295 lines
8.7 KiB
C
295 lines
8.7 KiB
C
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#include "inner.h"
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/*
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* Support functions for signatures (hash-to-point, norm).
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*
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* ==========================(LICENSE BEGIN)============================
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*
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* Copyright (c) 2017-2019 Falcon Project
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*
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* Permission is hereby granted, free of charge, to any person obtaining
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* a copy of this software and associated documentation files (the
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* "Software"), to deal in the Software without restriction, including
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* without limitation the rights to use, copy, modify, merge, publish,
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* distribute, sublicense, and/or sell copies of the Software, and to
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* permit persons to whom the Software is furnished to do so, subject to
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* the following conditions:
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*
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* The above copyright notice and this permission notice shall be
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* included in all copies or substantial portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
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* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
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* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
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* IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
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* CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
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* TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
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* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
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*
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* ===========================(LICENSE END)=============================
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*
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* @author Thomas Pornin <thomas.pornin@nccgroup.com>
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*/
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/* see inner.h */
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void
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PQCLEAN_FALCON512_AVX2_hash_to_point_vartime(
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inner_shake256_context *sc,
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uint16_t *x, unsigned logn) {
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/*
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* This is the straightforward per-the-spec implementation. It
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* is not constant-time, thus it might reveal information on the
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* plaintext (at least, enough to check the plaintext against a
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* list of potential plaintexts) in a scenario where the
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* attacker does not have access to the signature value or to
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* the public key, but knows the nonce (without knowledge of the
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* nonce, the hashed output cannot be matched against potential
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* plaintexts).
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*/
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size_t n;
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n = (size_t)1 << logn;
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while (n > 0) {
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uint8_t buf[2];
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uint32_t w;
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inner_shake256_extract(sc, (void *)buf, sizeof buf);
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w = ((unsigned)buf[0] << 8) | (unsigned)buf[1];
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if (w < 61445) {
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while (w >= 12289) {
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w -= 12289;
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}
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*x ++ = (uint16_t)w;
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n --;
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}
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}
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}
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/* see inner.h */
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void
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PQCLEAN_FALCON512_AVX2_hash_to_point_ct(
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inner_shake256_context *sc,
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uint16_t *x, unsigned logn, uint8_t *tmp) {
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/*
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* Each 16-bit sample is a value in 0..65535. The value is
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* kept if it falls in 0..61444 (because 61445 = 5*12289)
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* and rejected otherwise; thus, each sample has probability
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* about 0.93758 of being selected.
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*
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* We want to oversample enough to be sure that we will
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* have enough values with probability at least 1 - 2^(-256).
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* Depending on degree N, this leads to the following
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* required oversampling:
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*
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* logn n oversampling
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* 1 2 65
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* 2 4 67
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* 3 8 71
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* 4 16 77
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* 5 32 86
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* 6 64 100
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* 7 128 122
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* 8 256 154
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* 9 512 205
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* 10 1024 287
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*
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* If logn >= 7, then the provided temporary buffer is large
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* enough. Otherwise, we use a stack buffer of 63 entries
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* (i.e. 126 bytes) for the values that do not fit in tmp[].
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*/
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static const uint16_t overtab[] = {
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0, /* unused */
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65,
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67,
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71,
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77,
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86,
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100,
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122,
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154,
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205,
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287
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};
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unsigned n, n2, u, m, p, over;
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uint16_t *tt1, tt2[63];
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/*
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* We first generate m 16-bit value. Values 0..n-1 go to x[].
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* Values n..2*n-1 go to tt1[]. Values 2*n and later go to tt2[].
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* We also reduce modulo q the values; rejected values are set
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* to 0xFFFF.
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*/
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n = 1U << logn;
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n2 = n << 1;
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over = overtab[logn];
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m = n + over;
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tt1 = (uint16_t *)tmp;
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for (u = 0; u < m; u ++) {
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uint8_t buf[2];
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uint32_t w, wr;
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inner_shake256_extract(sc, buf, sizeof buf);
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w = ((uint32_t)buf[0] << 8) | (uint32_t)buf[1];
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wr = w - ((uint32_t)24578 & (((w - 24578) >> 31) - 1));
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wr = wr - ((uint32_t)24578 & (((wr - 24578) >> 31) - 1));
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wr = wr - ((uint32_t)12289 & (((wr - 12289) >> 31) - 1));
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wr |= ((w - 61445) >> 31) - 1;
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if (u < n) {
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x[u] = (uint16_t)wr;
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} else if (u < n2) {
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tt1[u - n] = (uint16_t)wr;
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} else {
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tt2[u - n2] = (uint16_t)wr;
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}
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}
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/*
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* Now we must "squeeze out" the invalid values. We do this in
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* a logarithmic sequence of passes; each pass computes where a
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* value should go, and moves it down by 'p' slots if necessary,
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* where 'p' uses an increasing powers-of-two scale. It can be
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* shown that in all cases where the loop decides that a value
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* has to be moved down by p slots, the destination slot is
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* "free" (i.e. contains an invalid value).
