/* computes a = B**n mod b without division or multiplication useful for
* normalizing numbers in a Montgomery system.
*/
void fp_montgomery_calc_normalization(z *a, z *b)
{
int x, bits;
/* how many bits of last digit does b use */
bits = zBits(b) % BITS_PER_DIGIT;
if (!bits) bits = BITS_PER_DIGIT;
/* compute A = B^(n-1) * 2^(bits-1) */
if (b->size > 1) {
fp_2expt (a, (b->size - 1) * BITS_PER_DIGIT + bits - 1);
} else {
//printf("b.size == 1\n");
sp2z((fp_digit)1,a);
bits = 1;
}
/* now compute C = A * B mod b */
for (x = bits - 1; x < (int)BITS_PER_DIGIT; x++) {
fp_mul_2 (a, a);
// zShiftLeft(a,a,1);
if (fp_cmp_mag (a, b) != FP_LT) {
s_fp_sub (a, b, a);
}
//if (zCompare(a,b) > 0)
// zSub(a,b,a);
}
}
/* computes a = B**n mod b without division or multiplication useful for
* normalizing numbers in a Montgomery system.
*/
void fp_montgomery_calc_normalization(fp_int *a, fp_int *b)
{
int x, bits;
/* how many bits of last digit does b use */
bits = fp_count_bits (b) % DIGIT_BIT;
if (!bits) bits = DIGIT_BIT;
/* compute A = B^(n-1) * 2^(bits-1) */
if (b->used > 1) {
fp_2expt (a, (b->used - 1) * DIGIT_BIT + bits - 1);
} else {
fp_set(a, 1);
bits = 1;
}
/* now compute C = A * B mod b */
for (x = bits - 1; x < (int)DIGIT_BIT; x++) {
fp_mul_2 (a, a);
if (fp_cmp_mag (a, b) != FP_LT) {
s_fp_sub (a, b, a);
}
}
}
