Setup SRTP decryption (#12)

.github/workflows/ci.yml

@@ -16,6 +16,7 @@ jobs:
         with:
           otp-version: ${{matrix.otp}}
           elixir-version: ${{matrix.elixir}}
+      - run: sudo apt install -y libsrtp2-dev
       - run: mix deps.get
       - run: mix credo
       - run: mix dialyzer
@@ -37,6 +38,7 @@ jobs:
         with:
           otp-version: ${{matrix.otp}}
           elixir-version: ${{matrix.elixir}}
+      - run: sudo apt install -y libsrtp2-dev
       - run: mix deps.get
       - run: mix coveralls.json
       - uses: codecov/codecov-action@v3

lib/ex_webrtc/dtls_transport.ex

@@ -14,6 +14,7 @@ defmodule ExWebRTC.DTLSTransport do
     :pkey,
     :fingerprint,
     :mode,
+    :srtp,
     finished: false
   ]
 
@@ -25,11 +26,14 @@ defmodule ExWebRTC.DTLSTransport do
     {:ok, fingerprint} = ExDTLS.get_cert_fingerprint(cert_client)
     :ok = ExDTLS.stop(cert_client)
 
+    srtp = ExLibSRTP.new()
+
     %__MODULE__{
       ice_agent: ice_agent,
       cert: cert,
       pkey: pkey,
-      fingerprint: fingerprint
+      fingerprint: fingerprint,
+      srtp: srtp
     }
   end
 
@@ -93,31 +97,65 @@ defmodule ExWebRTC.DTLSTransport do
     dtls
   end
 
-  def process_data(dtls, data) do
+  def process_data(dtls, <<f, _::binary>> = data) when f in 20..63 and not dtls.finished do
     case ExDTLS.process(dtls.client, data) do
       {:handshake_packets, packets} when dtls.ice_state in [:connected, :completed] ->
         :ok = ICEAgent.send_data(dtls.ice_agent, packets)
-        dtls
+        {:ok, dtls}
 
       {:handshake_packets, packets} ->
         Logger.debug("""
         Generated local DTLS packets but ICE is not in the connected or completed state yet.
         We will send those packets once ICE is ready.
         """)
 
-        %__MODULE__{dtls | buffered_packets: packets}
+        {:ok, %__MODULE__{dtls | buffered_packets: packets}}
 
-      {:handshake_finished, _keying_material, packets} ->
+      {:handshake_finished, keying_material, packets} ->
         Logger.debug("DTLS handshake finished")
         ICEAgent.send_data(dtls.ice_agent, packets)
-        %__MODULE__{dtls | finished: true}
+        # TODO: validate fingerprint
+        dtls = setup_srtp(dtls, keying_material)
+        {:ok, %__MODULE__{dtls | finished: true}}
 
-      {:handshake_finished, _keying_material} ->
+      {:handshake_finished, keying_material} ->
         Logger.debug("DTLS handshake finished")
-        %__MODULE__{dtls | finished: true}
+        dtls = setup_srtp(dtls, keying_material)
+        {:ok, %__MODULE__{dtls | finished: true}}
 
       :handshake_want_read ->
-        dtls
+        {:ok, dtls}
+    end
+  end
+
+  def process_data(dtls, <<f, _::binary>> = data) when f in 128..191 and dtls.finished do
+    case ExLibSRTP.unprotect(dtls.srtp, data) do
+      {:ok, payload} ->
+        {:ok, dtls, payload}
+
+      {:error, _reason} = err ->
+        err
     end
   end
+
+  def process_data(_dtls, _data) do
+    {:error, :invalid_data}
+  end
+
+  defp setup_srtp(dtls, keying_material) do
+    {_local_material, remote_material, profile} = keying_material
+
+    {:ok, crypto_profile} =
+      ExLibSRTP.Policy.crypto_profile_from_dtls_srtp_protection_profile(profile)
+
+    policy = %ExLibSRTP.Policy{
+      ssrc: :any_inbound,
+      key: remote_material,
+      rtp: crypto_profile,
+      rtcp: crypto_profile
+    }
+
+    :ok = ExLibSRTP.add_stream(dtls.srtp, policy)
+    dtls
+  end
 end

lib/ex_webrtc/peer_connection.ex

@@ -339,10 +339,19 @@ defmodule ExWebRTC.PeerConnection do
   end
 
   @impl true
-  def handle_info({:ex_ice, _from, {:data, data}}, state)
-      when not state.dtls_transport.finished do
-    dtls = DTLSTransport.process_data(state.dtls_transport, data)
-    {:noreply, %__MODULE__{state | dtls_transport: dtls}}
+  def handle_info({:ex_ice, _from, {:data, data}}, state) do
+    case DTLSTransport.process_data(state.dtls_transport, data) do
+      {:ok, dtls} ->
+        {:noreply, %__MODULE__{state | dtls_transport: dtls}}
+
+      {:ok, dtls, payload} ->
+        notify(state.owner, {:data, payload})
+        {:noreply, %__MODULE__{state | dtls_transport: dtls}}
+
+      {:error, reason} ->
+        Logger.error("Unable to process data, reason: #{inspect(reason)}")
+        {:noreply, state}
+    end
   end
 
