_ Git - BearSSL/blob - src/ssl/ssl_rec_gcm.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 /*
  28  * GCM initialisation. This does everything except setting the vtable,
  29  * which depends on whether this is a context for encrypting or for
  30  * decrypting.
  31  */
  32 static void
  33 gen_gcm_init(br_sslrec_gcm_context *cc,
  34         const br_block_ctr_class *bc_impl,
  35         const void *key, size_t key_len,
  36         br_ghash gh_impl,
  37         const void *iv)
  38 {
  39         unsigned char tmp[12];
  40
  41         cc->seq = 0;
  42         bc_impl->init(&cc->bc.vtable, key, key_len);
  43         cc->gh = gh_impl;
  44         memcpy(cc->iv, iv, sizeof cc->iv);
  45         memset(cc->h, 0, sizeof cc->h);
  46         memset(tmp, 0, sizeof tmp);
  47         bc_impl->run(&cc->bc.vtable, tmp, 0, cc->h, sizeof cc->h);
  48 }
  49
  50 static void
  51 in_gcm_init(br_sslrec_gcm_context *cc,
  52         const br_block_ctr_class *bc_impl,
  53         const void *key, size_t key_len,
  54         br_ghash gh_impl,
  55         const void *iv)
  56 {
  57         cc->vtable.in = &br_sslrec_in_gcm_vtable;
  58         gen_gcm_init(cc, bc_impl, key, key_len, gh_impl, iv);
  59 }
  60
  61 static int
  62 gcm_check_length(const br_sslrec_gcm_context *cc, size_t rlen)
  63 {
  64         /*
  65          * GCM adds a fixed overhead:
  66          *   8 bytes for the nonce_explicit (before the ciphertext)
  67          *  16 bytes for the authentication tag (after the ciphertext)
  68          */
  69         (void)cc;
  70         return rlen >= 24 && rlen <= (16384 + 24);
  71 }
  72
  73 /*
  74  * Compute the authentication tag. The value written in 'tag' must still
  75  * be CTR-encrypted.
  76  */
  77 static void
  78 do_tag(br_sslrec_gcm_context *cc,
  79         int record_type, unsigned version,
  80         void *data, size_t len, void *tag)
  81 {
  82         unsigned char header[13];
  83         unsigned char footer[16];
  84
  85         /*
  86          * Compute authentication tag. Three elements must be injected in
  87          * sequence, each possibly 0-padded to reach a length multiple
  88          * of the block size: the 13-byte header (sequence number, record
  89          * type, protocol version, record length), the cipher text, and
  90          * the word containing the encodings of the bit lengths of the two
  91          * other elements.
  92          */
  93         br_enc64be(header, cc->seq ++);
  94         header[8] = (unsigned char)record_type;
  95         br_enc16be(header + 9, version);
  96         br_enc16be(header + 11, len);
  97         br_enc64be(footer, (uint64_t)(sizeof header) << 3);
  98         br_enc64be(footer + 8, (uint64_t)len << 3);
  99         memset(tag, 0, 16);
 100         cc->gh(tag, cc->h, header, sizeof header);
 101         cc->gh(tag, cc->h, data, len);
 102         cc->gh(tag, cc->h, footer, sizeof footer);
 103 }
 104
 105 /*
 106  * Do CTR encryption. This also does CTR encryption of a single block at
 107  * address 'xortag' with the counter value appropriate for the final
 108  * processing of the authentication tag.
 109  */
 110 static void
 111 do_ctr(br_sslrec_gcm_context *cc, const void *nonce, void *data, size_t len,
 112         void *xortag)
 113 {
 114         unsigned char iv[12];
 115
 116         memcpy(iv, cc->iv, 4);
 117         memcpy(iv + 4, nonce, 8);
 118         cc->bc.vtable->run(&cc->bc.vtable, iv, 2, data, len);
 119         cc->bc.vtable->run(&cc->bc.vtable, iv, 1, xortag, 16);
 120 }
 121
 122 static unsigned char *
 123 gcm_decrypt(br_sslrec_gcm_context *cc,
 124         int record_type, unsigned version, void *data, size_t *data_len)
 125 {
 126         unsigned char *buf;
 127         size_t len, u;
 128         uint32_t bad;
 129         unsigned char tag[16];
 130
 131         buf = (unsigned char *)data + 8;
 132         len = *data_len - 24;
 133         do_tag(cc, record_type, version, buf, len, tag);
 134         do_ctr(cc, data, buf, len, tag);
 135
 136         /*
 137          * Compare the computed tag with the value from the record. It
 138          * is possibly useless to do a constant-time comparison here,
 139          * but it does not hurt.
