1 | #include <tommath.h> |
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2 | #ifdef BN_S_MP_MUL_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, tomstdenis@gmail.com, http://math.libtomcrypt.com |
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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 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 | } |
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86 | #endif |
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87 | |
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88 | /* $Source: /cvsroot/tcl/libtommath/bn_s_mp_mul_digs.c,v $ */ |
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89 | /* $Revision: 1.1.1.4 $ */ |
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90 | /* $Date: 2006/12/01 00:08:11 $ */ |
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