_ Git - BearSSL/blob - src/int/i15_ext2.c

   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 /*
  28  * This file contains some additional functions for "i15" big integers.
  29  * These functions are needed to support RSA.
  30  */
  31
  32 /* see inner.h */
  33 void
  34 br_i15_decode_reduce(uint16_t *x,
  35         const void *src, size_t len, const uint16_t *m)
  36 {
  37         uint32_t m_ebitlen, m_rbitlen;
  38         size_t mblen, k;
  39         const unsigned char *buf;
  40         uint32_t acc;
  41         int acc_len;
  42
  43         /*
  44          * Get the encoded bit length.
  45          */
  46         m_ebitlen = m[0];
  47
  48         /*
  49          * Special case for an invalid (null) modulus.
  50          */
  51         if (m_ebitlen == 0) {
  52                 x[0] = 0;
  53                 return;
  54         }
  55
  56         /*
  57          * Clear the destination.
  58          */
  59         br_i15_zero(x, m_ebitlen);
  60
  61         /*
  62          * First decode directly as many bytes as possible. This requires
  63          * computing the actual bit length.
  64          */
  65         m_rbitlen = m_ebitlen >> 4;
  66         m_rbitlen = (m_ebitlen & 15) + (m_rbitlen << 4) - m_rbitlen;
  67         mblen = (m_rbitlen + 7) >> 3;
  68         k = mblen - 1;
  69         if (k >= len) {
  70                 br_i15_decode(x, src, len);
  71                 x[0] = m_ebitlen;
  72                 return;
  73         }
  74         buf = src;
  75         br_i15_decode(x, buf, k);
  76         x[0] = m_ebitlen;
  77
  78         /*
  79          * Input remaining bytes, using 15-bit words.
  80          */
  81         acc = 0;
  82         acc_len = 0;
  83         while (k < len) {
  84                 uint32_t v;
  85
  86                 v = buf[k ++];
  87                 acc = (acc << 8) | v;
  88                 acc_len += 8;
  89                 if (acc_len >= 15) {
  90                         br_i15_muladd_small(x, acc >> (acc_len - 15), m);
  91                         acc_len -= 15;
  92                         acc &= ~((uint32_t)-1 << acc_len);
  93                 }
  94         }
  95
  96         /*
  97          * We may have some bits accumulated. We then perform a shift to
  98          * be able to inject these bits as a full 15-bit word.
  99          */
 100         if (acc_len != 0) {
 101                 acc = (acc | (x[1] << acc_len)) & 0x7FFF;
 102                 br_i15_rshift(x, 15 - acc_len);
 103                 br_i15_muladd_small(x, acc, m);
 104         }
 105 }
 106
 107 /* see inner.h */
 108 void
 109 br_i15_reduce(uint16_t *x, const uint16_t *a, const uint16_t *m)
 110 {
 111         uint32_t m_bitlen, a_bitlen;
 112         size_t mlen, alen, u;
 113
 114         m_bitlen = m[0];
 115         mlen = (m_bitlen + 15) >> 4;
 116
 117         x[0] = m_bitlen;
 118         if (m_bitlen == 0) {
 119                 return;
 120         }
 121
 122         /*
 123          * If the source is shorter, then simply copy all words from a[]
 124          * and zero out the upper words.
 125          */
 126         a_bitlen = a[0];
 127         alen = (a_bitlen + 15) >> 4;
 128         if (a_bitlen < m_bitlen) {
 129                 memcpy(x + 1, a + 1, alen * sizeof *a);
 130                 for (u = alen; u < mlen; u ++) {
 131                         x[u + 1] = 0;
 132                 }
 133                 return;
 134         }
 135
 136         /*
 137          * The source length is at least equal to that of the modulus.
 138          * We must thus copy N-1 words, and input the remaining words
 139          * one by one.
 140          */
 141         memcpy(x + 1, a + 2 + (alen - mlen), (mlen - 1) * sizeof *a);
 142         x[mlen] = 0;
 143         for (u = 1 + alen - mlen; u > 0; u --) {
 144                 br_i15_muladd_small(x, a[u], m);
 145         }
 146 }
 147
 148 /* see inner.h */
 149 void
 150 br_i15_mulacc(uint16_t *d, const uint16_t *a, const uint16_t *b)
 151 {
 152         size_t alen, blen, u;
 153
 154         alen = (a[0] + 15) >> 4;
 155         blen = (b[0] + 15) >> 4;
 156         d[0] = a[0] + b[0];
 157         for (u = 0; u < blen; u ++) {
 158                 uint32_t f;
 159                 size_t v;
 160                 uint32_t cc;
 161
 162                 f = b[1 + u];
 163                 cc = 0;
 164                 for (v = 0; v < alen; v ++) {
 165                         uint32_t z;
 166
 167                         z = (uint32_t)d[1 + u + v] + MUL15(f, a[1 + v]) + cc;
 168                         cc = z >> 15;
 169                         d[1 + u + v] = z & 0x7FFF;
 170                 }
 171                 d[1 + u + alen] = cc;
 172         }
 173 }