4e3f1344a5
tommath.h contains declarations for the public part of the library. tommath_private.h contains the functions which are private to ltm and should not be exposed to the public.
97 lines
2.3 KiB
C
97 lines
2.3 KiB
C
#include <tommath_private.h>
|
|
#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, tstdenis82@gmail.com, http://libtom.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) {
|
|
if ((err = s_mp_sub(x, n, x)) != MP_OKAY) {
|
|
return err;
|
|
}
|
|
goto top;
|
|
}
|
|
return MP_OKAY;
|
|
}
|
|
#endif
|
|
|
|
/* $Source$ */
|
|
/* $Revision$ */
|
|
/* $Date$ */
|