1 /* 2 * Copyright (c) 2017 Thomas Pornin <pornin@bolet.org> 3 * 4 * Permission is hereby granted, free of charge, to any person obtaining 5 * a copy of this software and associated documentation files (the 6 * "Software"), to deal in the Software without restriction, including 7 * without limitation the rights to use, copy, modify, merge, publish, 8 * distribute, sublicense, and/or sell copies of the Software, and to 9 * permit persons to whom the Software is furnished to do so, subject to 10 * the following conditions: 11 * 12 * The above copyright notice and this permission notice shall be 13 * included in all copies or substantial portions of the Software. 14 * 15 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, 16 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF 17 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND 18 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS 19 * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN 20 * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN 21 * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE 22 * SOFTWARE. 23 */ 24 25 #include "inner.h" 26 27 /* see inner.h */ 28 void 29 br_i15_reduce(uint16_t *x, const uint16_t *a, const uint16_t *m) 30 { 31 uint32_t m_bitlen, a_bitlen; 32 size_t mlen, alen, u; 33 34 m_bitlen = m[0]; 35 mlen = (m_bitlen + 15) >> 4; 36 37 x[0] = m_bitlen; 38 if (m_bitlen == 0) { 39 return; 40 } 41 42 /* 43 * If the source is shorter, then simply copy all words from a[] 44 * and zero out the upper words. 45 */ 46 a_bitlen = a[0]; 47 alen = (a_bitlen + 15) >> 4; 48 if (a_bitlen < m_bitlen) { 49 memcpy(x + 1, a + 1, alen * sizeof *a); 50 for (u = alen; u < mlen; u ++) { 51 x[u + 1] = 0; 52 } 53 return; 54 } 55 56 /* 57 * The source length is at least equal to that of the modulus. 58 * We must thus copy N-1 words, and input the remaining words 59 * one by one. 60 */ 61 memcpy(x + 1, a + 2 + (alen - mlen), (mlen - 1) * sizeof *a); 62 x[mlen] = 0; 63 for (u = 1 + alen - mlen; u > 0; u --) { 64 br_i15_muladd_small(x, a[u], m); 65 } 66 } 67