libavformat/srtp.c

   1 /*
   2  * SRTP encryption/decryption
   3  * Copyright (c) 2012 Martin Storsjo
   4  *
   5  * This file is part of FFmpeg.
   6  *
   7  * FFmpeg is free software; you can redistribute it and/or
   8  * modify it under the terms of the GNU Lesser General Public
   9  * License as published by the Free Software Foundation; either
  10  * version 2.1 of the License, or (at your option) any later version.
  11  *
  12  * FFmpeg is distributed in the hope that it will be useful,
  13  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  14  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  15  * Lesser General Public License for more details.
  16  *
  17  * You should have received a copy of the GNU Lesser General Public
  18  * License along with FFmpeg; if not, write to the Free Software
  19  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  20  */
  21
  22 #include "libavutil/base64.h"
  23 #include "libavutil/aes.h"
  24 #include "libavutil/hmac.h"
  25 #include "libavutil/intreadwrite.h"
  26 #include "libavutil/log.h"
  27 #include "rtp.h"
  28 #include "rtpdec.h"
  29 #include "srtp.h"
  30
  31 void ff_srtp_free(struct SRTPContext *s)
  32 {
  33     if (!s)
  34         return;
  35     av_freep(&s->aes);
  36     if (s->hmac)
  37         av_hmac_free(s->hmac);
  38     s->hmac = NULL;
  39 }
  40
  41 static void encrypt_counter(struct AVAES *aes, uint8_t *iv, uint8_t *outbuf,
  42                             int outlen)
  43 {
  44     int i, j, outpos;
  45     for (i = 0, outpos = 0; outpos < outlen; i++) {
  46         uint8_t keystream[16];
  47         AV_WB16(&iv[14], i);
  48         av_aes_crypt(aes, keystream, iv, 1, NULL, 0);
  49         for (j = 0; j < 16 && outpos < outlen; j++, outpos++)
  50             outbuf[outpos] ^= keystream[j];
  51     }
  52 }
  53
  54 static void derive_key(struct AVAES *aes, const uint8_t *salt, int label,
  55                        uint8_t *out, int outlen)
  56 {
  57     uint8_t input[16] = { 0 };
  58     memcpy(input, salt, 14);
  59     // Key derivation rate assumed to be zero
  60     input[14 - 7] ^= label;
  61     memset(out, 0, outlen);
  62     encrypt_counter(aes, input, out, outlen);
  63 }
  64
  65 int ff_srtp_set_crypto(struct SRTPContext *s, const char *suite,
  66                        const char *params)
  67 {
  68     uint8_t buf[30];
  69
  70     ff_srtp_free(s);
  71
  72     // RFC 4568
  73     if (!strcmp(suite, "AES_CM_128_HMAC_SHA1_80") ||
  74         !strcmp(suite, "SRTP_AES128_CM_HMAC_SHA1_80")) {
  75         s->rtp_hmac_size = s->rtcp_hmac_size = 10;
  76     } else if (!strcmp(suite, "AES_CM_128_HMAC_SHA1_32")) {
  77         s->rtp_hmac_size = s->rtcp_hmac_size = 4;
  78     } else if (!strcmp(suite, "SRTP_AES128_CM_HMAC_SHA1_32")) {
  79         // RFC 5764 section 4.1.2
  80         s->rtp_hmac_size  = 4;
  81         s->rtcp_hmac_size = 10;
  82     } else {
  83         av_log(NULL, AV_LOG_WARNING, "SRTP Crypto suite %s not supported\n",
  84                                      suite);
  85         return AVERROR(EINVAL);
  86     }
  87     if (av_base64_decode(buf, params, sizeof(buf)) != sizeof(buf)) {
  88         av_log(NULL, AV_LOG_WARNING, "Incorrect amount of SRTP params\n");
  89         return AVERROR(EINVAL);
  90     }
  91     // MKI and lifetime not handled yet
  92     s->aes  = av_aes_alloc();
  93     s->hmac = av_hmac_alloc(AV_HMAC_SHA1);
  94     if (!s->aes || !s->hmac)
  95         return AVERROR(ENOMEM);
  96     memcpy(s->master_key, buf, 16);
