_ Git - BearSSL/blob - src/aead/gcm.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  * Implementation Notes
  29  * ====================
  30  *
  31  * Since CTR and GHASH implementations can handle only full blocks, a
  32  * 16-byte buffer (buf[]) is maintained in the context:
  33  *
  34  *  - When processing AAD, buf[] contains the 0-15 unprocessed bytes.
  35  *
  36  *  - When doing CTR encryption / decryption, buf[] contains the AES output
  37  *    for the last partial block, to be used with the next few bytes of
  38  *    data, as well as the already encrypted bytes. For instance, if the
  39  *    processed data length so far is 21 bytes, then buf[0..4] contains
  40  *    the five last encrypted bytes, and buf[5..15] contains the next 11
  41  *    AES output bytes to be XORed with the next 11 bytes of input.
  42  *
  43  *    The recorded AES output bytes are used to complete the block when
  44  *    the corresponding bytes are obtained. Note that buf[] always
  45  *    contains the _encrypted_ bytes, whether we apply encryption or
  46  *    decryption: these bytes are used as input to GHASH when the block
  47  *    is complete.
  48  *
  49  * In both cases, the low bits of the data length counters (count_aad,
  50  * count_ctr) are used to work out the current situation.
  51  */
  52
  53 /* see bearssl_aead.h */
  54 void
  55 br_gcm_init(br_gcm_context *ctx, const br_block_ctr_class **bctx, br_ghash gh)
  56 {
  57         unsigned char iv[12];
  58
  59         ctx->bctx = bctx;
  60         ctx->gh = gh;
  61
  62         /*
  63          * The GHASH key h[] is the raw encryption of the all-zero
  64          * block. Since we only have a CTR implementation, we use it
  65          * with an all-zero IV and a zero counter, to CTR-encrypt an
  66          * all-zero block.
  67          */
  68         memset(ctx->h, 0, sizeof ctx->h);
  69         memset(iv, 0, sizeof iv);
  70         (*bctx)->run(bctx, iv, 0, ctx->h, sizeof ctx->h);
  71 }
  72
  73 /* see bearssl_aead.h */
  74 void
  75 br_gcm_reset(br_gcm_context *ctx, const void *iv, size_t len)
  76 {
  77         /*
  78          * If the provided nonce is 12 bytes, then this is the initial
  79          * IV for CTR mode; it will be used with a counter that starts
  80          * at 2 (value 1 is for encrypting the GHASH output into the tag).
  81          *
  82          * If the provided nonce has any other length, then it is hashed
  83          * (with GHASH) into a 16-byte value that will be the IV for CTR
  84          * (both 12-byte IV and 32-bit counter).
  85          */
  86         if (len == 12) {
  87                 memcpy(ctx->j0_1, iv, 12);
  88                 ctx->j0_2 = 1;
  89         } else {
  90                 unsigned char ty[16], tmp[16];
  91
  92                 memset(ty, 0, sizeof ty);
  93                 ctx->gh(ty, ctx->h, iv, len);
  94                 memset(tmp, 0, 8);
  95                 br_enc64be(tmp + 8, (uint64_t)len << 3);
  96                 ctx->gh(ty, ctx->h, tmp, 16);
  97                 memcpy(ctx->j0_1, ty, 12);
  98                 ctx->j0_2 = br_dec32be(ty + 12);
  99         }
 100         ctx->jc = ctx->j0_2 + 1;
 101         memset(ctx->y, 0, sizeof ctx->y);
 102         ctx->count_aad = 0;
 103         ctx->count_ctr = 0;
 104 }
 105
 106 /* see bearssl_aead.h */
 107 void
 108 br_gcm_aad_inject(br_gcm_context *ctx, const void *data, size_t len)
 109 {
 110         size_t ptr, dlen;
 111
 112         ptr = (size_t)ctx->count_aad & (size_t)15;
 113         if (ptr != 0) {
 114                 /*
 115                  * If there is a partial block, then we first try to
 116                  * complete it.
 117                  */
 118                 size_t clen;
 119
 120                 clen = 16 - ptr;
 121                 if (len < clen) {
 122                         memcpy(ctx->buf + ptr, data, len);
 123                         ctx->count_aad += (uint64_t)len;
 124                         return;
 125                 }
 126                 memcpy(ctx->buf + ptr, data, clen);
 127                 ctx->gh(ctx->y, ctx->h, ctx->buf, 16);
 128                 data = (const unsigned char *)data + clen;
 129                 len -= clen;
 130                 ctx->count_aad += (uint64_t)clen;
 131         }
 132
 133         /*
 134          * Now AAD is aligned on a 16-byte block (with regards to GHASH).
 135          * We process all complete blocks, and save the last partial
 136          * block.
 137          */
 138         dlen = len & ~(size_t)15;
 139         ctx->gh(ctx->y, ctx->h, data, dlen);
 140         memcpy(ctx->buf, (const unsigned char *)data + dlen, len - dlen);
 141         ctx->count_aad += (uint64_t)len;
 142 }
 143
 144 /* see bearssl_aead.h */
 145 void
 146 br_gcm_flip(br_gcm_context *ctx)
 147 {
 148         /*
 149          * We complete the GHASH computation if there is a partial block.
 150          * The GHASH implementation automatically applies padding with
 151          * zeros.
