Secrecy in Clipper Chip

The serial number of the clipper chip is encrypted with the system key and
transmitted as one part of a three part message.

Presumably the protocol can be recovered (if by nothing else, differential
analysis).

Postulate if you will, a chip (or logic) sitting between the clipper chip
and its communications channel.  The purpose of this chip is twofold:
    
	1) Transmit Channel

	    The spoof chip XORs the 30 bit encrypted serial number with
	    a secondary keying variable.  This renders the serial number
	    unrecoverable with just the system key

	2) Recieve Channel

	    The spoof chip XORs the incoming encrypted serial number
	    with a secondary keying variable (assuming the serial number
	    is necessary for local operation).

This has the net result of hiding the serial number.  This gets more
interesting when the number of serial numbers issued becomes large,
making it difficult to distinguish between valid serial numbers and
a spoofed serial number.  Without knowing the system key, you could
lie about the serial number, but risk detection of the lie.

IF you had the system key and the encryption algorithm you could
successfully lie with a lower probability of detection by emulating the
format of your own serial number.