Chromecast sender as a PlayReady license provider

Question

Let's assume we have an encrypted stream (SmoothStreaming + Playready) and a custom receiver build on googlecast/cast-custom-receiver.

I can see that device tries to get the license from the LA_URL (license acquisition URL) - extracted from stream PlayReady ProtectionHeader.

I wonder is there a way to override this behaviour? I want receiver to delegate license acquisition to sender - iOS app in my case.

Answer 1

You can achieve this by modifying the Media Player Library. The only thing that needs to be done - exposing challenge, key system, init data & sessionId though prepareLicenseRequest().

<    this.c.prepareLicenseRequest && !this.c.prepareLicenseRequest() || this.gb()
---
>    this.c.prepareLicenseRequest && !this.c.prepareLicenseRequest(this.ef, Df[this.vc], this.qb, this.hf) || this.gb()

Please verify the variable names cause they can be different (they can be found in webkitAddKey() call later in the media_player.js code).

In your receiver just define the your own prepareLicenseRequest implementation:

window.mediaHost.prepareLicenseRequest = function(challenge,keySystem, initData, sessionId) {
  debug('drm', 'prepareLicenseRequest');
  window.initData = initData;
  window.sessionId = sessionId;
  window.keySystem = keySystem;

  var base64challenge = window.btoa(pack(challenge));
  window.messageBus.broadcast(base64challenge); // TODO send only to initiator
  return false // receiver will stop not request license by itself
}                 

When license acquired by sender it can send it back via the same channel. In this case receiver needs to add the license to the env:

window.messageBus.onMessage = function(event) {
    var base64key = event['data'];
    var key = unpack(window.atob(base64key));
    window.mediaElement.webkitAddKey(window.keySystem, new Uint8Array(unpack(unbase64)), window.initData, window.sessionId);
    window.mediaElement.play(); 
}

window.messageBus in this case is a custom channel (urn:x-cast:me.trnl.cast.key)

That's it. It works for us and it's quite fast.