Mercurial > dropbear

annotate libtommath/bn_s_mp_mul_high_digs.c @ 291:55a99934db87 DROPBEAR_0.48

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0.48 bump
author Matt Johnston <matt@ucc.asn.au>
date Thu, 09 Mar 2006 15:03:22 +0000
parents eed26cff980b
children 5ff8218bcee9
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284
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1 #include <tommath.h>
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2 #ifdef BN_S_MP_MUL_HIGH_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, [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 does not compute the lower digs digits
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19 * [meant to get the higher part of the product]
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20 */
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21 int
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22 s_mp_mul_high_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 #ifdef BN_FAST_S_MP_MUL_HIGH_DIGS_C
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32 if (((a->used + b->used + 1) < MP_WARRAY)
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33 && MIN (a->used, b->used) < (1 << ((CHAR_BIT * sizeof (mp_word)) - (2 * DIGIT_BIT)))) {
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34 return fast_s_mp_mul_high_digs (a, b, c, digs);
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35 }
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36 #endif
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37
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38 if ((res = mp_init_size (&t, a->used + b->used + 1)) != MP_OKAY) {
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39 return res;
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40 }
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41 t.used = a->used + b->used + 1;
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42
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43 pa = a->used;
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44 pb = b->used;
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45 for (ix = 0; ix < pa; ix++) {
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46 /* clear the carry */
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47 u = 0;
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48
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49 /* left hand side of A[ix] * B[iy] */
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50 tmpx = a->dp[ix];
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51
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52 /* alias to the address of where the digits will be stored */
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53 tmpt = &(t.dp[digs]);
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54
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55 /* alias for where to read the right hand side from */
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56 tmpy = b->dp + (digs - ix);
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57
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58 for (iy = digs - ix; iy < pb; iy++) {
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59 /* calculate the double precision result */
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60 r = ((mp_word)*tmpt) +
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61 ((mp_word)tmpx) * ((mp_word)*tmpy++) +
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62 ((mp_word) u);
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63
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64 /* get the lower part */
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65 *tmpt++ = (mp_digit) (r & ((mp_word) MP_MASK));
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66
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67 /* carry the carry */
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68 u = (mp_digit) (r >> ((mp_word) DIGIT_BIT));
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69 }
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70 *tmpt = u;
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71 }
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72 mp_clamp (&t);
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73 mp_exch (&t, c);
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74 mp_clear (&t);
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75 return MP_OKAY;
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76 }
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77 #endif