Imported Upstream version 4.8.0.309

Former-commit-id: 5f9c6ae75f295e057a7d2971f3a6df4656fa8850

external/boringssl/crypto/newhope/reduce.c

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+/* Copyright (c) 2016, Google Inc.
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+ * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+ * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+ * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
+
+#include "internal.h"
+
+
+/* Incomplete-reduction routines; for details on allowed input ranges
+ * and produced output ranges, see the description in the paper:
+ * https://cryptojedi.org/papers/#newhope */
+
+static const uint32_t kQInv = 12287; /* -inverse_mod(p,2^18) */
+static const uint32_t kRLog = 18;
+
+uint16_t newhope_montgomery_reduce(uint32_t a) {
+  uint32_t u;
+
+  u = (a * kQInv);
+  u &= ((1 << kRLog) - 1);
+  u *= PARAM_Q;
+  a = a + u;
+  return a >> 18;
+}
+
+uint16_t newhope_barrett_reduce(uint16_t a) {
+  uint32_t u;
+
+  u = ((uint32_t)a * 5) >> 16;
+  u *= PARAM_Q;
+  a -= u;
+  return a;
+}