   @impl true

mix.exs

@@ -48,6 +48,7 @@ defmodule ExWebRTC.MixProject do
       {:ex_sdp, "~> 0.13"},
       {:ex_ice, "~> 0.1"},
       {:ex_dtls, "~> 0.13"},
+      {:ex_libsrtp, "~> 0.6"},
 
       # dev/test
       {:excoveralls, "~> 0.14", only: [:dev, :test], runtime: false},

mix.lock

@@ -11,6 +11,7 @@
   "ex_doc": {:hex, :ex_doc, "0.30.6", "5f8b54854b240a2b55c9734c4b1d0dd7bdd41f71a095d42a70445c03cf05a281", [:mix], [{:earmark_parser, "~> 1.4.31", [hex: :earmark_parser, repo: "hexpm", optional: false]}, {:makeup_elixir, "~> 0.14", [hex: :makeup_elixir, repo: "hexpm", optional: false]}, {:makeup_erlang, "~> 0.1", [hex: :makeup_erlang, repo: "hexpm", optional: false]}], "hexpm", "bd48f2ddacf4e482c727f9293d9498e0881597eae6ddc3d9562bd7923375109f"},
   "ex_dtls": {:hex, :ex_dtls, "0.13.0", "4d7631eefc19a8820d4f79883f379ff2ad642976bda55493d4ec4e5d10d6c078", [:mix], [{:unifex, "~> 1.0", [hex: :unifex, repo: "hexpm", optional: false]}], "hexpm", "3ece30967006ec12a4088e60514cb08847814fba8b8a21aca3862e5d1fd4a6bc"},
   "ex_ice": {:hex, :ex_ice, "0.1.0", "2653c884872d8769cf9fc655c74002a63ed6c21be1b3c2badfa42bdc74de2355", [:mix], [{:ex_stun, "~> 0.1.0", [hex: :ex_stun, repo: "hexpm", optional: false]}], "hexpm", "e2539a321f87f31997ba974d532d00511e5828f2f113b550b1ef6aa799dd2ffe"},
+  "ex_libsrtp": {:hex, :ex_libsrtp, "0.6.0", "d96cd7fc1780157614f0bf47d31587e5eab953b43067f4885849f8177ec452a9", [:mix], [{:bunch, "~> 1.3", [hex: :bunch, repo: "hexpm", optional: false]}, {:unifex, "~> 1.0", [hex: :unifex, repo: "hexpm", optional: false]}], "hexpm", "e9ce8a507a658f7e2df72fae82a4b3ba0a056c175f0bc490e79ab03058e094d5"},
   "ex_sdp": {:hex, :ex_sdp, "0.13.0", "b464cf5f6b70433159be243115857599f82b07234ee022997868c85ae1f225f7", [:mix], [{:bunch, "~> 1.3", [hex: :bunch, repo: "hexpm", optional: false]}, {:uuid, "~> 1.1", [hex: :uuid, repo: "hexpm", optional: false]}], "hexpm", "975ca4d274240c51ee85909bc0618bd4dd940e69f7d8c8f0d701f1524eaffaad"},
   "ex_stun": {:hex, :ex_stun, "0.1.0", "252474bf4c8519fbf4bc0fbfc6a1b846a634b1478c65dbbfb4b6ab4e33c2a95a", [:mix], [], "hexpm", "629fc8be45b624a92522f81d85ba001877b1f0745889a2419bdb678790d7480c"},
   "excoveralls": {:hex, :excoveralls, "0.17.1", "83fa7906ef23aa7fc8ad7ee469c357a63b1b3d55dd701ff5b9ce1f72442b2874", [:mix], [{:castore, "~> 1.0", [hex: :castore, repo: "hexpm", optional: true]}, {:jason, "~> 1.0", [hex: :jason, repo: "hexpm", optional: false]}], "hexpm", "95bc6fda953e84c60f14da4a198880336205464e75383ec0f570180567985ae0"},