_ Git - BearSSL/blob - src/mac/hmac_ct.c

   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 static inline size_t
  28 hash_size(const br_hash_class *dig)
  29 {
  30         return (unsigned)(dig->desc >> BR_HASHDESC_OUT_OFF)
  31                 & BR_HASHDESC_OUT_MASK;
  32 }
  33
  34 static inline size_t
  35 block_size(const br_hash_class *dig)
  36 {
  37         unsigned ls;
  38
  39         ls = (unsigned)(dig->desc >> BR_HASHDESC_LBLEN_OFF)
  40                 & BR_HASHDESC_LBLEN_MASK;
  41         return (size_t)1 << ls;
  42 }
  43
  44 /* see bearssl.h */
  45 size_t
  46 br_hmac_outCT(const br_hmac_context *ctx,
  47         const void *data, size_t len, size_t min_len, size_t max_len,
  48         void *out)
  49 {
  50         /*
  51          * Method implemented here is inspired from the descriptions on:
  52          *    https://www.imperialviolet.org/2013/02/04/luckythirteen.html
  53          *
  54          * Principle: we input bytes one by one. We use a MUX to push
  55          * padding bytes instead of data bytes when appropriate. At each
  56          * block limit, we get the current hash function state: this is
  57          * a potential output, since we handle MD padding ourselves.
  58          *
  59          * be     1 for big-endian, 0 for little-endian
  60          * po     minimal MD padding length
  61          * bs     block size (always a power of 2)
  62          * hlen   hash output size
  63          */
  64
  65         const br_hash_class *dig;
  66         br_hash_compat_context hc;
  67         int be;
  68         uint32_t po, bs;
  69         uint32_t kr, km, kl, kz, u;
  70         uint64_t count, ncount, bit_len;
  71         unsigned char tmp1[64], tmp2[64];
  72         size_t hlen;
  73
  74         /*
  75          * Copy the current hash context.
  76          */
  77         hc = ctx->dig;
  78
  79         /*
  80          * Get function-specific information.
  81          */
  82         dig = hc.vtable;
  83         be = (dig->desc & BR_HASHDESC_MD_PADDING_BE) != 0;
  84         po = 9;
  85         if (dig->desc & BR_HASHDESC_MD_PADDING_128) {
  86                 po += 8;
  87         }
  88         bs = block_size(dig);
  89         hlen = hash_size(dig);
  90
  91         /*
  92          * Get current input length and compute total bit length.
  93          */
  94         count = dig->state(&hc.vtable, tmp1);
  95         bit_len = (count + (uint64_t)len) << 3;
  96
  97         /*
  98          * We can input the blocks that we are sure we will use.
  99          * This offers better performance (no MUX for these blocks)
 100          * and also ensures that the remaining lengths fit on 32 bits.
 101          */
 102         ncount = (count + (uint64_t)min_len) & ~(uint64_t)(bs - 1);
 103         if (ncount > count) {
 104                 size_t zlen;
 105
 106                 zlen = (size_t)(ncount - count);
 107                 dig->update(&hc.vtable, data, zlen);
 108                 data = (const unsigned char *)data + zlen;
 109                 len -= zlen;
 110                 max_len -= zlen;
 111                 count = ncount;
 112         }
 113
 114         /*
 115          * At that point:
 116          * -- 'count' contains the number of bytes already processed
 117          * (in total).
 118          * -- We must input 'len' bytes. 'min_len' is unimportant: we
 119          * used it to know how many full blocks we could process
 120          * directly. Now only len and max_len matter.
 121          *
 122          * We compute kr, kl, kz and km.
 123          *  kr   number of input bytes already in the current block
 124          *  km   index of the first byte after the end of the last padding
 125          *       block, if length is max_len
 126          *  kz   index of the last byte of the actual last padding block
 127          *  kl   index of the start of the encoded length
 128          *
 129          * km, kz and kl are counted from the current offset in the
 130          * input data.
 131          */
 132         kr = (uint32_t)count & (bs - 1);
 133         kz = ((kr + (uint32_t)len + po + bs - 1) & ~(bs - 1)) - 1 - kr;
 134         kl = kz - 7;
 135         km = ((kr + (uint32_t)max_len + po + bs - 1) & ~(bs - 1)) - kr;
 136
 137         /*
 138          * We must now process km bytes. For index u from 0 to km-1:
 139          *   d is from data[] if u < max_len, 0x00 otherwise
 140          *   e is an encoded length byte or 0x00, depending on u
 141          * The tests for d and e need not be constant-time, since
 142          * they relate only to u and max_len, not to the actual length.
 143          *
 144          * Actual input length is then:
 145          *   d      if u < len
 146          *   0x80   if u == len
 147          *   0x00   if u > len and u < kl
 148          *   e      if u >= kl
 149          *
 150          * Hash state is obtained whenever we reach a full block. This
 151          * is the result we want if and only if u == kz.
 152          */
 153         for (u = 0; u < km; u ++) {
 154                 uint32_t v;
 155                 uint32_t d, e, x0, x1;
 156                 unsigned char x[1];
 157
 158                 d = (u < max_len) ? ((const unsigned char *)data)[u] : 0x00;
 159                 v = (kr + u) & (bs - 1);
 160                 if (v >= (bs - 8)) {
 161                         unsigned j;
 162
 163                         j = (v - (bs - 8)) << 3;
 164                         if (be) {
 165                                 e = (uint32_t)(bit_len >> (56 - j));
 166                         } else {
 167                                 e = (uint32_t)(bit_len >> j);
 168                         }
 169                         e &= 0xFF;
 170                 } else {
 171                         e = 0x00;
 172                 }
 173                 x0 = MUX(EQ(u, (uint32_t)len), 0x80, d);
 174                 x1 = MUX(LT(u, kl), 0x00, e);
 175                 x[0] = MUX(LE(u, (uint32_t)len), x0, x1);
 176                 dig->update(&hc.vtable, x, 1);
 177                 if (v == (bs - 1)) {
 178                         dig->state(&hc.vtable, tmp1);
 179                         CCOPY(EQ(u, kz), tmp2, tmp1, hlen);
 180                 }
 181         }
 182
 183         /*
 184          * Inner hash output is in tmp2[]; we finish processing.
 185          */
 186         dig->init(&hc.vtable);
 187         dig->set_state(&hc.vtable, ctx->kso, (uint64_t)bs);
 188         dig->update(&hc.vtable, tmp2, hlen);
 189         dig->out(&hc.vtable, tmp2);
 190         memcpy(out, tmp2, ctx->out_len);
 191         return ctx->out_len;
 192 }