	[Prof. Denning's description of SkipJack mostly omitted]

	CHIP STRUCTURE
	The Clipper Chip contains a classified 64-bit block encryption
	algorithm called "Skipjack."  The algorithm uses 80 bit keys (compared
	with 56 for the DES) and has 32 rounds of scrambling (compared with 16
	for the DES).  It supports all 4 DES modes of operation.  Throughput is
	16 Mbits a second. [...]

	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
	The key K and message stream M (i.e., digitized voice) are then
	fed into the Clipper Chip to produce two values:

  	   E[M; K], the encrypted message stream, and 
	   E[E[K; U] + N; F], a law enforcement block.  

Three questions:
1) It looks like each 64 bits of input gives you 4*64 bits of output:
		E[M;K] = 64 bits
		E[K;U] = E[ 80 bits ] = 128 bits
		E[ E[K;U], N ; F ] = E[ 128 + 30 bits ] = 192 bits
   Do you really need to transmit all 256 bits each time,
   or do you only transmit the 192 bits of wiretap block at the beginning?   
   All 256 would be really obnoxious for bandwidth-limited applications
   like cellular phones (or even regular phones over 

2) how do the 4 DES modes interact with the two-part output?
   Do the various feedback modes only apply to the message block,
   or also to the wiretap block?  Or, if the wiretap block is only
   transmitted at the beginning, does it get incorporated into
   everything through feedback modes, but not during ECB mode?

3) Does the Clipper Chip check the wiretap block itself?
   Does the block have to be present at all?
   Since the receiving chip doesn't know the transmitter's U,
   it presumably can't check the validity of E[K;U], so it's 
   limited to checking the *form* of the wiretap block,
   and maybe checking the serial number for reasonableness
   (unless there's some sort of back-door structure that lets
   it recognize a valid E[K;U].)
   
   In that case, can you replace the wiretap block with a DIFFERENT
   wiretap block, presumably an old valid one to avoid attracting attention?
   (The chip won't do it, so you postprocess the output.)
   Regular people can do one with their own serial number and a dummy key;
   paranoid people can use someone else's serial number.

   On the other hand, if I could think of that solution so easily,
   presumably the NSA could too - have they done something to block it,
   like use message encryption that's really E[M; K,U,N] ?


	Thanks!