WrmHeader

A WRMHEADER ready to be turned into a license challenge.

This wraps ktvine's PlayreadyHeader parse with the two things only a CDM needs: the protocol version to declare, and the ability to check a decrypted content key against the header's advertised checksum.

xml is the document verbatim. A challenge embeds it inside signed bytes, so re-serializing it — even into equivalent XML — would change what the signature covers.