1 | #include <tommath.h> |
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2 | #ifdef BN_S_MP_SUB_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 | /* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */ |
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19 | int |
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20 | s_mp_sub (mp_int * a, mp_int * b, mp_int * c) |
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21 | { |
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22 | int olduse, res, min, max; |
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23 | |
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24 | /* find sizes */ |
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25 | min = b->used; |
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26 | max = a->used; |
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27 | |
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28 | /* init result */ |
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29 | if (c->alloc < max) { |
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30 | if ((res = mp_grow (c, max)) != MP_OKAY) { |
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31 | return res; |
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32 | } |
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33 | } |
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34 | olduse = c->used; |
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35 | c->used = max; |
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36 | |
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37 | { |
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38 | register mp_digit u, *tmpa, *tmpb, *tmpc; |
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39 | register int i; |
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40 | |
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41 | /* alias for digit pointers */ |
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42 | tmpa = a->dp; |
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43 | tmpb = b->dp; |
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44 | tmpc = c->dp; |
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45 | |
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46 | /* set carry to zero */ |
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47 | u = 0; |
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48 | for (i = 0; i < min; i++) { |
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49 | /* T[i] = A[i] - B[i] - U */ |
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50 | *tmpc = *tmpa++ - *tmpb++ - u; |
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51 | |
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52 | /* U = carry bit of T[i] |
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53 | * Note this saves performing an AND operation since |
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54 | * if a carry does occur it will propagate all the way to the |
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55 | * MSB. As a result a single shift is enough to get the carry |
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56 | */ |
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57 | u = *tmpc >> ((mp_digit)(CHAR_BIT * sizeof (mp_digit) - 1)); |
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58 | |
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59 | /* Clear carry from T[i] */ |
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60 | *tmpc++ &= MP_MASK; |
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61 | } |
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62 | |
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63 | /* now copy higher words if any, e.g. if A has more digits than B */ |
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64 | for (; i < max; i++) { |
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65 | /* T[i] = A[i] - U */ |
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66 | *tmpc = *tmpa++ - u; |
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67 | |
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68 | /* U = carry bit of T[i] */ |
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69 | u = *tmpc >> ((mp_digit)(CHAR_BIT * sizeof (mp_digit) - 1)); |
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70 | |
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71 | /* Clear carry from T[i] */ |
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72 | *tmpc++ &= MP_MASK; |
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73 | } |
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74 | |
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75 | /* clear digits above used (since we may not have grown result above) */ |
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76 | for (i = c->used; i < olduse; i++) { |
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77 | *tmpc++ = 0; |
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78 | } |
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79 | } |
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80 | |
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81 | mp_clamp (c); |
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82 | return MP_OKAY; |
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83 | } |
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84 | |
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85 | #endif |
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86 | |
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87 | /* $Source: /cvsroot/tcl/libtommath/bn_s_mp_sub.c,v $ */ |
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88 | /* $Revision: 1.1.1.3 $ */ |
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89 | /* $Date: 2006/12/01 00:08:11 $ */ |
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