_ Git - BearSSL/blob - src/symcipher/poly1305_i15.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 is a "reference" implementation of Poly1305 that uses the
  29  * generic "i15" code for big integers. It is slow, but it handles all
  30  * big-integer operations with generic code, thereby avoiding most
  31  * tricky situations with carry propagation and modular reduction.
  32  */
  33
  34 /*
  35  * Modulus: 2^130-5.
  36  */
  37 static const uint16_t P1305[] = {
  38         0x008A,
  39         0x7FFB, 0x7FFF, 0x7FFF, 0x7FFF, 0x7FFF, 0x7FFF, 0x7FFF, 0x7FFF, 0x03FF
  40 };
  41
  42 /*
  43  * -p mod 2^15.
  44  */
  45 #define P0I   0x4CCD
  46
  47 /*
  48  * R^2 mod p, for conversion to Montgomery representation (R = 2^135,
  49  * since we use 9 words of 15 bits each, and 15*9 = 135).
  50  */
  51 static const uint16_t R2[] = {
  52         0x008A,
  53         0x6400, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000
  54 };
  55
  56 /*
  57  * Perform the inner processing of blocks for Poly1305. The "r" array
  58  * is in Montgomery representation, while the "a" array is not.
  59  */
  60 static void
  61 poly1305_inner(uint16_t *a, const uint16_t *r, const void *data, size_t len)
  62 {
  63         const unsigned char *buf;
  64
  65         buf = data;
  66         while (len > 0) {
  67                 unsigned char tmp[16], rev[16];
  68                 uint16_t b[10];
  69                 uint32_t ctl;
  70                 int i;
  71
  72                 /*
  73                  * If there is a partial block, right-pad it with zeros.
  74                  */
  75                 if (len < 16) {
  76                         memset(tmp, 0, sizeof tmp);
  77                         memcpy(tmp, buf, len);
  78                         buf = tmp;
  79                         len = 16;
  80                 }
  81
  82                 /*
  83                  * Decode next block and apply the "high bit". Since
  84                  * decoding is little-endian, we must byte-swap the buffer.
  85                  */
  86                 for (i = 0; i < 16; i ++) {
  87                         rev[i] = buf[15 - i];
  88                 }
  89                 br_i15_decode_mod(b, rev, sizeof rev, P1305);
  90                 b[9] |= 0x0100;
  91
  92                 /*
  93                  * Add the accumulator to the decoded block (modular
  94                  * addition).
  95                  */
  96                 ctl = br_i15_add(b, a, 1);
  97                 ctl |= NOT(br_i15_sub(b, P1305, 0));
  98                 br_i15_sub(b, P1305, ctl);
  99
 100                 /*
 101                  * Multiply by r, result is the new accumulator value.
 102                  */
 103                 br_i15_montymul(a, b, r, P1305, P0I);
 104
 105                 buf += 16;
 106                 len -= 16;
 107         }
 108 }
 109
 110 /*
 111  * Byteswap a 16-byte value.
 112  */
 113 static void
 114 byteswap16(unsigned char *buf)
 115 {
 116         int i;
 117
 118         for (i = 0; i < 8; i ++) {
 119                 unsigned x;
 120
 121                 x = buf[i];
 122                 buf[i] = buf[15 - i];
 123                 buf[15 - i] = x;
 124         }
 125 }
 126
 127 /* see bearssl_block.h */
 128 void
 129 br_poly1305_i15_run(const void *key, const void *iv,
 130         void *data, size_t len, const void *aad, size_t aad_len,
 131         void *tag, br_chacha20_run ichacha, int encrypt)
 132 {
 133         unsigned char pkey[32], foot[16];
 134         uint16_t t[10], r[10], acc[10];
 135
 136         /*
 137          * Compute the MAC key. The 'r' value is the first 16 bytes of
 138          * pkey[].
 139          */
 140         memset(pkey, 0, sizeof pkey);
 141         ichacha(key, iv, 0, pkey, sizeof pkey);
 142
 143         /*
 144          * If encrypting, ChaCha20 must run first, followed by Poly1305.
 145          * When decrypting, the operations are reversed.
 146          */
 147         if (encrypt) {
 148                 ichacha(key, iv, 1, data, len);
 149         }
 150
 151         /*
 152          * Run Poly1305. We must process the AAD, then ciphertext, then
 153          * the footer (with the lengths). Note that the AAD and ciphertext
 154          * are meant to be padded with zeros up to the next multiple of 16,
 155          * and the length of the footer is 16 bytes as well.
 156          */
 157
 158         /*
 159          * Apply the "clamping" operation on the encoded 'r' value.
 160          */
 161         pkey[ 3] &= 0x0F;
 162         pkey[ 7] &= 0x0F;
 163         pkey[11] &= 0x0F;
 164         pkey[15] &= 0x0F;
 165         pkey[ 4] &= 0xFC;
 166         pkey[ 8] &= 0xFC;
 167         pkey[12] &= 0xFC;
 168
 169         /*
 170          * Decode the clamped 'r' value. Decoding should use little-endian
 171          * so we must byteswap the value first.
 172          */
 173         byteswap16(pkey);
 174         br_i15_decode_mod(t, pkey, 16, P1305);
 175
 176         /*
 177          * Convert 'r' to Montgomery representation.
 178          */
 179         br_i15_montymul(r, t, R2, P1305, P0I);
 180
 181         /*
 182          * Accumulator is 0.
 183          */
 184         br_i15_zero(acc, 0x8A);
 185
 186         /*
 187          * Process the additional authenticated data, ciphertext, and
 188          * footer in due order.
 189          */
 190         br_enc64le(foot, (uint64_t)aad_len);
 191         br_enc64le(foot + 8, (uint64_t)len);
 192         poly1305_inner(acc, r, aad, aad_len);
 193         poly1305_inner(acc, r, data, len);
 194         poly1305_inner(acc, r, foot, sizeof foot);
 195
 196         /*
 197          * Decode the value 's'. Again, a byteswap is needed.
 198          */
 199         byteswap16(pkey + 16);
 200         br_i15_decode_mod(t, pkey + 16, 16, P1305);
 201
 202         /*
 203          * Add the value 's' to the accumulator. That addition is done
 204          * modulo 2^128, so we just ignore the carry.
 205          */
 206         br_i15_add(acc, t, 1);
 207
 208         /*
 209          * Encode the result (128 low bits) to the tag. Encoding should
 210          * be little-endian.
 211          */
 212         br_i15_encode(tag, 16, acc);
 213         byteswap16(tag);
 214
 215         /*
 216          * If decrypting, then ChaCha20 runs _after_ Poly1305.
 217          */
 218         if (!encrypt) {
 219                 ichacha(key, iv, 1, data, len);
 220         }
 221 }