  97     memcpy(s->master_salt, buf + 16, 14);
  98
  99     // RFC 3711
 100     av_aes_init(s->aes, s->master_key, 128, 0);
 101
 102     derive_key(s->aes, s->master_salt, 0x00, s->rtp_key, sizeof(s->rtp_key));
 103     derive_key(s->aes, s->master_salt, 0x02, s->rtp_salt, sizeof(s->rtp_salt));
 104     derive_key(s->aes, s->master_salt, 0x01, s->rtp_auth, sizeof(s->rtp_auth));
 105
 106     derive_key(s->aes, s->master_salt, 0x03, s->rtcp_key, sizeof(s->rtcp_key));
 107     derive_key(s->aes, s->master_salt, 0x05, s->rtcp_salt, sizeof(s->rtcp_salt));
 108     derive_key(s->aes, s->master_salt, 0x04, s->rtcp_auth, sizeof(s->rtcp_auth));
 109     return 0;
 110 }
 111
 112 static void create_iv(uint8_t *iv, const uint8_t *salt, uint64_t index,
 113                       uint32_t ssrc)
 114 {
 115     uint8_t indexbuf[8];
 116     int i;
 117     memset(iv, 0, 16);
 118     AV_WB32(&iv[4], ssrc);
 119     AV_WB64(indexbuf, index);
 120     for (i = 0; i < 8; i++) // index << 16
 121         iv[6 + i] ^= indexbuf[i];
 122     for (i = 0; i < 14; i++)
 123         iv[i] ^= salt[i];
 124 }
 125
 126 int ff_srtp_decrypt(struct SRTPContext *s, uint8_t *buf, int *lenptr)
 127 {
 128     uint8_t iv[16] = { 0 }, hmac[20];
 129     int len = *lenptr;
 130     int av_uninit(seq_largest);
 131     uint32_t ssrc, av_uninit(roc);
 132     uint64_t index;
 133     int rtcp, hmac_size;
 134
 135     // TODO: Missing replay protection
 136
 137     if (len < 2)
 138         return AVERROR_INVALIDDATA;
 139
 140     rtcp = RTP_PT_IS_RTCP(buf[1]);
 141     hmac_size = rtcp ? s->rtcp_hmac_size : s->rtp_hmac_size;
 142
 143     if (len < hmac_size)
 144         return AVERROR_INVALIDDATA;
 145
 146     // Authentication HMAC
 147     av_hmac_init(s->hmac, rtcp ? s->rtcp_auth : s->rtp_auth, sizeof(s->rtp_auth));
 148     // If MKI is used, this should exclude the MKI as well
 149     av_hmac_update(s->hmac, buf, len - hmac_size);
 150
 151     if (!rtcp) {
 152         int seq = AV_RB16(buf + 2);
 153         uint32_t v;
 154         uint8_t rocbuf[4];
 155
 156         // RFC 3711 section 3.3.1, appendix A
 157         seq_largest = s->seq_initialized ? s->seq_largest : seq;
 158         v = roc = s->roc;
 159         if (seq_largest < 32768) {
 160             if (seq - seq_largest > 32768)
 161                 v = roc - 1;
 162         } else {
 163             if (seq_largest - 32768 > seq)
 164                 v = roc + 1;
 165         }
 166         if (v == roc) {
 167             seq_largest = FFMAX(seq_largest, seq);
 168         } else if (v == roc + 1) {
 169             seq_largest = seq;
 170             roc = v;
 171         }
 172         index = seq + (((uint64_t)v) << 16);
 173
 174         AV_WB32(rocbuf, roc);
 175         av_hmac_update(s->hmac, rocbuf, 4);
 176     }
 177
 178     av_hmac_final(s->hmac, hmac, sizeof(hmac));
 179     if (memcmp(hmac, buf + len - hmac_size, hmac_size)) {
 180         av_log(NULL, AV_LOG_WARNING, "HMAC mismatch\n");
 181         return AVERROR_INVALIDDATA;
 182     }
 183
 184     len -= hmac_size;
 185     *lenptr = len;
 186
 187     if (len < 12)
 188         return AVERROR_INVALIDDATA;
 189
 190     if (rtcp) {
 191         uint32_t srtcp_index = AV_RB32(buf + len - 4);
 192         len -= 4;
 193         *lenptr = len;
 194
 195         ssrc = AV_RB32(buf + 4);
 196         index = srtcp_index & 0x7fffffff;
 197
 198         buf += 8;
 199         len -= 8;
 200         if (!(srtcp_index & 0x80000000))
 201             return 0;
 202     } else {
 203         int ext, csrc;
