits above the top */ |
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63 while (ix++ < olduse) { |
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64 *tmpc++ = 0; |
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65 } |
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66 |
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67 /* set used count */ |
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68 c->used = a->used + 1; |
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69 mp_clamp(c); |
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70 |
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71 return MP_OKAY; |
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72 } |
