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1 #include <tommath.h> |
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2 #ifdef BN_S_MP_MUL_DIGS_C |
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2
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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 /* multiplies |a| * |b| and only computes upto digs digits of result |
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19 * HAC pp. 595, Algorithm 14.12 Modified so you can control how |
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20 * many digits of output are created. |
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21 */ |
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22 int |
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23 s_mp_mul_digs (mp_int * a, mp_int * b, mp_int * c, int digs) |
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24 { |
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25 mp_int t; |
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26 int res, pa, pb, ix, iy; |
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27 mp_digit u; |
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28 mp_word r; |
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29 mp_digit tmpx, *tmpt, *tmpy; |
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30 |
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31 /* can we use the fast multiplier? */ |
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32 if (((digs) < MP_WARRAY) && |
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33 MIN (a->used, b->used) < |
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34 (1 << ((CHAR_BIT * sizeof (mp_word)) - (2 * DIGIT_BIT)))) { |
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35 return fast_s_mp_mul_digs (a, b, c, digs); |
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36 } |
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37 |
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38 if ((res = mp_init_size (&t, digs)) != MP_OKAY) { |
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39 return res; |
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40 } |
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41 t.used = digs; |
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42 |
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43 /* compute the digits of the product directly */ |
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44 pa = a->used; |
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45 for (ix = 0; ix < pa; ix++) { |
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46 /* set the carry to zero */ |
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47 u = 0; |
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48 |
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49 /* limit ourselves to making digs digits of output */ |
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50 pb = MIN (b->used, digs - ix); |
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51 |
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52 /* setup some aliases */ |
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53 /* copy of the digit from a used within the nested loop */ |
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54 tmpx = a->dp[ix]; |
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55 |
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56 /* an alias for the destination shifted ix places */ |
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57 tmpt = t.dp + ix; |
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58 |
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59 /* an alias for the digits of b */ |
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60 tmpy = b->dp; |
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61 |
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62 /* compute the columns of the output and propagate the carry */ |
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63 for (iy = 0; iy < pb; iy++) { |
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64 /* compute the column as a mp_word */ |
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65 r = ((mp_word)*tmpt) + |
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66 ((mp_word)tmpx) * ((mp_word)*tmpy++) + |
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67 ((mp_word) u); |
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68 |
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69 /* the new column is the lower part of the result */ |
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70 *tmpt++ = (mp_digit) (r & ((mp_word) MP_MASK)); |
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71 |
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72 /* get the carry word from the result */ |
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73 u = (mp_digit) (r >> ((mp_word) DIGIT_BIT)); |
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74 } |
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75 /* set carry if it is placed below digs */ |
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76 if (ix + iy < digs) { |
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77 *tmpt = u; |
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78 } |
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79 } |
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80 |
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81 mp_clamp (&t); |
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82 mp_exch (&t, c); |
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83 |
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84 mp_clear (&t); |
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85 return MP_OKAY; |
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86 } |
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87 #endif |
