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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 does not compute the lower digs digits |
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18 * [meant to get the higher part of the product] |
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19 */ |
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20 int |
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21 s_mp_mul_high_digs (mp_int * a, mp_int * b, mp_int * c, int digs) |
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22 { |
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23 mp_int t; |
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24 int res, pa, pb, ix, iy; |
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25 mp_digit u; |
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26 mp_word r; |
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27 mp_digit tmpx, *tmpt, *tmpy; |
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28 |
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29 /* can we use the fast multiplier? */ |
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30 if (((a->used + b->used + 1) < MP_WARRAY) |
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31 && MIN (a->used, b->used) < (1 << ((CHAR_BIT * sizeof (mp_word)) - (2 * DIGIT_BIT)))) { |
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32 return fast_s_mp_mul_high_digs (a, b, c, digs); |
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33 } |
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34 |
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35 if ((res = mp_init_size (&t, a->used + b->used + 1)) != MP_OKAY) { |
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36 return res; |
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37 } |
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38 t.used = a->used + b->used + 1; |
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39 |
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40 pa = a->used; |
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41 pb = b->used; |
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42 for (ix = 0; ix < pa; ix++) { |
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43 /* clear the carry */ |
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44 u = 0; |
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45 |
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46 /* left hand side of A[ix] * B[iy] */ |
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47 tmpx = a->dp[ix]; |
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48 |
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49 /* alias to the address of where the digits will be stored */ |
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50 tmpt = &(t.dp[digs]); |
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51 |
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52 /* alias for where to read the right hand side from */ |
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53 tmpy = b->dp + (digs - ix); |
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54 |
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55 for (iy = digs - ix; iy < pb; iy++) { |
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56 /* calculate the double precision result */ |
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57 r = ((mp_word)*tmpt) + |
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58 ((mp_word)tmpx) * ((mp_word)*tmpy++) + |
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59 ((mp_word) u); |
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60 |
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61 /* get the lower part */ |
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62 *tmpt++ = (mp_digit) (r & ((mp_word) MP_MASK)); |
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63 |
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64 /* carry the carry */ |
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65 u = (mp_digit) (r >> ((mp_word) DIGIT_BIT)); |
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66 } |
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67 *tmpt = u; |
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68 } |
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69 mp_clamp (&t); |
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70 mp_exch (&t, c); |
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71 mp_clear (&t); |
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72 return MP_OKAY; |
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73 } |
