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srtp.c
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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();
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 }
326 
327 #ifdef TEST
328 #include <stdio.h>
329 
330 static const char *aes128_80_key = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmn";
331 
332 static const uint8_t rtp_aes128_80[] = {
333  // RTP header
334  0x80, 0xe0, 0x12, 0x34, 0x12, 0x34, 0x56, 0x78, 0x12, 0x34, 0x56, 0x78,
335  // encrypted payload
336  0x62, 0x69, 0x76, 0xca, 0xc5,
337  // HMAC
338  0xa1, 0xac, 0x1b, 0xb4, 0xa0, 0x1c, 0xd5, 0x49, 0x28, 0x99,
339 };
340 
341 static const uint8_t rtcp_aes128_80[] = {
342  // RTCP header
343  0x81, 0xc9, 0x00, 0x07, 0x12, 0x34, 0x56, 0x78,
344  // encrypted payload
345  0x8a, 0xac, 0xdc, 0xa5, 0x4c, 0xf6, 0x78, 0xa6, 0x62, 0x8f, 0x24, 0xda,
346  0x6c, 0x09, 0x3f, 0xa9, 0x28, 0x7a, 0xb5, 0x7f, 0x1f, 0x0f, 0xc9, 0x35,
347  // RTCP index
348  0x80, 0x00, 0x00, 0x03,
349  // HMAC
350  0xe9, 0x3b, 0xc0, 0x5c, 0x0c, 0x06, 0x9f, 0xab, 0xc0, 0xde,
351 };
352 
353 static const char *aes128_32_key = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmn";
354 
355 static const uint8_t rtp_aes128_32[] = {
356  // RTP header
357  0x80, 0xe0, 0x12, 0x34, 0x12, 0x34, 0x56, 0x78, 0x12, 0x34, 0x56, 0x78,
358  // encrypted payload
359  0x62, 0x69, 0x76, 0xca, 0xc5,
360  // HMAC
361  0xa1, 0xac, 0x1b, 0xb4,
362 };
363 
364 static const uint8_t rtcp_aes128_32[] = {
365  // RTCP header
366  0x81, 0xc9, 0x00, 0x07, 0x12, 0x34, 0x56, 0x78,
367  // encrypted payload
368  0x35, 0xe9, 0xb5, 0xff, 0x0d, 0xd1, 0xde, 0x70, 0x74, 0x10, 0xaa, 0x1b,
369  0xb2, 0x8d, 0xf0, 0x20, 0x02, 0x99, 0x6b, 0x1b, 0x0b, 0xd0, 0x47, 0x34,
370  // RTCP index
371  0x80, 0x00, 0x00, 0x04,
372  // HMAC
373  0x5b, 0xd2, 0xa9, 0x9d,
374 };
375 
376 static const char *aes128_80_32_key = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmn";
377 
378 static const uint8_t rtp_aes128_80_32[] = {
379  // RTP header
380  0x80, 0xe0, 0x12, 0x34, 0x12, 0x34, 0x56, 0x78, 0x12, 0x34, 0x56, 0x78,
381  // encrypted payload
382  0x62, 0x69, 0x76, 0xca, 0xc5,
383  // HMAC
384  0xa1, 0xac, 0x1b, 0xb4,
385 };
386 
387 static const uint8_t rtcp_aes128_80_32[] = {
388  // RTCP header
389  0x81, 0xc9, 0x00, 0x07, 0x12, 0x34, 0x56, 0x78,
390  // encrypted payload
391  0xd6, 0xae, 0xc1, 0x58, 0x63, 0x70, 0xc9, 0x88, 0x66, 0x26, 0x1c, 0x53,
392  0xff, 0x5d, 0x5d, 0x2b, 0x0f, 0x8c, 0x72, 0x3e, 0xc9, 0x1d, 0x43, 0xf9,
393  // RTCP index
394  0x80, 0x00, 0x00, 0x05,
395  // HMAC
396  0x09, 0x16, 0xb4, 0x27, 0x9a, 0xe9, 0x92, 0x26, 0x4e, 0x10,
397 };
398 
