1#include <tommath.h>
2#ifdef BN_MP_DR_REDUCE_C
3/* LibTomMath, multiple-precision integer library -- Tom St Denis
4 *
5 * LibTomMath is a library that provides multiple-precision
6 * integer arithmetic as well as number theoretic functionality.
7 *
8 * The library was designed directly after the MPI library by
9 * Michael Fromberger but has been written from scratch with
10 * additional optimizations in place.
11 *
12 * The library is free for all purposes without any express
13 * guarantee it works.
14 *
15 * Tom St Denis, tomstdenis@gmail.com, http://libtom.org
16 */
17
18/* reduce "x" in place modulo "n" using the Diminished Radix algorithm.
19 *
20 * Based on algorithm from the paper
21 *
22 * "Generating Efficient Primes for Discrete Log Cryptosystems"
23 *                 Chae Hoon Lim, Pil Joong Lee,
24 *          POSTECH Information Research Laboratories
25 *
26 * The modulus must be of a special format [see manual]
27 *
28 * Has been modified to use algorithm 7.10 from the LTM book instead
29 *
30 * Input x must be in the range 0 <= x <= (n-1)**2
31 */
32int
33mp_dr_reduce (mp_int * x, mp_int * n, mp_digit k)
34{
35  int      err, i, m;
36  mp_word  r;
37  mp_digit mu, *tmpx1, *tmpx2;
38
39  /* m = digits in modulus */
40  m = n->used;
41
42  /* ensure that "x" has at least 2m digits */
43  if (x->alloc < m + m) {
44    if ((err = mp_grow (x, m + m)) != MP_OKAY) {
45      return err;
46    }
47  }
48
49/* top of loop, this is where the code resumes if
50 * another reduction pass is required.
51 */
52top:
53  /* aliases for digits */
54  /* alias for lower half of x */
55  tmpx1 = x->dp;
56
57  /* alias for upper half of x, or x/B**m */
58  tmpx2 = x->dp + m;
59
60  /* set carry to zero */
61  mu = 0;
62
63  /* compute (x mod B**m) + k * [x/B**m] inline and inplace */
64  for (i = 0; i < m; i++) {
65      r         = ((mp_word)*tmpx2++) * ((mp_word)k) + *tmpx1 + mu;
66      *tmpx1++  = (mp_digit)(r & MP_MASK);
67      mu        = (mp_digit)(r >> ((mp_word)DIGIT_BIT));
68  }
69
70  /* set final carry */
71  *tmpx1++ = mu;
72
73  /* zero words above m */
74  for (i = m + 1; i < x->used; i++) {
75      *tmpx1++ = 0;
76  }
77
78  /* clamp, sub and return */
79  mp_clamp (x);
80
81  /* if x >= n then subtract and reduce again
82   * Each successive "recursion" makes the input smaller and smaller.
83   */
84  if (mp_cmp_mag (x, n) != MP_LT) {
85    s_mp_sub(x, n, x);
86    goto top;
87  }
88  return MP_OKAY;
89}
90#endif
91
92/* $Source: /cvs/libtom/libtommath/bn_mp_dr_reduce.c,v $ */
93/* $Revision: 1.4 $ */
94/* $Date: 2006/12/28 01:25:13 $ */
95