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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 /* this is a modified version of fast_s_mp_mul_digs that only produces |
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18 * output digits *above* digs. See the comments for fast_s_mp_mul_digs |
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19 * to see how it works. |
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20 * |
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21 * This is used in the Barrett reduction since for one of the multiplications |
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22 * only the higher digits were needed. This essentially halves the work. |
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23 * |
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24 * Based on Algorithm 14.12 on pp.595 of HAC. |
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25 */ |
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26 int |
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27 fast_s_mp_mul_high_digs (mp_int * a, mp_int * b, mp_int * c, int digs) |
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28 { |
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29 int oldused, newused, res, pa, pb, ix; |
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30 mp_word W[MP_WARRAY]; |
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31 |
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32 /* calculate size of product and allocate more space if required */ |
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33 newused = a->used + b->used + 1; |
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34 if (c->alloc < newused) { |
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35 if ((res = mp_grow (c, newused)) != MP_OKAY) { |
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36 return res; |
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37 } |
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38 } |
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39 |
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40 /* like the other comba method we compute the columns first */ |
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41 pa = a->used; |
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42 pb = b->used; |
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43 memset (W + digs, 0, (pa + pb + 1 - digs) * sizeof (mp_word)); |
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44 for (ix = 0; ix < pa; ix++) { |
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45 { |
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46 register mp_digit tmpx, *tmpy; |
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47 register int iy; |
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48 register mp_word *_W; |
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49 |
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50 /* work todo, that is we only calculate digits that are at "digs" or above */ |
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51 iy = digs - ix; |
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52 |
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53 /* copy of word on the left of A[ix] * B[iy] */ |
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54 tmpx = a->dp[ix]; |
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55 |
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56 /* alias for right side */ |
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57 tmpy = b->dp + iy; |
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58 |
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59 /* alias for the columns of output. Offset to be equal to or above the |
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60 * smallest digit place requested |
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61 */ |
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62 _W = W + digs; |
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63 |
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64 /* skip cases below zero where ix > digs */ |
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65 if (iy < 0) { |
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66 iy = abs(iy); |
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67 tmpy += iy; |
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68 _W += iy; |
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69 iy = 0; |
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70 } |
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71 |
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72 /* compute column products for digits above the minimum */ |
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73 for (; iy < pb; iy++) { |
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74 *_W++ += ((mp_word) tmpx) * ((mp_word)*tmpy++); |
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75 } |
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76 } |
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77 } |
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78 |
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79 /* setup dest */ |
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80 oldused = c->used; |
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81 c->used = newused; |
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82 |
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83 /* now convert the array W downto what we need |
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84 * |
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85 * See comments in bn_fast_s_mp_mul_digs.c |
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86 */ |
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87 for (ix = digs + 1; ix < newused; ix++) { |
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88 W[ix] += (W[ix - 1] >> ((mp_word) DIGIT_BIT)); |
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89 c->dp[ix - 1] = (mp_digit) (W[ix - 1] & ((mp_word) MP_MASK)); |
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90 } |
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91 c->dp[newused - 1] = (mp_digit) (W[newused - 1] & ((mp_word) MP_MASK)); |
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92 |
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93 for (; ix < oldused; ix++) { |
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94 c->dp[ix] = 0; |
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95 } |
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96 mp_clamp (c); |
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97 return MP_OKAY; |
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98 } |
