Support getting external HMAC auth context with libsrtp 2.1.0.

webrtc/pc/srtpfilter.cc

Index: webrtc/pc/srtpfilter.cc
diff --git a/webrtc/pc/srtpfilter.cc b/webrtc/pc/srtpfilter.cc
index 549410510311650f302facf75876e67c21988bd3..fa310afdf6a1c0cca24aa42e804389fcaf0736f3 100644
--- a/webrtc/pc/srtpfilter.cc
+++ b/webrtc/pc/srtpfilter.cc
@@ -565,10 +565,21 @@ bool SrtpSession::GetRtpAuthParams(uint8_t** key, int* key_len, int* tag_len) {
   // stream_template will be the reference context for other streams.
   // Let's use it for getting the keys.
   srtp_stream_ctx_t* srtp_context = session_->stream_template;
+#if defined(SRTP_MAX_MKI_LEN)

[joachim, 2017/06/27 21:15:16]

The #define has been introduced in libsrtp 2.1.0

+  // libsrtp 2.1.0
+  if (srtp_context && srtp_context->session_keys &&
+      srtp_context->session_keys->rtp_auth) {
+    external_hmac = reinterpret_cast<ExternalHmacContext*>(
+        srtp_context->session_keys->rtp_auth->state);
+  }
+#else
+  // libsrtp 2.0.0
+  // TODO(jbauch): Remove after switching to libsrtp 2.1.0
   if (srtp_context && srtp_context->rtp_auth) {
     external_hmac = reinterpret_cast<ExternalHmacContext*>(
         srtp_context->rtp_auth->state);
   }
+#endif
 
   if (!external_hmac) {
     LOG(LS_ERROR) << "Failed to get auth keys from libsrtp!.";