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