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Diffstat (limited to 'libtommath/bn_mp_reduce_2k.c')
-rw-r--r-- | libtommath/bn_mp_reduce_2k.c | 57 |
1 files changed, 57 insertions, 0 deletions
diff --git a/libtommath/bn_mp_reduce_2k.c b/libtommath/bn_mp_reduce_2k.c new file mode 100644 index 0000000..428f2ff --- /dev/null +++ b/libtommath/bn_mp_reduce_2k.c @@ -0,0 +1,57 @@ +#include <tommath.h> +#ifdef BN_MP_REDUCE_2K_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 a modulo n where n is of the form 2**p - d */ +int mp_reduce_2k(mp_int *a, mp_int *n, mp_digit d) +{ + mp_int q; + int p, res; + + if ((res = mp_init(&q)) != MP_OKAY) { + return res; + } + + p = mp_count_bits(n); +top: + /* q = a/2**p, a = a mod 2**p */ + if ((res = mp_div_2d(a, p, &q, a)) != MP_OKAY) { + goto ERR; + } + + if (d != 1) { + /* q = q * d */ + if ((res = mp_mul_d(&q, d, &q)) != MP_OKAY) { + goto ERR; + } + } + + /* a = a + q */ + if ((res = s_mp_add(a, &q, a)) != MP_OKAY) { + goto ERR; + } + + if (mp_cmp_mag(a, n) != MP_LT) { + s_mp_sub(a, n, a); + goto top; + } + +ERR: + mp_clear(&q); + return res; +} + +#endif |