Fix RTP header extension encryption

Previously, RTP header extensions with encryption had been filtered
if the encryption had been activated (not the other way around) which
was likely an unintended logic inversion.

In addition, it ensures that encrypted RTP header extensions are only
negotiated if RTP header extension encryption is turned on. Formerly,
which extensions had been negotiated depended on the order in which
they were inserted, regardless of whether or not header encryption was
actually enabled, leading to no extensions being sent on the wire.

Further changes:

- If RTP header encryption enabled, prefer encrypted extensions over
  non-encrypted extensions
- Add most extensions to list of extensions supported for encryption
- Discard encrypted extensions in a session description in case encryption
  is not supported for that extension

Note that this depends on https://github.com/cisco/libsrtp/pull/491 to get
into libwebrtc (cherry-pick or bump libsrtp version). Otherwise, two-byte
header extensions will prevent any RTP packets being sent/received.

Bug: webrtc:11713
Change-Id: Ia0779453d342fa11e06996d9bc2d3c826f3466d3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/177980
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Taylor <deadbeef@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33723}

api/rtp_parameters.cc

@@ -170,63 +170,115 @@ bool RtpExtension::IsSupportedForVideo(absl::string_view uri) {
 }
 
 bool RtpExtension::IsEncryptionSupported(absl::string_view uri) {
-  return uri == webrtc::RtpExtension::kAudioLevelUri ||
-         uri == webrtc::RtpExtension::kTimestampOffsetUri ||
-#if !defined(ENABLE_EXTERNAL_AUTH)
-         // TODO(jbauch): Figure out a way to always allow "kAbsSendTimeUri"
-         // here and filter out later if external auth is really used in
-         // srtpfilter. External auth is used by Chromium and replaces the
-         // extension header value of "kAbsSendTimeUri", so it must not be
-         // encrypted (which can't be done by Chromium).
-         uri == webrtc::RtpExtension::kAbsSendTimeUri ||
+  return
+#if defined(ENABLE_EXTERNAL_AUTH)
+      // TODO(jbauch): Figure out a way to always allow "kAbsSendTimeUri"
+      // here and filter out later if external auth is really used in
+      // srtpfilter. External auth is used by Chromium and replaces the
+      // extension header value of "kAbsSendTimeUri", so it must not be
+      // encrypted (which can't be done by Chromium).
+      uri != webrtc::RtpExtension::kAbsSendTimeUri &&
 #endif
-         uri == webrtc::RtpExtension::kAbsoluteCaptureTimeUri ||
-         uri == webrtc::RtpExtension::kVideoRotationUri ||
-         uri == webrtc::RtpExtension::kTransportSequenceNumberUri ||
-         uri == webrtc::RtpExtension::kTransportSequenceNumberV2Uri ||
-         uri == webrtc::RtpExtension::kPlayoutDelayUri ||
-         uri == webrtc::RtpExtension::kVideoContentTypeUri ||
-         uri == webrtc::RtpExtension::kMidUri ||
-         uri == webrtc::RtpExtension::kRidUri ||
-         uri == webrtc::RtpExtension::kRepairedRidUri ||
-         uri == webrtc::RtpExtension::kVideoLayersAllocationUri;
+      uri != webrtc::RtpExtension::kEncryptHeaderExtensionsUri;
 }
 
-const RtpExtension* RtpExtension::FindHeaderExtensionByUri(
+// Returns whether a header extension with the given URI exists.
+// Note: This does not differentiate between encrypted and non-encrypted
+// extensions, so use with care!
+static bool HeaderExtensionWithUriExists(
     const std::vector<RtpExtension>& extensions,
     absl::string_view uri) {
   for (const auto& extension : extensions) {
     if (extension.uri == uri) {
+      return true;
+    }
+  }
+  return false;
+}
+
+const RtpExtension* RtpExtension::FindHeaderExtensionByUri(
+    const std::vector<RtpExtension>& extensions,
+    absl::string_view uri,
+    Filter filter) {
+  const webrtc::RtpExtension* fallback_extension = nullptr;
+  for (const auto& extension : extensions) {
+    if (extension.uri != uri) {
+      continue;
+    }
+
+    switch (filter) {
+      case kDiscardEncryptedExtension:
+        // We only accept an unencrypted extension.
+        if (!extension.encrypt) {
+          return &extension;
+        }
+        break;
+
+      case kPreferEncryptedExtension:
+        // We prefer an encrypted extension but we can fall back to an
+        // unencrypted extension.
+        if (extension.encrypt) {
+          return &extension;
+        } else {
+          fallback_extension = &extension;
+        }
+        break;
+
+      case kRequireEncryptedExtension:
+        // We only accept an encrypted extension.
+        if (extension.encrypt) {
+          return &extension;
+        }
+        break;
+    }
+  }
+
+  // Returning fallback extension (if any)
+  return fallback_extension;
+}
+
+const RtpExtension* RtpExtension::FindHeaderExtensionByUriAndEncryption(
+    const std::vector<RtpExtension>& extensions,
+    absl::string_view uri,
+    bool encrypt) {
+  for (const auto& extension : extensions) {
+    if (extension.uri == uri && extension.encrypt == encrypt) {
       return &extension;
     }
   }
   return nullptr;
 }
 
-std::vector<RtpExtension> RtpExtension::FilterDuplicateNonEncrypted(
-    const std::vector<RtpExtension>& extensions) {
+const std::vector<RtpExtension> RtpExtension::DeduplicateHeaderExtensions(
+    const std::vector<RtpExtension>& extensions,
+    Filter filter) {
   std::vector<RtpExtension> filtered;
-  for (auto extension = extensions.begin(); extension != extensions.end();
-       ++extension) {
-    if (extension->encrypt) {
-      filtered.push_back(*extension);
-      continue;
-    }
 
-    // Only add non-encrypted extension if no encrypted with the same URI
-    // is also present...
-    if (std::any_of(extension + 1, extensions.end(),
-                    [&](const RtpExtension& check) {
-                      return extension->uri == check.uri;
-                    })) {
-      continue;
-    }
-
-    // ...and has not been added before.
-    if (!FindHeaderExtensionByUri(filtered, extension->uri)) {
-      filtered.push_back(*extension);
+  // If we do not discard encrypted extensions, add them first
+  if (filter != kDiscardEncryptedExtension) {
+    for (const auto& extension : extensions) {
+      if (!extension.encrypt) {
+        continue;
+      }
+      if (!HeaderExtensionWithUriExists(filtered, extension.uri)) {
+        filtered.push_back(extension);
+      }
     }
   }
+
+  // If we do not require encrypted extensions, add missing, non-encrypted
+  // extensions.
+  if (filter != kRequireEncryptedExtension) {
+    for (const auto& extension : extensions) {
+      if (extension.encrypt) {
+        continue;
+      }
+      if (!HeaderExtensionWithUriExists(filtered, extension.uri)) {
+        filtered.push_back(extension);
+      }
+    }
+  }
+
   return filtered;
 }
 }  // namespace webrtc