 204         s->seq_initialized = 1;
 205         s->seq_largest     = seq_largest;
 206         s->roc             = roc;
 207
 208         csrc = buf[0] & 0x0f;
 209         ext  = buf[0] & 0x10;
 210         ssrc = AV_RB32(buf + 8);
 211
 212         buf += 12;
 213         len -= 12;
 214
 215         buf += 4 * csrc;
 216         len -= 4 * csrc;
 217         if (len < 0)
 218             return AVERROR_INVALIDDATA;
 219
 220         if (ext) {
 221             if (len < 4)
 222                 return AVERROR_INVALIDDATA;
 223             ext = (AV_RB16(buf + 2) + 1) * 4;
 224             if (len < ext)
 225                 return AVERROR_INVALIDDATA;
 226             len -= ext;
 227             buf += ext;
 228         }
 229     }
 230
 231     create_iv(iv, rtcp ? s->rtcp_salt : s->rtp_salt, index, ssrc);
 232     av_aes_init(s->aes, rtcp ? s->rtcp_key : s->rtp_key, 128, 0);
 233     encrypt_counter(s->aes, iv, buf, len);
 234
 235     return 0;
 236 }
 237
 238 int ff_srtp_encrypt(struct SRTPContext *s, const uint8_t *in, int len,
 239                     uint8_t *out, int outlen)
 240 {
 241     uint8_t iv[16] = { 0 }, hmac[20];
 242     uint64_t index;
 243     uint32_t ssrc;
 244     int rtcp, hmac_size, padding;
 245     uint8_t *buf;
 246
 247     if (len < 8)
 248         return AVERROR_INVALIDDATA;
 249
 250     rtcp = RTP_PT_IS_RTCP(in[1]);
 251     hmac_size = rtcp ? s->rtcp_hmac_size : s->rtp_hmac_size;
 252     padding = hmac_size;
 253     if (rtcp)
 254         padding += 4; // For the RTCP index
 255
 256     if (len + padding > outlen)
 257         return 0;
 258
 259     memcpy(out, in, len);
 260     buf = out;
 261
 262     if (rtcp) {
 263         ssrc = AV_RB32(buf + 4);
 264         index = s->rtcp_index++;
 265
 266         buf += 8;
 267         len -= 8;
 268     } else {
 269         int ext, csrc;
 270         int seq = AV_RB16(buf + 2);
 271
 272         if (len < 12)
 273             return AVERROR_INVALIDDATA;
 274
 275         ssrc = AV_RB32(buf + 8);
 276
 277         if (seq < s->seq_largest)
 278             s->roc++;
 279         s->seq_largest = seq;
 280         index = seq + (((uint64_t)s->roc) << 16);
 281
 282         csrc = buf[0] & 0x0f;
 283         ext = buf[0] & 0x10;
 284
 285         buf += 12;
 286         len -= 12;
 287
 288         buf += 4 * csrc;
 289         len -= 4 * csrc;
 290         if (len < 0)
 291             return AVERROR_INVALIDDATA;
 292
 293         if (ext) {
 294             if (len < 4)
 295                 return AVERROR_INVALIDDATA;
 296             ext = (AV_RB16(buf + 2) + 1) * 4;
 297             if (len < ext)
 298                 return AVERROR_INVALIDDATA;
 299             len -= ext;
 300             buf += ext;
 301         }
 302     }
 303
 304     create_iv(iv, rtcp ? s->rtcp_salt : s->rtp_salt, index, ssrc);
 305     av_aes_init(s->aes, rtcp ? s->rtcp_key : s->rtp_key, 128, 0);
 306     encrypt_counter(s->aes, iv, buf, len);
 307
 308     if (rtcp) {
 309         AV_WB32(buf + len, 0x80000000 | index);
 310         len += 4;
 311     }
 312
 313     av_hmac_init(s->hmac, rtcp ? s->rtcp_auth : s->rtp_auth, sizeof(s->rtp_auth));
 314     av_hmac_update(s->hmac, out, buf + len - out);
 315     if (!rtcp) {
 316         uint8_t rocbuf[4];
 317         AV_WB32(rocbuf, s->roc);
 318         av_hmac_update(s->hmac, rocbuf, 4);
 319     }
 320     av_hmac_final(s->hmac, hmac, sizeof(hmac));
 321
 322     memcpy(buf + len, hmac, hmac_size);
 323     len += hmac_size;
 324     return buf + len - out;
 325 }