#include #ifdef BN_MP_DR_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 */ /* reduce "x" in place modulo "n" using the Diminished Radix algorithm. * * Based on algorithm from the paper * * "Generating Efficient Primes for Discrete Log Cryptosystems" * Chae Hoon Lim, Pil Joong Lee, * POSTECH Information Research Laboratories * * The modulus must be of a special format [see manual] * * Has been modified to use algorithm 7.10 from the LTM book instead * * Input x must be in the range 0 <= x <= (n-1)**2 */ int mp_dr_reduce (mp_int * x, mp_int * n, mp_digit k) { int err, i, m; mp_word r; mp_digit mu, *tmpx1, *tmpx2; /* m = digits in modulus */ m = n->used; /* ensure that "x" has at least 2m digits */ if (x->alloc < m + m) { if ((err = mp_grow (x, m + m)) != MP_OKAY) { return err; } } /* top of loop, this is where the code resumes if * another reduction pass is required. */ top: /* aliases for digits */ /* alias for lower half of x */ tmpx1 = x->dp; /* alias for upper half of x, or x/B**m */ tmpx2 = x->dp + m; /* set carry to zero */ mu = 0; /* compute (x mod B**m) + k * [x/B**m] inline and inplace */ for (i = 0; i < m; i++) { r = ((mp_word)*tmpx2++) * ((mp_word)k) + *tmpx1 + mu; *tmpx1++ = (mp_digit)(r & MP_MASK); mu = (mp_digit)(r >> ((mp_word)DIGIT_BIT)); } /* set final carry */ *tmpx1++ = mu; /* zero words above m */ for (i = m + 1; i < x->used; i++) { *tmpx1++ = 0; } /* clamp, sub and return */ mp_clamp (x); /* if x >= n then subtract and reduce again * Each successive "recursion" makes the input smaller and smaller. */ if (mp_cmp_mag (x, n) != MP_LT) { s_mp_sub(x, n, x); goto top; } return MP_OKAY; } #endif /* $Source: /root/tcl/repos-to-convert/tcl/libtommath/bn_mp_dr_reduce.c,v $ */ /* $Revision: 1.1.1.2 $ */ /* $Date: 2005/09/26 16:31:56 $ */