2 #ifdef BN_MP_MONTGOMERY_CALC_NORMALIZATION_C
3 /* LibTomMath, multiple-precision integer library -- Tom St Denis
5 * LibTomMath is a library that provides multiple-precision
6 * integer arithmetic as well as number theoretic functionality.
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.
12 * The library is free for all purposes without any express
15 * Tom St Denis, tomstdenis@iahu.ca, http://math.libtomcrypt.org
19 * shifts with subtractions when the result is greater than b.
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.
24 int mp_montgomery_calc_normalization (mp_int * a, mp_int * b)
28 /* how many bits of last digit does b use */
29 bits = mp_count_bits (b) % DIGIT_BIT;
32 if ((res = mp_2expt (a, (b->used - 1) * DIGIT_BIT + bits - 1)) != MP_OKAY) {
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) {
46 if (mp_cmp_mag (a, b) != MP_LT) {
47 if ((res = s_mp_sub (a, b, a)) != MP_OKAY) {