399 static void print_data(const uint8_t *buf, int len)
400 {
401  int i;
402  for (i = 0; i < len; i++)
403  printf("%02x", buf[i]);
404  printf("\n");
405 }
406 
407 static int test_decrypt(struct SRTPContext *srtp, const uint8_t *in, int len,
408  uint8_t *out)
409 {
410  memcpy(out, in, len);
411  if (!ff_srtp_decrypt(srtp, out, &len)) {
412  print_data(out, len);
413  return len;
414  } else
415  return -1;
416 }
417 
418 static void test_encrypt(const uint8_t *data, int in_len, const char *suite,
419  const char *key)
420 {
421  struct SRTPContext enc = { 0 }, dec = { 0 };
422  int len;
423  char buf[RTP_MAX_PACKET_LENGTH];
424  ff_srtp_set_crypto(&enc, suite, key);
425  ff_srtp_set_crypto(&dec, suite, key);
426  len = ff_srtp_encrypt(&enc, data, in_len, buf, sizeof(buf));
427  if (!ff_srtp_decrypt(&dec, buf, &len)) {
428  if (len == in_len && !memcmp(buf, data, len))
429  printf("Decrypted content matches input\n");
430  else
431  printf("Decrypted content doesn't match input\n");
432  } else {
433  printf("Decryption failed\n");
434  }
435  ff_srtp_free(&enc);
436  ff_srtp_free(&dec);
437 }
438 
439 int main(void)
440 {
441  static const char *aes128_80_suite = "AES_CM_128_HMAC_SHA1_80";
442  static const char *aes128_32_suite = "AES_CM_128_HMAC_SHA1_32";
443  static const char *aes128_80_32_suite = "SRTP_AES128_CM_HMAC_SHA1_32";
444  static const char *test_key = "abcdefghijklmnopqrstuvwxyz1234567890ABCD";
446  struct SRTPContext srtp = { 0 };
447  int len;
448  ff_srtp_set_crypto(&srtp, aes128_80_suite, aes128_80_key);
449  len = test_decrypt(&srtp, rtp_aes128_80, sizeof(rtp_aes128_80), buf);
450  test_encrypt(buf, len, aes128_80_suite, test_key);
451  test_encrypt(buf, len, aes128_32_suite, test_key);
452  test_encrypt(buf, len, aes128_80_32_suite, test_key);
453  test_decrypt(&srtp, rtcp_aes128_80, sizeof(rtcp_aes128_80), buf);
454  test_encrypt(buf, len, aes128_80_suite, test_key);
455  test_encrypt(buf, len, aes128_32_suite, test_key);
456  test_encrypt(buf, len, aes128_80_32_suite, test_key);
457  ff_srtp_free(&srtp);
458 
459  memset(&srtp, 0, sizeof(srtp)); // Clear the context
460  ff_srtp_set_crypto(&srtp, aes128_32_suite, aes128_32_key);
461  test_decrypt(&srtp, rtp_aes128_32, sizeof(rtp_aes128_32), buf);
462  test_decrypt(&srtp, rtcp_aes128_32, sizeof(rtcp_aes128_32), buf);
463  ff_srtp_free(&srtp);
464 
465  memset(&srtp, 0, sizeof(srtp)); // Clear the context
466  ff_srtp_set_crypto(&srtp, aes128_80_32_suite, aes128_80_32_key);
467  test_decrypt(&srtp, rtp_aes128_80_32, sizeof(rtp_aes128_80_32), buf);
468  test_decrypt(&srtp, rtcp_aes128_80_32, sizeof(rtcp_aes128_80_32), buf);
469  ff_srtp_free(&srtp);
470  return 0;
471 }
472 #endif /* TEST */