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1 #include <tommath.h> |
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2 #ifdef BN_FAST_S_MP_MUL_HIGH_DIGS_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 /* this is a modified version of fast_s_mul_digs that only produces |
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19 * output digits *above* digs. See the comments for fast_s_mul_digs |
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20 * to see how it works. |
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21 * |
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22 * This is used in the Barrett reduction since for one of the multiplications |
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23 * only the higher digits were needed. This essentially halves the work. |
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24 * |
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25 * Based on Algorithm 14.12 on pp.595 of HAC. |
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26 */ |
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27 int |
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28 fast_s_mp_mul_high_digs (mp_int * a, mp_int * b, mp_int * c, int digs) |
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29 { |
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30 int olduse, res, pa, ix, iz; |
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31 mp_digit W[MP_WARRAY]; |
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32 mp_word _W; |
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33 |
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34 /* grow the destination as required */ |
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35 pa = a->used + b->used; |
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36 if (c->alloc < pa) { |
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37 if ((res = mp_grow (c, pa)) != MP_OKAY) { |
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38 return res; |
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39 } |
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40 } |
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41 |
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42 /* number of output digits to produce */ |
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43 pa = a->used + b->used; |
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44 _W = 0; |
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45 for (ix = digs; ix <= pa; ix++) { |
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46 int tx, ty, iy; |
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47 mp_digit *tmpx, *tmpy; |
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48 |
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49 /* get offsets into the two bignums */ |
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50 ty = MIN(b->used-1, ix); |
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51 tx = ix - ty; |
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52 |
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53 /* setup temp aliases */ |
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54 tmpx = a->dp + tx; |
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55 tmpy = b->dp + ty; |
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56 |
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57 /* this is the number of times the loop will iterrate, essentially its |
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58 while (tx++ < a->used && ty-- >= 0) { ... } |
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59 */ |
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60 iy = MIN(a->used-tx, ty+1); |
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61 |
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62 /* execute loop */ |
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63 for (iz = 0; iz < iy; iz++) { |
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64 _W += ((mp_word)*tmpx++)*((mp_word)*tmpy--); |
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65 } |
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66 |
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67 /* store term */ |
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68 W[ix] = ((mp_digit)_W) & MP_MASK; |
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69 |
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70 /* make next carry */ |
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71 _W = _W >> ((mp_word)DIGIT_BIT); |
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72 } |
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73 |
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74 /* setup dest */ |
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75 olduse = c->used; |
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76 c->used = pa; |
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77 |
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78 { |
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79 register mp_digit *tmpc; |
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80 |
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81 tmpc = c->dp + digs; |
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82 for (ix = digs; ix <= pa; ix++) { |
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83 /* now extract the previous digit [below the carry] */ |
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84 *tmpc++ = W[ix]; |
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85 } |
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86 |
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87 /* clear unused digits [that existed in the old copy of c] */ |
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88 for (; ix < olduse; ix++) { |
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89 *tmpc++ = 0; |
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90 } |
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91 } |
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92 mp_clamp (c); |
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93 return MP_OKAY; |
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94 } |
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95 #endif |
