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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 |
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18 /* this is a shell function that calls either the normal or Montgomery |
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19 * exptmod functions. Originally the call to the montgomery code was |
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20 * embedded in the normal function but that wasted alot of stack space |
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21 * for nothing (since 99% of the time the Montgomery code would be called) |
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22 */ |
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23 int mp_exptmod (mp_int * G, mp_int * X, mp_int * P, mp_int * Y) |
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24 { |
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25 int dr; |
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26 |
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27 /* modulus P must be positive */ |
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28 if (P->sign == MP_NEG) { |
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29 return MP_VAL; |
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30 } |
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31 |
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32 /* if exponent X is negative we have to recurse */ |
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33 if (X->sign == MP_NEG) { |
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34 mp_int tmpG, tmpX; |
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35 int err; |
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36 |
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37 /* first compute 1/G mod P */ |
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38 if ((err = mp_init(&tmpG)) != MP_OKAY) { |
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39 return err; |
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40 } |
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41 if ((err = mp_invmod(G, P, &tmpG)) != MP_OKAY) { |
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42 mp_clear(&tmpG); |
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43 return err; |
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44 } |
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45 |
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46 /* now get |X| */ |
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47 if ((err = mp_init(&tmpX)) != MP_OKAY) { |
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48 mp_clear(&tmpG); |
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49 return err; |
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50 } |
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51 if ((err = mp_abs(X, &tmpX)) != MP_OKAY) { |
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52 mp_clear_multi(&tmpG, &tmpX, NULL); |
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53 return err; |
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54 } |
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55 |
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56 /* and now compute (1/G)**|X| instead of G**X [X < 0] */ |
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57 err = mp_exptmod(&tmpG, &tmpX, P, Y); |
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58 mp_clear_multi(&tmpG, &tmpX, NULL); |
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59 return err; |
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60 } |
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61 |
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62 /* is it a DR modulus? */ |
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63 dr = mp_dr_is_modulus(P); |
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64 |
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65 /* if not, is it a uDR modulus? */ |
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66 if (dr == 0) { |
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67 dr = mp_reduce_is_2k(P) << 1; |
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68 } |
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69 |
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70 /* if the modulus is odd or dr != 0 use the fast method */ |
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71 #ifndef NO_FAST_EXPTMOD |
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72 if (mp_isodd (P) == 1 || dr != 0) { |
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73 return mp_exptmod_fast (G, X, P, Y, dr); |
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74 } |
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75 else |
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76 #endif |
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77 { |
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78 /* otherwise use the generic Barrett reduction technique */ |
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79 return s_mp_exptmod (G, X, P, Y); |
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80 } |
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81 } |
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82 |
