1 #include <tommath.h> 2 #ifdef BN_MP_REDUCE_2K_L_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 a modulo n where n is of the form 2**p - d 19 This differs from reduce_2k since "d" can be larger 20 than a single digit. 21 */ 22 int mp_reduce_2k_l(mp_int *a, mp_int *n, mp_int *d) 23 { 24 mp_int q; 25 int p, res; 26 27 if ((res = mp_init(&q)) != MP_OKAY) { 28 return res; 29 } 30 31 p = mp_count_bits(n); 32 top: 33 /* q = a/2**p, a = a mod 2**p */ 34 if ((res = mp_div_2d(a, p, &q, a)) != MP_OKAY) { 35 goto ERR; 36 } 37 38 /* q = q * d */ 39 if ((res = mp_mul(&q, d, &q)) != MP_OKAY) { 40 goto ERR; 41 } 42 43 /* a = a + q */ 44 if ((res = s_mp_add(a, &q, a)) != MP_OKAY) { 45 goto ERR; 46 } 47 48 if (mp_cmp_mag(a, n) != MP_LT) { 49 s_mp_sub(a, n, a); 50 goto top; 51 } 52 53 ERR: 54 mp_clear(&q); 55 return res; 56 } 57 58 #endif 59 60 /* $Source: /cvs/libtom/libtommath/bn_mp_reduce_2k_l.c,v $ */ 61 /* $Revision: 1.3 $ */ 62 /* $Date: 2006/03/31 14:18:44 $ */ 63
