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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 /* b = a*2 */ |
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18 int mp_mul_2(mp_int * a, mp_int * b) |
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19 { |
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20 int x, res, oldused; |
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21 |
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22 /* grow to accomodate result */ |
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23 if (b->alloc < a->used + 1) { |
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24 if ((res = mp_grow (b, a->used + 1)) != MP_OKAY) { |
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25 return res; |
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26 } |
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27 } |
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28 |
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29 oldused = b->used; |
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30 b->used = a->used; |
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31 |
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32 { |
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33 register mp_digit r, rr, *tmpa, *tmpb; |
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34 |
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35 /* alias for source */ |
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36 tmpa = a->dp; |
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37 |
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38 /* alias for dest */ |
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39 tmpb = b->dp; |
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40 |
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41 /* carry */ |
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42 r = 0; |
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43 for (x = 0; x < a->used; x++) { |
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44 |
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45 /* get what will be the *next* carry bit from the |
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46 * MSB of the current digit |
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47 */ |
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48 rr = *tmpa >> ((mp_digit)(DIGIT_BIT - 1)); |
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49 |
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50 /* now shift up this digit, add in the carry [from the previous] */ |
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51 *tmpb++ = ((*tmpa++ << ((mp_digit)1)) | r) & MP_MASK; |
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52 |
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53 /* copy the carry that would be from the source |
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54 * digit into the next iteration |
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55 */ |
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56 r = rr; |
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57 } |
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58 |
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59 /* new leading digit? */ |
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60 if (r != 0) { |
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61 /* add a MSB which is always 1 at this point */ |
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62 *tmpb = 1; |
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63 ++(b->used); |
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64 } |
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65 |
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66 /* now zero any excess digits on the destination |
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67 * that we didn't write to |
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68 */ |
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69 tmpb = b->dp + b->used; |
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70 for (x = b->used; x < oldused; x++) { |
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71 *tmpb++ = 0; |
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72 } |
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73 } |
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74 b->sign = a->sign; |
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75 return MP_OKAY; |
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76 } |