 152          */
 153         size_t ptr;
 154
 155         ptr = (size_t)ctx->count_aad & (size_t)15;
 156         if (ptr != 0) {
 157                 ctx->gh(ctx->y, ctx->h, ctx->buf, ptr);
 158         }
 159 }
 160
 161 /* see bearssl_aead.h */
 162 void
 163 br_gcm_run(br_gcm_context *ctx, int encrypt, void *data, size_t len)
 164 {
 165         unsigned char *buf;
 166         size_t ptr, dlen;
 167
 168         buf = data;
 169         ptr = (size_t)ctx->count_ctr & (size_t)15;
 170         if (ptr != 0) {
 171                 /*
 172                  * If we have a partial block, then we try to complete it.
 173                  */
 174                 size_t u, clen;
 175
 176                 clen = 16 - ptr;
 177                 if (len < clen) {
 178                         clen = len;
 179                 }
 180                 for (u = 0; u < clen; u ++) {
 181                         unsigned x, y;
 182
 183                         x = buf[u];
 184                         y = x ^ ctx->buf[ptr + u];
 185                         ctx->buf[ptr + u] = encrypt ? y : x;
 186                         buf[u] = y;
 187                 }
 188                 ctx->count_ctr += (uint64_t)clen;
 189                 buf += clen;
 190                 len -= clen;
 191                 if (ptr + clen < 16) {
 192                         return;
 193                 }
 194                 ctx->gh(ctx->y, ctx->h, ctx->buf, 16);
 195         }
 196
 197         /*
 198          * Process full blocks.
 199          */
 200         dlen = len & ~(size_t)15;
 201         if (!encrypt) {
 202                 ctx->gh(ctx->y, ctx->h, buf, dlen);
 203         }
 204         ctx->jc = (*ctx->bctx)->run(ctx->bctx, ctx->j0_1, ctx->jc, buf, dlen);
 205         if (encrypt) {
 206                 ctx->gh(ctx->y, ctx->h, buf, dlen);
 207         }
 208         buf += dlen;
 209         len -= dlen;
 210         ctx->count_ctr += (uint64_t)dlen;
 211
 212         if (len > 0) {
 213                 /*
 214                  * There is a partial block.
 215                  */
 216                 size_t u;
 217
 218                 memset(ctx->buf, 0, sizeof ctx->buf);
 219                 ctx->jc = (*ctx->bctx)->run(ctx->bctx, ctx->j0_1,
 220                         ctx->jc, ctx->buf, 16);
 221                 for (u = 0; u < len; u ++) {
 222                         unsigned x, y;
 223
 224                         x = buf[u];
 225                         y = x ^ ctx->buf[u];
 226                         ctx->buf[u] = encrypt ? y : x;
 227                         buf[u] = y;
 228                 }
 229                 ctx->count_ctr += (uint64_t)len;
 230         }
 231 }
 232
 233 /* see bearssl_aead.h */
 234 void
 235 br_gcm_get_tag(br_gcm_context *ctx, void *tag)
 236 {
 237         size_t ptr;
 238         unsigned char tmp[16];
 239
 240         ptr = (size_t)ctx->count_ctr & (size_t)15;
 241         if (ptr > 0) {
 242                 /*
 243                  * There is a partial block: encrypted/decrypted data has
 244                  * been produced, but the encrypted bytes must still be
 245                  * processed by GHASH.
 246                  */
 247                 ctx->gh(ctx->y, ctx->h, ctx->buf, ptr);
 248         }
 249
 250         /*
 251          * Final block for GHASH: the AAD and plaintext lengths (in bits).
 252          */
 253         br_enc64be(tmp, ctx->count_aad << 3);
 254         br_enc64be(tmp + 8, ctx->count_ctr << 3);
 255         ctx->gh(ctx->y, ctx->h, tmp, 16);
 256
 257         /*
 258          * Tag is the GHASH output XORed with the encryption of the
 259          * nonce with the initial counter value.
 260          */
 261         memcpy(tag, ctx->y, 16);
 262         (*ctx->bctx)->run(ctx->bctx, ctx->j0_1, ctx->j0_2, tag, 16);
 263 }
 264
 265 /* see bearssl_aead.h */
 266 uint32_t
 267 br_gcm_check_tag(br_gcm_context *ctx, const void *tag)
 268 {
 269         unsigned char tmp[16];
 270         size_t u;
 271         int x;
 272
 273         br_gcm_get_tag(ctx, tmp);
 274         x = 0;
 275         for (u = 0; u < sizeof tmp; u ++) {
 276                 x |= tmp[u] ^ ((const unsigned char *)tag)[u];
 277         }
 278         return EQ0(x);
 279 }
 280
 281 /* see bearssl_aead.h */
 282 const br_aead_class br_gcm_vtable = {
 283         16,
 284         (void (*)(const br_aead_class **, const void *, size_t))
 285                 &br_gcm_reset,
 286         (void (*)(const br_aead_class **, const void *, size_t))
 287                 &br_gcm_aad_inject,
 288         (void (*)(const br_aead_class **))
 289                 &br_gcm_flip,
 290         (void (*)(const br_aead_class **, int, void *, size_t))
 291                 &br_gcm_run,
 292         (void (*)(const br_aead_class **, void *))
 293                 &br_gcm_get_tag,
 294         (uint32_t (*)(const br_aead_class **, const void *))
 295                 &br_gcm_check_tag
 296 };