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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 /* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */ |
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18 int |
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19 s_mp_sub (mp_int * a, mp_int * b, mp_int * c) |
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20 { |
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21 int olduse, res, min, max; |
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22 |
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23 /* find sizes */ |
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24 min = b->used; |
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25 max = a->used; |
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26 |
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27 /* init result */ |
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28 if (c->alloc < max) { |
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29 if ((res = mp_grow (c, max)) != MP_OKAY) { |
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30 return res; |
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31 } |
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32 } |
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33 olduse = c->used; |
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34 c->used = max; |
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35 |
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36 { |
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37 register mp_digit u, *tmpa, *tmpb, *tmpc; |
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38 register int i; |
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39 |
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40 /* alias for digit pointers */ |
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41 tmpa = a->dp; |
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42 tmpb = b->dp; |
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43 tmpc = c->dp; |
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44 |
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45 /* set carry to zero */ |
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46 u = 0; |
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47 for (i = 0; i < min; i++) { |
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48 /* T[i] = A[i] - B[i] - U */ |
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49 *tmpc = *tmpa++ - *tmpb++ - u; |
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50 |
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51 /* U = carry bit of T[i] |
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52 * Note this saves performing an AND operation since |
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53 * if a carry does occur it will propagate all the way to the |
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54 * MSB. As a result a single shift is enough to get the carry |
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55 */ |
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56 u = *tmpc >> ((mp_digit)(CHAR_BIT * sizeof (mp_digit) - 1)); |
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57 |
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58 /* Clear carry from T[i] */ |
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59 *tmpc++ &= MP_MASK; |
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60 } |
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61 |
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62 /* now copy higher words if any, e.g. if A has more digits than B */ |
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63 for (; i < max; i++) { |
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64 /* T[i] = A[i] - U */ |
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65 *tmpc = *tmpa++ - u; |
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66 |
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67 /* U = carry bit of T[i] */ |
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68 u = *tmpc >> ((mp_digit)(CHAR_BIT * sizeof (mp_digit) - 1)); |
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69 |
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70 /* Clear carry from T[i] */ |
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71 *tmpc++ &= MP_MASK; |
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72 } |
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73 |
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74 /* clear digits above used (since we may not have grown result above) */ |
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75 for (i = c->used; i < olduse; i++) { |
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76 *tmpc++ = 0; |
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77 } |
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78 } |
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79 |
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80 mp_clamp (c); |
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81 return MP_OKAY; |
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82 } |
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83 |
