Mercurial > dropbear
view libtommath/bn_mp_montgomery_setup.c @ 530:22a0d8355c2c
merge of 'a101cbd046507cf723e6362a49196dbd4b924042'
and 'c8e1b84cfe874887ad7df0dd95a00de46dbc0136'
author | Matt Johnston <matt@ucc.asn.au> |
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date | Thu, 26 Feb 2009 12:18:34 +0000 |
parents | 5ff8218bcee9 |
children | 60fc6476e044 |
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#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, [email protected], 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 $ */