/*
* Copyright (c) 2017 Thomas Pornin
*
* 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"
/*
* Constant-time division. The divisor must not be larger than 16 bits,
* and the quotient must fit on 17 bits.
*/
static uint32_t
divrem16(uint32_t x, uint32_t d, uint32_t *r)
{
int i;
uint32_t q;
q = 0;
d <<= 16;
for (i = 16; i >= 0; i --) {
uint32_t ctl;
ctl = LE(d, x);
q |= ctl << i;
x -= (-ctl) & d;
d >>= 1;
}
if (r != NULL) {
*r = x;
}
return q;
}
/* see inner.h */
void
br_i15_muladd_small(uint16_t *x, uint16_t z, const uint16_t *m)
{
/*
* Constant-time: we accept to leak the exact bit length of the
* modulus m.
*/
unsigned m_bitlen, mblr;
size_t u, mlen;
uint32_t hi, a0, a, b, q;
uint32_t cc, tb, over, under;
/*
* Simple case: the modulus fits on one word.
*/
m_bitlen = m[0];
if (m_bitlen == 0) {
return;
}
if (m_bitlen <= 15) {
uint32_t rem;
divrem16(((uint32_t)x[1] << 15) | z, m[1], &rem);
x[1] = rem;
return;
}
mlen = (m_bitlen + 15) >> 4;
mblr = m_bitlen & 15;
/*
* Principle: we estimate the quotient (x*2^15+z)/m by
* doing a 30/15 division with the high words.
*
* Let:
* w = 2^15
* a = (w*a0 + a1) * w^N + a2
* b = b0 * w^N + b2
* such that:
* 0 <= a0 < w
* 0 <= a1 < w
* 0 <= a2 < w^N
* w/2 <= b0 < w
* 0 <= b2 < w^N
* a < w*b
* I.e. the two top words of a are a0:a1, the top word of b is
* b0, we ensured that b0 is "full" (high bit set), and a is
* such that the quotient q = a/b fits on one word (0 <= q < w).
*
* If a = b*q + r (with 0 <= r < q), then we can estimate q by
* using a division on the top words:
* a0*w + a1 = b0*u + v (with 0 <= v < b0)
* Then the following holds:
* 0 <= u <= w
* u-2 <= q <= u
*/
hi = x[mlen];
if (mblr == 0) {
a0 = x[mlen];
memmove(x + 2, x + 1, (mlen - 1) * sizeof *x);
x[1] = z;
a = (a0 << 15) + x[mlen];
b = m[mlen];
} else {
a0 = (x[mlen] << (15 - mblr)) | (x[mlen - 1] >> mblr);
memmove(x + 2, x + 1, (mlen - 1) * sizeof *x);
x[1] = z;
a = (a0 << 15) | (((x[mlen] << (15 - mblr))
| (x[mlen - 1] >> mblr)) & 0x7FFF);
b = (m[mlen] << (15 - mblr)) | (m[mlen - 1] >> mblr);
}
q = divrem16(a, b, NULL);
/*
* We computed an estimate for q, but the real one may be q,
* q-1 or q-2; moreover, the division may have returned a value
* 8000 or even 8001 if the two high words were identical, and
* we want to avoid values beyond 7FFF. We thus adjust q so
* that the "true" multiplier will be q+1, q or q-1, and q is
* in the 0000..7FFF range.
*/
q = MUX(EQ(b, a0), 0x7FFF, q - 1 + ((q - 1) >> 31));
/*
* We subtract q*m from x (x has an extra high word of value 'hi').
* Since q may be off by 1 (in either direction), we may have to
* add or subtract m afterwards.
*
* The 'tb' flag will be true (1) at the end of the loop if the
* result is greater than or equal to the modulus (not counting
* 'hi' or the carry).
*/
cc = 0;
tb = 1;
for (u = 1; u <= mlen; u ++) {
uint32_t mw, zl, xw, nxw;
mw = m[u];
zl = MUL15(mw, q) + cc;
cc = zl >> 15;
zl &= 0x7FFF;
xw = x[u];
nxw = xw - zl;
cc += nxw >> 31;
nxw &= 0x7FFF;
x[u] = nxw;
tb = MUX(EQ(nxw, mw), tb, GT(nxw, mw));
}
/*
* If we underestimated q, then either cc < hi (one extra bit
* beyond the top array word), or cc == hi and tb is true (no
* extra bit, but the result is not lower than the modulus).
*
* If we overestimated q, then cc > hi.
*/
over = GT(cc, hi);
under = ~over & (tb | LT(cc, hi));
br_i15_add(x, m, over);
br_i15_sub(x, m, under);
}