 140          */
 141         bad = 0;
 142         for (u = 0; u < 16; u ++) {
 143                 bad |= tag[u] ^ buf[len + u];
 144         }
 145         if (bad) {
 146                 return NULL;
 147         }
 148         *data_len = len;
 149         return buf;
 150 }
 151
 152 /* see bearssl_ssl.h */
 153 const br_sslrec_in_gcm_class br_sslrec_in_gcm_vtable = {
 154         {
 155                 sizeof(br_sslrec_gcm_context),
 156                 (int (*)(const br_sslrec_in_class *const *, size_t))
 157                         &gcm_check_length,
 158                 (unsigned char *(*)(const br_sslrec_in_class **,
 159                         int, unsigned, void *, size_t *))
 160                         &gcm_decrypt
 161         },
 162         (void (*)(const br_sslrec_in_gcm_class **,
 163                 const br_block_ctr_class *, const void *, size_t,
 164                 br_ghash, const void *))
 165                 &in_gcm_init
 166 };
 167
 168 static void
 169 out_gcm_init(br_sslrec_gcm_context *cc,
 170         const br_block_ctr_class *bc_impl,
 171         const void *key, size_t key_len,
 172         br_ghash gh_impl,
 173         const void *iv)
 174 {
 175         cc->vtable.out = &br_sslrec_out_gcm_vtable;
 176         gen_gcm_init(cc, bc_impl, key, key_len, gh_impl, iv);
 177 }
 178
 179 static void
 180 gcm_max_plaintext(const br_sslrec_gcm_context *cc,
 181         size_t *start, size_t *end)
 182 {
 183         size_t len;
 184
 185         (void)cc;
 186         *start += 8;
 187         len = *end - *start - 16;
 188         if (len > 16384) {
 189                 len = 16384;
 190         }
 191         *end = *start + len;
 192 }
 193
 194 static unsigned char *
 195 gcm_encrypt(br_sslrec_gcm_context *cc,
 196         int record_type, unsigned version, void *data, size_t *data_len)
 197 {
 198         unsigned char *buf;
 199         size_t u, len;
 200         unsigned char tmp[16];
 201
 202         buf = (unsigned char *)data;
 203         len = *data_len;
 204         memset(tmp, 0, sizeof tmp);
 205         br_enc64be(buf - 8, cc->seq);
 206         do_ctr(cc, buf - 8, buf, len, tmp);
 207         do_tag(cc, record_type, version, buf, len, buf + len);
 208         for (u = 0; u < 16; u ++) {
 209                 buf[len + u] ^= tmp[u];
 210         }
 211         len += 24;
 212         buf -= 13;
 213         buf[0] = (unsigned char)record_type;
 214         br_enc16be(buf + 1, version);
 215         br_enc16be(buf + 3, len);
 216         *data_len = len + 5;
 217         return buf;
 218 }
 219
 220 /* see bearssl_ssl.h */
 221 const br_sslrec_out_gcm_class br_sslrec_out_gcm_vtable = {
 222         {
 223                 sizeof(br_sslrec_gcm_context),
 224                 (void (*)(const br_sslrec_out_class *const *,
 225                         size_t *, size_t *))
 226                         &gcm_max_plaintext,
 227                 (unsigned char *(*)(const br_sslrec_out_class **,
 228                         int, unsigned, void *, size_t *))
 229                         &gcm_encrypt
 230         },
 231         (void (*)(const br_sslrec_out_gcm_class **,
 232                 const br_block_ctr_class *, const void *, size_t,
 233                 br_ghash, const void *))
 234                 &out_gcm_init
 235 };