diff options
Diffstat (limited to 'libtommath/bn_mp_reduce.c')
-rw-r--r-- | libtommath/bn_mp_reduce.c | 100 |
1 files changed, 100 insertions, 0 deletions
diff --git a/libtommath/bn_mp_reduce.c b/libtommath/bn_mp_reduce.c new file mode 100644 index 0000000..e4c8842 --- /dev/null +++ b/libtommath/bn_mp_reduce.c @@ -0,0 +1,100 @@ +#include <tommath.h> +#ifdef BN_MP_REDUCE_C +/* LibTomMath, multiple-precision integer library -- Tom St Denis + * + * LibTomMath is a library that provides multiple-precision + * integer arithmetic as well as number theoretic functionality. + * + * The library was designed directly after the MPI library by + * Michael Fromberger but has been written from scratch with + * additional optimizations in place. + * + * The library is free for all purposes without any express + * guarantee it works. + * + * Tom St Denis, tomstdenis@gmail.com, http://math.libtomcrypt.com + */ + +/* reduces x mod m, assumes 0 < x < m**2, mu is + * precomputed via mp_reduce_setup. + * From HAC pp.604 Algorithm 14.42 + */ +int mp_reduce (mp_int * x, mp_int * m, mp_int * mu) +{ + mp_int q; + int res, um = m->used; + + /* q = x */ + if ((res = mp_init_copy (&q, x)) != MP_OKAY) { + return res; + } + + /* q1 = x / b**(k-1) */ + mp_rshd (&q, um - 1); + + /* according to HAC this optimization is ok */ + if (((unsigned long) um) > (((mp_digit)1) << (DIGIT_BIT - 1))) { + if ((res = mp_mul (&q, mu, &q)) != MP_OKAY) { + goto CLEANUP; + } + } else { +#ifdef BN_S_MP_MUL_HIGH_DIGS_C + if ((res = s_mp_mul_high_digs (&q, mu, &q, um)) != MP_OKAY) { + goto CLEANUP; + } +#elif defined(BN_FAST_S_MP_MUL_HIGH_DIGS_C) + if ((res = fast_s_mp_mul_high_digs (&q, mu, &q, um)) != MP_OKAY) { + goto CLEANUP; + } +#else + { + res = MP_VAL; + goto CLEANUP; + } +#endif + } + + /* q3 = q2 / b**(k+1) */ + mp_rshd (&q, um + 1); + + /* x = x mod b**(k+1), quick (no division) */ + if ((res = mp_mod_2d (x, DIGIT_BIT * (um + 1), x)) != MP_OKAY) { + goto CLEANUP; + } + + /* q = q * m mod b**(k+1), quick (no division) */ + if ((res = s_mp_mul_digs (&q, m, &q, um + 1)) != MP_OKAY) { + goto CLEANUP; + } + + /* x = x - q */ + if ((res = mp_sub (x, &q, x)) != MP_OKAY) { + goto CLEANUP; + } + + /* If x < 0, add b**(k+1) to it */ + if (mp_cmp_d (x, 0) == MP_LT) { + mp_set (&q, 1); + if ((res = mp_lshd (&q, um + 1)) != MP_OKAY) + goto CLEANUP; + if ((res = mp_add (x, &q, x)) != MP_OKAY) + goto CLEANUP; + } + + /* Back off if it's too big */ + while (mp_cmp (x, m) != MP_LT) { + if ((res = s_mp_sub (x, m, x)) != MP_OKAY) { + goto CLEANUP; + } + } + +CLEANUP: + mp_clear (&q); + + return res; +} +#endif + +/* $Source: /root/tcl/repos-to-convert/tcl/libtommath/bn_mp_reduce.c,v $ */ +/* $Revision: 1.1.1.4 $ */ +/* $Date: 2006/12/01 00:08:11 $ */ |