# moritz/libtommath forked from libtom/libtommath

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 #include #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://libtom.org */ /* 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\$ */ /* \$Revision\$ */ /* \$Date\$ */