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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_i32_decode_reduce(uint32_t *x,
const void *src, size_t len, const uint32_t *m)
{
uint32_t m_bitlen;
size_t mblen, k, q;
const unsigned char *buf;
m_bitlen = m[0];
/*
* Special case for an invalid modulus.
*/
if (m_bitlen == 0) {
x[0] = 0;
return;
}
/*
* Clear the destination.
*/
br_i32_zero(x, m_bitlen);
/*
* First decode directly as many bytes as possible without
* reduction, taking care to leave a number of bytes which
* is a multiple of 4.
*/
mblen = (m_bitlen + 7) >> 3;
k = mblen - 1;
/*
* Up to k bytes can be safely decoded.
*/
if (k >= len) {
br_i32_decode(x, src, len);
x[0] = m_bitlen;
return;
}
/*
* We want to first inject some bytes with direct decoding,
* then extra bytes by whole 32-bit words. First compute
* the size that should be injected that way.
*/
buf = src;
q = (len - k + 3) & ~(size_t)3;
/*
* It may happen that this is more than what we already have
* (by at most 3 bytes). Such a case may happen only with
* a very short modulus. In that case, we must process the first
* bytes "manually".
*/
if (q > len) {
int i;
uint32_t w;
w = 0;
for (i = 0; i < 4; i ++) {
w <<= 8;
if (q <= len) {
w |= buf[len - q];
}
q --;
}
br_i32_muladd_small(x, w, m);
} else {
br_i32_decode(x, buf, len - q);
x[0] = m_bitlen;
}
/*
* At that point, we have exactly q bytes to inject, and q is
* a multiple of 4.
*/
for (k = len - q; k < len; k += 4) {
br_i32_muladd_small(x, br_dec32be(buf + k), m);
}
}
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