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*/
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for (p = 1; p <= over; p <<= 1) {
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unsigned v;
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/*
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* In the loop below:
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*
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* - v contains the index of the final destination of
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* the value; it is recomputed dynamically based on
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* whether values are valid or not.
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*
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* - u is the index of the value we consider ("source");
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* its address is s.
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*
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* - The loop may swap the value with the one at index
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* u-p. The address of the swap destination is d.
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*/
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v = 0;
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for (u = 0; u < m; u ++) {
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uint16_t *s, *d;
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unsigned j, sv, dv, mk;
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if (u < n) {
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s = &x[u];
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} else if (u < n2) {
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s = &tt1[u - n];
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} else {
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s = &tt2[u - n2];
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}
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sv = *s;
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/*
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* The value in sv should ultimately go to
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* address v, i.e. jump back by u-v slots.
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*/
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j = u - v;
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/*
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* We increment v for the next iteration, but
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* only if the source value is valid. The mask
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* 'mk' is -1 if the value is valid, 0 otherwise,
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* so we _subtract_ mk.
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*/
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mk = (sv >> 15) - 1U;
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v -= mk;
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/*
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* In this loop we consider jumps by p slots; if
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* u < p then there is nothing more to do.
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*/
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if (u < p) {
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continue;
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}
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/*
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* Destination for the swap: value at address u-p.
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*/
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if ((u - p) < n) {
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d = &x[u - p];
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} else if ((u - p) < n2) {
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d = &tt1[(u - p) - n];
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} else {
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d = &tt2[(u - p) - n2];
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}
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dv = *d;
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/*
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* The swap should be performed only if the source
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* is valid AND the jump j has its 'p' bit set.
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*/
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mk &= -(((j & p) + 0x1FF) >> 9);
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*s = (uint16_t)(sv ^ (mk & (sv ^ dv)));
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*d = (uint16_t)(dv ^ (mk & (sv ^ dv)));
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}
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}
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}
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/* see inner.h */
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int
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PQCLEAN_FALCON512_AVX2_is_short(
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const int16_t *s1, const int16_t *s2, unsigned logn) {
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/*
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* We use the l2-norm. Code below uses only 32-bit operations to
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* compute the square of the norm with saturation to 2^32-1 if
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* the value exceeds 2^31-1.
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*/
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size_t n, u;
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uint32_t s, ng;
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n = (size_t)1 << logn;
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s = 0;
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ng = 0;
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for (u = 0; u < n; u ++) {
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int32_t z;
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z = s1[u];
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s += (uint32_t)(z * z);
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ng |= s;
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z = s2[u];
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s += (uint32_t)(z * z);
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ng |= s;
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}
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s |= -(ng >> 31);
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/*
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* Acceptance bound on the l2-norm is:
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* 1.2*1.55*sqrt(q)*sqrt(2*N)
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* Value 7085 is floor((1.2^2)*(1.55^2)*2*1024).
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*/
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return s < (((uint32_t)7085 * (uint32_t)12289) >> (10 - logn));
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}
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/* see inner.h */
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int
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PQCLEAN_FALCON512_AVX2_is_short_half(
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uint32_t sqn, const int16_t *s2, unsigned logn) {
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size_t n, u;
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uint32_t ng;
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n = (size_t)1 << logn;
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ng = -(sqn >> 31);
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for (u = 0; u < n; u ++) {
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int32_t z;
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z = s2[u];
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sqn += (uint32_t)(z * z);
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ng |= sqn;
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}
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sqn |= -(ng >> 31);
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/*
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* Acceptance bound on the l2-norm is:
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* 1.2*1.55*sqrt(q)*sqrt(2*N)
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* Value 7085 is floor((1.2^2)*(1.55^2)*2*1024).
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*/
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return sqn < (((uint32_t)7085 * (uint32_t)12289) >> (10 - logn));
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}
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