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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();
  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. }
  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";
  445.     uint8_t buf[RTP_MAX_PACKET_LENGTH];
  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 */
  473.