Mercurial > dropbear
annotate bn_fast_mp_invmod.c @ 200:c5c969ed76f3 libtommath
propagate from branch 'au.asn.ucc.matt.ltm-orig' (head 7fa10cba9535de3461cedb14b877c24858826204)
to branch 'au.asn.ucc.matt.dropbear.ltm' (head fc26f60de0370ab0a281fa41a2d13fb17c9d90a8)
| author | Matt Johnston <matt@ucc.asn.au> |
|---|---|
| date | Wed, 11 May 2005 16:15:27 +0000 |
| parents | d8254fc979e9 |
| children |
| rev | line source |
|---|---|
| 142 | 1 #include <tommath.h> |
| 2 #ifdef BN_FAST_MP_INVMOD_C | |
| 2 | 3 /* LibTomMath, multiple-precision integer library -- Tom St Denis |
| 4 * | |
| 5 * LibTomMath is a library that provides multiple-precision | |
| 6 * integer arithmetic as well as number theoretic functionality. | |
| 7 * | |
| 8 * The library was designed directly after the MPI library by | |
| 9 * Michael Fromberger but has been written from scratch with | |
| 10 * additional optimizations in place. | |
| 11 * | |
| 12 * The library is free for all purposes without any express | |
| 13 * guarantee it works. | |
| 14 * | |
| 15 * Tom St Denis, [email protected], http://math.libtomcrypt.org | |
| 16 */ | |
| 17 | |
| 18 /* computes the modular inverse via binary extended euclidean algorithm, | |
| 19 * that is c = 1/a mod b | |
| 20 * | |
| 142 | 21 * Based on slow invmod except this is optimized for the case where b is |
| 2 | 22 * odd as per HAC Note 14.64 on pp. 610 |
| 23 */ | |
|
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24 int fast_mp_invmod (mp_int * a, mp_int * b, mp_int * c) |
| 2 | 25 { |
| 26 mp_int x, y, u, v, B, D; | |
| 27 int res, neg; | |
| 28 | |
| 29 /* 2. [modified] b must be odd */ | |
| 30 if (mp_iseven (b) == 1) { | |
| 31 return MP_VAL; | |
| 32 } | |
| 33 | |
| 34 /* init all our temps */ | |
| 35 if ((res = mp_init_multi(&x, &y, &u, &v, &B, &D, NULL)) != MP_OKAY) { | |
| 36 return res; | |
| 37 } | |
| 38 | |
| 39 /* x == modulus, y == value to invert */ | |
| 40 if ((res = mp_copy (b, &x)) != MP_OKAY) { | |
|
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41 goto LBL_ERR; |
| 2 | 42 } |
| 43 | |
| 44 /* we need y = |a| */ | |
|
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45 if ((res = mp_mod (a, b, &y)) != MP_OKAY) { |
|
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46 goto LBL_ERR; |
| 2 | 47 } |
| 48 | |
| 49 /* 3. u=x, v=y, A=1, B=0, C=0,D=1 */ | |
| 50 if ((res = mp_copy (&x, &u)) != MP_OKAY) { | |
|
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51 goto LBL_ERR; |
| 2 | 52 } |
| 53 if ((res = mp_copy (&y, &v)) != MP_OKAY) { | |
|
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54 goto LBL_ERR; |
| 2 | 55 } |
| 56 mp_set (&D, 1); | |
| 57 | |
| 58 top: | |
| 59 /* 4. while u is even do */ | |
| 60 while (mp_iseven (&u) == 1) { | |
| 61 /* 4.1 u = u/2 */ | |
| 62 if ((res = mp_div_2 (&u, &u)) != MP_OKAY) { | |
|
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63 goto LBL_ERR; |
| 2 | 64 } |
| 65 /* 4.2 if B is odd then */ | |
| 66 if (mp_isodd (&B) == 1) { | |
| 67 if ((res = mp_sub (&B, &x, &B)) != MP_OKAY) { | |
|
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68 goto LBL_ERR; |
| 2 | 69 } |
| 70 } | |
| 71 /* B = B/2 */ | |
| 72 if ((res = mp_div_2 (&B, &B)) != MP_OKAY) { | |
|
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73 goto LBL_ERR; |
| 2 | 74 } |
| 75 } | |
| 76 | |
| 77 /* 5. while v is even do */ | |
| 78 while (mp_iseven (&v) == 1) { | |
| 79 /* 5.1 v = v/2 */ | |
| 80 if ((res = mp_div_2 (&v, &v)) != MP_OKAY) { | |
|
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81 goto LBL_ERR; |
| 2 | 82 } |
| 83 /* 5.2 if D is odd then */ | |
| 84 if (mp_isodd (&D) == 1) { | |
| 85 /* D = (D-x)/2 */ | |
| 86 if ((res = mp_sub (&D, &x, &D)) != MP_OKAY) { | |
|
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87 goto LBL_ERR; |
| 2 | 88 } |
| 89 } | |
| 90 /* D = D/2 */ | |
| 91 if ((res = mp_div_2 (&D, &D)) != MP_OKAY) { | |
|
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92 goto LBL_ERR; |
| 2 | 93 } |
| 94 } | |
| 95 | |
| 96 /* 6. if u >= v then */ | |
| 97 if (mp_cmp (&u, &v) != MP_LT) { | |
| 98 /* u = u - v, B = B - D */ | |
| 99 if ((res = mp_sub (&u, &v, &u)) != MP_OKAY) { | |
|
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100 goto LBL_ERR; |
| 2 | 101 } |
| 102 | |
| 103 if ((res = mp_sub (&B, &D, &B)) != MP_OKAY) { | |
|
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104 goto LBL_ERR; |
| 2 | 105 } |
| 106 } else { | |
| 107 /* v - v - u, D = D - B */ | |
| 108 if ((res = mp_sub (&v, &u, &v)) != MP_OKAY) { | |
|
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109 goto LBL_ERR; |
| 2 | 110 } |
| 111 | |
| 112 if ((res = mp_sub (&D, &B, &D)) != MP_OKAY) { | |
|
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113 goto LBL_ERR; |
| 2 | 114 } |
| 115 } | |
| 116 | |
| 117 /* if not zero goto step 4 */ | |
| 118 if (mp_iszero (&u) == 0) { | |
| 119 goto top; | |
| 120 } | |
| 121 | |
| 122 /* now a = C, b = D, gcd == g*v */ | |
| 123 | |
| 124 /* if v != 1 then there is no inverse */ | |
| 125 if (mp_cmp_d (&v, 1) != MP_EQ) { | |
| 126 res = MP_VAL; | |
|
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127 goto LBL_ERR; |
| 2 | 128 } |
| 129 | |
| 130 /* b is now the inverse */ | |
| 131 neg = a->sign; | |
| 132 while (D.sign == MP_NEG) { | |
| 133 if ((res = mp_add (&D, b, &D)) != MP_OKAY) { | |
|
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134 goto LBL_ERR; |
| 2 | 135 } |
| 136 } | |
| 137 mp_exch (&D, c); | |
| 138 c->sign = neg; | |
| 139 res = MP_OKAY; | |
| 140 | |
|
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141 LBL_ERR:mp_clear_multi (&x, &y, &u, &v, &B, &D, NULL); |
| 2 | 142 return res; |
| 143 } | |
| 142 | 144 #endif |