1 /* 2 * Copyright (c) 2016 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_i32_decode_reduce(uint32_t *x, 30 const void *src, size_t len, const uint32_t *m) 31 { 32 uint32_t m_bitlen; 33 size_t mblen, k, q; 34 const unsigned char *buf; 35 36 m_bitlen = m[0]; 37 38 /* 39 * Special case for an invalid modulus. 40 */ 41 if (m_bitlen == 0) { 42 x[0] = 0; 43 return; 44 } 45 46 /* 47 * Clear the destination. 48 */ 49 br_i32_zero(x, m_bitlen); 50 51 /* 52 * First decode directly as many bytes as possible without 53 * reduction, taking care to leave a number of bytes which 54 * is a multiple of 4. 55 */ 56 mblen = (m_bitlen + 7) >> 3; 57 k = mblen - 1; 58 59 /* 60 * Up to k bytes can be safely decoded. 61 */ 62 if (k >= len) { 63 br_i32_decode(x, src, len); 64 x[0] = m_bitlen; 65 return; 66 } 67 68 /* 69 * We want to first inject some bytes with direct decoding, 70 * then extra bytes by whole 32-bit words. First compute 71 * the size that should be injected that way. 72 */ 73 buf = src; 74 q = (len - k + 3) & ~(size_t)3; 75 76 /* 77 * It may happen that this is more than what we already have 78 * (by at most 3 bytes). Such a case may happen only with 79 * a very short modulus. In that case, we must process the first 80 * bytes "manually". 81 */ 82 if (q > len) { 83 int i; 84 uint32_t w; 85 86 w = 0; 87 for (i = 0; i < 4; i ++) { 88 w <<= 8; 89 if (q <= len) { 90 w |= buf[len - q]; 91 } 92 q --; 93 } 94 br_i32_muladd_small(x, w, m); 95 } else { 96 br_i32_decode(x, buf, len - q); 97 x[0] = m_bitlen; 98 } 99 100 /* 101 * At that point, we have exactly q bytes to inject, and q is 102 * a multiple of 4. 103 */ 104 for (k = len - q; k < len; k += 4) { 105 br_i32_muladd_small(x, br_dec32be(buf + k), m); 106 } 107 } 108