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/*
* Copyright (c) 2016 Thomas Pornin <pornin@bolet.org>
*
* Permission is hereby granted, free of charge, to any person obtaining
* a copy of this software and associated documentation files (the
* "Software"), to deal in the Software without restriction, including
* without limitation the rights to use, copy, modify, merge, publish,
* distribute, sublicense, and/or sell copies of the Software, and to
* permit persons to whom the Software is furnished to do so, subject to
* the following conditions:
*
* The above copyright notice and this permission notice shall be
* included in all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
* NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
* BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
* ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
* CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
* SOFTWARE.
*/
#include "inner.h"
/* see inner.h */
void
br_i31_modpow(uint32_t *x,
const unsigned char *e, size_t elen,
const uint32_t *m, uint32_t m0i, uint32_t *t1, uint32_t *t2)
{
size_t mlen;
uint32_t k;
/*
* 'mlen' is the length of m[] expressed in bytes (including
* the "bit length" first field).
*/
mlen = ((m[0] + 63) >> 5) * sizeof m[0];
/*
* Throughout the algorithm:
* -- t1[] is in Montgomery representation; it contains x, x^2,
* x^4, x^8...
* -- The result is accumulated, in normal representation, in
* the x[] array.
* -- t2[] is used as destination buffer for each multiplication.
*
* Note that there is no need to call br_i32_from_monty().
*/
memcpy(t1, x, mlen);
br_i31_to_monty(t1, m);
br_i31_zero(x, m[0]);
x[1] = 1;
for (k = 0; k < ((uint32_t)elen << 3); k ++) {
uint32_t ctl;
ctl = (e[elen - 1 - (k >> 3)] >> (k & 7)) & 1;
br_i31_montymul(t2, x, t1, m, m0i);
CCOPY(ctl, x, t2, mlen);
br_i31_montymul(t2, t1, t1, m, m0i);
memcpy(t1, t2, mlen);
}
}
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