1#include <tommath.h>
2#ifdef BN_MP_MONTGOMERY_CALC_NORMALIZATION_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/*
19 * shifts with subtractions when the result is greater than b.
20 *
21 * The method is slightly modified to shift B unconditionally upto just under
22 * the leading bit of b.  This saves alot of multiple precision shifting.
23 */
24int mp_montgomery_calc_normalization (mp_int * a, mp_int * b)
25{
26  int     x, bits, res;
27
28  /* how many bits of last digit does b use */
29  bits = mp_count_bits (b) % DIGIT_BIT;
30
31  if (b->used > 1) {
32     if ((res = mp_2expt (a, (b->used - 1) * DIGIT_BIT + bits - 1)) != MP_OKAY) {
33        return res;
34     }
35  } else {
36     mp_set(a, 1);
37     bits = 1;
38  }
39
40
41  /* now compute C = A * B mod b */
42  for (x = bits - 1; x < (int)DIGIT_BIT; x++) {
43    if ((res = mp_mul_2 (a, a)) != MP_OKAY) {
44      return res;
45    }
46    if (mp_cmp_mag (a, b) != MP_LT) {
47      if ((res = s_mp_sub (a, b, a)) != MP_OKAY) {
48        return res;
49      }
50    }
51  }
52
53  return MP_OKAY;
54}
55#endif
56
57/* $Source: /cvs/libtom/libtommath/bn_mp_montgomery_calc_normalization.c,v $ */
58/* $Revision: 1.4 $ */
59/* $Date: 2006/12/28 01:25:13 $ */
60