1 #include <tommath.h> 2 #ifdef BN_MP_REDUCE_C 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, tomstdenis (at) gmail.com, http://math.libtomcrypt.com 16 */ 17 18 /* reduces x mod m, assumes 0 < x < m**2, mu is 19 * precomputed via mp_reduce_setup. 20 * From HAC pp.604 Algorithm 14.42 21 */ 22 int mp_reduce (mp_int * x, mp_int * m, mp_int * mu) 23 { 24 mp_int q; 25 int res, um = m->used; 26 27 /* q = x */ 28 if ((res = mp_init_copy (&q, x)) != MP_OKAY) { 29 return res; 30 } 31 32 /* q1 = x / b**(k-1) */ 33 mp_rshd (&q, um - 1); 34 35 /* according to HAC this optimization is ok */ 36 if (((unsigned long) um) > (((mp_digit)1) << (DIGIT_BIT - 1))) { 37 if ((res = mp_mul (&q, mu, &q)) != MP_OKAY) { 38 goto CLEANUP; 39 } 40 } else { 41 #ifdef BN_S_MP_MUL_HIGH_DIGS_C 42 if ((res = s_mp_mul_high_digs (&q, mu, &q, um)) != MP_OKAY) { 43 goto CLEANUP; 44 } 45 #elif defined(BN_FAST_S_MP_MUL_HIGH_DIGS_C) 46 if ((res = fast_s_mp_mul_high_digs (&q, mu, &q, um)) != MP_OKAY) { 47 goto CLEANUP; 48 } 49 #else 50 { 51 res = MP_VAL; 52 goto CLEANUP; 53 } 54 #endif 55 } 56 57 /* q3 = q2 / b**(k+1) */ 58 mp_rshd (&q, um + 1); 59 60 /* x = x mod b**(k+1), quick (no division) */ 61 if ((res = mp_mod_2d (x, DIGIT_BIT * (um + 1), x)) != MP_OKAY) { 62 goto CLEANUP; 63 } 64 65 /* q = q * m mod b**(k+1), quick (no division) */ 66 if ((res = s_mp_mul_digs (&q, m, &q, um + 1)) != MP_OKAY) { 67 goto CLEANUP; 68 } 69 70 /* x = x - q */ 71 if ((res = mp_sub (x, &q, x)) != MP_OKAY) { 72 goto CLEANUP; 73 } 74 75 /* If x < 0, add b**(k+1) to it */ 76 if (mp_cmp_d (x, 0) == MP_LT) { 77 mp_set (&q, 1); 78 if ((res = mp_lshd (&q, um + 1)) != MP_OKAY) 79 goto CLEANUP; 80 if ((res = mp_add (x, &q, x)) != MP_OKAY) 81 goto CLEANUP; 82 } 83 84 /* Back off if it's too big */ 85 while (mp_cmp (x, m) != MP_LT) { 86 if ((res = s_mp_sub (x, m, x)) != MP_OKAY) { 87 goto CLEANUP; 88 } 89 } 90 91 CLEANUP: 92 mp_clear (&q); 93 94 return res; 95 } 96 #endif 97 98 /* $Source: /cvs/libtom/libtommath/bn_mp_reduce.c,v $ */ 99 /* $Revision: 1.3 $ */ 100 /* $Date: 2006/03/31 14:18:44 $ */ 101
