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Diffstat (limited to 'libtommath/bn_mp_montgomery_setup.c')
| -rw-r--r-- | libtommath/bn_mp_montgomery_setup.c | 59 |
1 files changed, 59 insertions, 0 deletions
diff --git a/libtommath/bn_mp_montgomery_setup.c b/libtommath/bn_mp_montgomery_setup.c new file mode 100644 index 0000000..0dc800e --- /dev/null +++ b/libtommath/bn_mp_montgomery_setup.c @@ -0,0 +1,59 @@ +#include <tommath.h> +#ifdef BN_MP_MONTGOMERY_SETUP_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 + */ + +/* setups the montgomery reduction stuff */ +int +mp_montgomery_setup (mp_int * n, mp_digit * rho) +{ + mp_digit x, b; + +/* fast inversion mod 2**k + * + * Based on the fact that + * + * XA = 1 (mod 2**n) => (X(2-XA)) A = 1 (mod 2**2n) + * => 2*X*A - X*X*A*A = 1 + * => 2*(1) - (1) = 1 + */ + b = n->dp[0]; + + if ((b & 1) == 0) { + return MP_VAL; + } + + x = (((b + 2) & 4) << 1) + b; /* here x*a==1 mod 2**4 */ + x *= 2 - b * x; /* here x*a==1 mod 2**8 */ +#if !defined(MP_8BIT) + x *= 2 - b * x; /* here x*a==1 mod 2**16 */ +#endif +#if defined(MP_64BIT) || !(defined(MP_8BIT) || defined(MP_16BIT)) + x *= 2 - b * x; /* here x*a==1 mod 2**32 */ +#endif +#ifdef MP_64BIT + x *= 2 - b * x; /* here x*a==1 mod 2**64 */ +#endif + + /* rho = -1/m mod b */ + *rho = (unsigned long)(((mp_word)1 << ((mp_word) DIGIT_BIT)) - x) & MP_MASK; + + return MP_OKAY; +} +#endif + +/* $Source: /cvs/libtom/libtommath/bn_mp_montgomery_setup.c,v $ */ +/* $Revision: 1.4 $ */ +/* $Date: 2006/12/04 21:34:03 $ */ |