From Denning:     the Skipjack encryption algorithm    F, an 80-bit family key that is common to all chips    N, a 30-bit serial number    U, an 80-bit secret key that unlocks all messages encrypted with the chip     E[M; K], the encrypted message stream, and     E[E[K; U] + N; F], a law enforcement block.    Where the session key is K, and is transmitted encrypted in the unit Key U. Which along with the serial number N is encrypted in the Family key F.  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 function of this spoof 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 family key  	2) Receive Channel  	    The spoof chip XORs the incoming encrypted serial number 	    with a secondary keying variable (assuming integrity of the 	    law enforcement block is necessary for local operation - 	    checksums, sequence control, etc.).  This has the net result of hiding the serial number.  It is probable theere is a known plaintext pattern used as a filler in the block containing N (34 bits as used in generating U, U1,U2) correctness of the law enforcement block can be determined with only the family key F.  Whereas, no one has proposed Federal Agencies be denied F, and because they could recover it themselves, The correctness of the serial number can be tested by examining the pad bits of N in E[N; F].  The one could selectively alter the law enforcement block as above, but the mutilation could be detected.  A better approach would be to mutilate the entire law enforcement block.  If it were done with a group encryption scheme such as DES or (presumably) Skipjack, the chances the law enforcement block can be recovered are lessened.  What do you want to bet the transmission protocol can be recognized and the serial numbers decrypted in a target search?  When digital transmission becomes widely available, would there be a requirement that clipper protocol transmissions be refused when containing mutilated law enforcement blocks?  One way to avoid notice, would be to spoof protocol information of the block containing M, as well as spoofing the law enforcement block.  The goal is to use a secure communications scheme, without redress to  detection or key K interception (contained encrypted within the law enforcement block).  The data stream is returned to its original state for use by the clipper chip (or system) if required, for proper operation.  It is somewhat improbable that the entire protocol will be contained within the clipper chip, yet likely that sequence of events will be tested for, requiring a valid law enforcement block to be received before accepting and decrypting E(M; K);  The spoof chip could be implemented anywhere in the protocols, including on the resulting serial data stream.  Existing clipper products could be subborned.  After all, they are high security encryption systems right?  Super encipherment/encryption could allow the chip to be used without redress to detection of the use of the chip, or disclosure of the serial number.  Security must be adequate to deny the serial number, which should not be recoverable by other means.  One can see the use of cut outs for procurring clipper phones, or once the number of units is high enough, stealing them.  It would be a mistake on the part of authority, but nice from a point of privacy, if the serial number N were not associated with a particular clipper chip or lot of chips through the manufacturing and  distribution process.  Hopefully the list of known missing or stolen clipper serial numbers N encrypted with F, and the protocols are not  sufficient plaintext to attact the super encrypted clipper stream. This could be further made difficult by altering the temporal and or spatial relationship of the clipper stream to that of the super encrypted stream.  Detection of an encrypted stream could tip off the use of the aforementioned scheme.  ******************************************************************************  If you could capture valid law enforcement blocks not your own, and use them in a codebook sustitution with your own, where they point to a valid law enforcement block stored in a library utilizing a session key matching the remainder of the transmission, you could simply out and out lie, yet deliver to monitoring and/or hostile forces a seemingly valid law enforcement block.   These captured law enforcement blocks would be used as authenticators, such as in a manually keyed encryption system.  Fending this off would require escalation in examining the protocols and blocks in the transmission.  The M code stream  might be independently attacked based on knowledge of clipper chip protocols as revealed plaintext.  This could be invalidated by changing the temporal and or spatial relationship of the clipper M stream and the actual transmitted stream, under the control of a secure key generator synchronized between endpoints.  The useful life time of captured law enforcement blocks might be limited based on hostile forces using them as targets following transmission interception.  You would need a large number of them, but, hey there's supposed to be millions of these things, right?  Adding time stamps to the encrypted law enforcement block is probably impractical, who wants an encryption chip with a real time clock?  *****************************************************************************