#include #ifdef BN_MP_MONTGOMERY_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@iahu.ca, http://math.libtomcrypt.org */ /* computes xR**-1 == x (mod N) via Montgomery Reduction */ int mp_montgomery_reduce (mp_int * x, mp_int * n, mp_digit rho) { int ix, res, digs; mp_digit mu; /* can the fast reduction [comba] method be used? * * Note that unlike in mul you're safely allowed *less* * than the available columns [255 per default] since carries * are fixed up in the inner loop. */ digs = n->used * 2 + 1; if ((digs < MP_WARRAY) && n->used < (1 << ((CHAR_BIT * sizeof (mp_word)) - (2 * DIGIT_BIT)))) { return fast_mp_montgomery_reduce (x, n, rho); } /* grow the input as required */ if (x->alloc < digs) { if ((res = mp_grow (x, digs)) != MP_OKAY) { return res; } } x->used = digs; for (ix = 0; ix < n->used; ix++) { /* mu = ai * rho mod b * * The value of rho must be precalculated via * montgomery_setup() such that * it equals -1/n0 mod b this allows the * following inner loop to reduce the * input one digit at a time */ mu = (mp_digit) (((mp_word)x->dp[ix]) * ((mp_word)rho) & MP_MASK); /* a = a + mu * m * b**i */ { register int iy; register mp_digit *tmpn, *tmpx, u; register mp_word r; /* alias for digits of the modulus */ tmpn = n->dp; /* alias for the digits of x [the input] */ tmpx = x->dp + ix; /* set the carry to zero */ u = 0; /* Multiply and add in place */ for (iy = 0; iy < n->used; iy++) { /* compute product and sum */ r = ((mp_word)mu) * ((mp_word)*tmpn++) + ((mp_word) u) + ((mp_word) * tmpx); /* get carry */ u = (mp_digit)(r >> ((mp_word) DIGIT_BIT)); /* fix digit */ *tmpx++ = (mp_digit)(r & ((mp_word) MP_MASK)); } /* At this point the ix'th digit of x should be zero */ /* propagate carries upwards as required*/ while (u) { *tmpx += u; u = *tmpx >> DIGIT_BIT; *tmpx++ &= MP_MASK; } } } /* at this point the n.used'th least * significant digits of x are all zero * which means we can shift x to the * right by n.used digits and the * residue is unchanged. */ /* x = x/b**n.used */ mp_clamp(x); mp_rshd (x, n->used); /* if x >= n then x = x - n */ if (mp_cmp_mag (x, n) != MP_LT) { return s_mp_sub (x, n, x); } return MP_OKAY; } #endif /* $Source: /root/tcl/repos-to-convert/tcl/libtommath/bn_mp_montgomery_reduce.c,v $ */ /* $Revision: 1.1.1.2 $ */ /* $Date: 2005/09/26 16:31:56 $ */