tommath/bn_mp_montgomery_calc_normalization.c

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#include "tommath_private.h"
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#ifdef BN_MP_MONTGOMERY_CALC_NORMALIZATION_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis
*
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* LibTomMath is a library that provides multiple-precision
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* integer arithmetic as well as number theoretic functionality.
*
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* The library was designed directly after the MPI library by
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* 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.
*/
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/*
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* shifts with subtractions when the result is greater than b.
*
* The method is slightly modified to shift B unconditionally upto just under
* the leading bit of b. This saves alot of multiple precision shifting.
*/
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int mp_montgomery_calc_normalization(mp_int *a, const mp_int *b)
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{
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int x, bits, res;
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/* how many bits of last digit does b use */
bits = mp_count_bits(b) % DIGIT_BIT;
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if (b->used > 1) {
if ((res = mp_2expt(a, ((b->used - 1) * DIGIT_BIT) + bits - 1)) != MP_OKAY) {
return res;
}
} else {
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mp_set(a, 1uL);
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bits = 1;
}
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/* now compute C = A * B mod b */
for (x = bits - 1; x < (int)DIGIT_BIT; x++) {
if ((res = mp_mul_2(a, a)) != MP_OKAY) {
return res;
}
if (mp_cmp_mag(a, b) != MP_LT) {
if ((res = s_mp_sub(a, b, a)) != MP_OKAY) {
return res;
}
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}
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}
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return MP_OKAY;
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}
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#endif
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/* ref: $Format:%D$ */
/* git commit: $Format:%H$ */
/* commit time: $Format:%ai$ */