encryption - WebRTC SRTP解密

Question

我正在尝试构建 SRTP至RTP流转换器，我在获取 Master Key 时遇到问题来自 WebRTC peerconnection我正在创作。
据我了解，使用 DES exchange ，关键是通过 SDP 进行交换交换并显示在 a=crypto field 。所以，这种情况看起来很简单(如果我错了，请纠正我)，但最终没用，因为 WebRTC标准化现在要求不应使用 DES(只有 Chrome 现在支持它，将来可能会被删除)。
对于DTLS SDP 中有指纹字段, 是 certificate desired 的哈希值吗？在 future 的交换中使用？[编辑:在阅读了一些内容后，我认为情况并非如此能够抢到Master Key解码 SRTP 流，但我碰壁了，因为我不知道在哪里看，甚至 100% 确定是否可能。
因此，简而言之，解码 WebRTC 是否可行(无需进入较低的 C++ API 并创建我自己的 SRTP 实现)使用 WebRTC PeerConnection 创建的提要在 Chrome和 FireFox (可能通过从 SDP 交换中收集的信息进行数据包嗅探)？[编辑:令人沮丧的是，似乎无法访问 key 的私有(private)部分(又名主 key )......如果我请更正我错了]

Answer 1

这是一些使用 openssl 和 libsrtp native api 的代码

#define SRTP_MASTER_KEY_KEY_LEN 16
#define SRTP_MASTER_KEY_SALT_LEN 14
static void dtls_srtp_init( struct transport_dtls *dtls )
{

/*
  When SRTP mode is in effect, different keys are used for ordinary
   DTLS record protection and SRTP packet protection.  These keys are
   generated using a TLS exporter [RFC5705] to generate

   2 * (SRTPSecurityParams.master_key_len +
        SRTPSecurityParams.master_salt_len) bytes of data

   which are assigned as shown below.  The per-association context value
   is empty.

   client_write_SRTP_master_key[SRTPSecurityParams.master_key_len];
   server_write_SRTP_master_key[SRTPSecurityParams.master_key_len];
   client_write_SRTP_master_salt[SRTPSecurityParams.master_salt_len];
   server_write_SRTP_master_salt[SRTPSecurityParams.master_salt_len];
*/
  int code;
  err_status_t     err;
  srtp_policy_t policy;
  char dtls_buffer[SRTP_MASTER_KEY_KEY_LEN * 2 + SRTP_MASTER_KEY_SALT_LEN * 2];
  char client_write_key[SRTP_MASTER_KEY_KEY_LEN + SRTP_MASTER_KEY_SALT_LEN];
  char server_write_key[SRTP_MASTER_KEY_KEY_LEN + SRTP_MASTER_KEY_SALT_LEN];
  size_t offset = 0;

  /*
   The exporter label for this usage is "EXTRACTOR-dtls_srtp".  (The
   "EXTRACTOR" prefix is for historical compatibility.)
   RFC 5764 4.2.  Key Derivation
  */
  const char * label = "EXTRACTOR-dtls_srtp";

  SRTP_PROTECTION_PROFILE * srtp_profile= SSL_get_selected_srtp_profile( dtls->ssl );

/* SSL_export_keying_material exports a value derived from the master secret,
 * as specified in RFC 5705. It writes |olen| bytes to |out| given a label and
 * optional context. (Since a zero length context is allowed, the |use_context|
 * flag controls whether a context is included.)
 *
 * It returns 1 on success and zero otherwise.
 */
  code = SSL_export_keying_material(dtls->ssl, 
                                    dtls_buffer, 
                                    sizeof(dtls_buffer),
                                    label, 
                                    strlen( label),
                                    NULL,
                                    0, 
                                    PJ_FALSE);

  memcpy(&client_write_key[0], &dtls_buffer[offset], SRTP_MASTER_KEY_KEY_LEN);
  offset += SRTP_MASTER_KEY_KEY_LEN;
  memcpy(&server_write_key[0], &dtls_buffer[offset], SRTP_MASTER_KEY_KEY_LEN);
  offset += SRTP_MASTER_KEY_KEY_LEN;
  memcpy(&client_write_key[SRTP_MASTER_KEY_KEY_LEN], &dtls_buffer[offset], SRTP_MASTER_KEY_SALT_LEN);
  offset += SRTP_MASTER_KEY_SALT_LEN;
  memcpy(&server_write_key[SRTP_MASTER_KEY_KEY_LEN], &dtls_buffer[offset], SRTP_MASTER_KEY_SALT_LEN); 

  switch( srtp_profile->id )
  {
  case SRTP_AES128_CM_SHA1_80:
    crypto_policy_set_aes_cm_128_hmac_sha1_80(&policy.rtp);
    crypto_policy_set_aes_cm_128_hmac_sha1_80(&policy.rtcp);
    break;
  case SRTP_AES128_CM_SHA1_32:
    crypto_policy_set_aes_cm_128_hmac_sha1_32(&policy.rtp);   // rtp is 32,
    crypto_policy_set_aes_cm_128_hmac_sha1_80(&policy.rtcp);  // rtcp still 80
    break;
  default:
    assert(0);
  }
  policy.ssrc.value = 0;
  policy.next = NULL;

  /* Init transmit direction */
  policy.ssrc.type = ssrc_any_outbound;  
  policy.key = client_write_key;    

  err = srtp_create(&dtls->srtp_ctx_rx, &policy);
  if (err != err_status_ok) {
    printf("not working\n");
  } 

  /* Init receive direction */
  policy.ssrc.type = ssrc_any_inbound;  
  policy.key = server_write_key;    

  err = srtp_create(&dtls->srtp_ctx_tx, &policy);
  if (err != err_status_ok) {
    printf("not working\n");
  } 

}