Interpret a block's nulldata back into a SHA256.  The first three bytes (2 magic + 1 opcode)
    will not be present in bin_payload.

def parse(bin_payload):    
    """
    Interpret a block's nulldata back into a SHA256.  The first three bytes (2 magic + 1 opcode)
    will not be present in bin_payload.
    """
    
    message_hash = hexlify(bin_payload)
    if not is_hex( message_hash ):
        log.warning("Not a message hash")
        return None 

    if len(message_hash) != 40:
        log.warning("Not a 160-bit hash")
        return None 

    return {
       'opcode': 'ANNOUNCE',
       'message_hash': message_hash
    }