The *security* of the system does depend only on the secrecy of the keys.

The ability to tap is an inherent *insecurity* which depends fundamentally on
the secrecy of the algorithm.  To spell it out, because so many people seem to
miss the point, if the algorithm were published, anyone could create a chip
which would work correctly with the Skipjack chips but which would not have
its key registered.

I believe there is no technical means of ensuring key escrow without the
government maintaining a secret of some kind.  For example, the secret could be
the private keys of the escrow agencies; their digital signatures would be
required before a key could be used.  In this case, the NSA/NIST/whoever has
decided that protecting the entire algorithm is easier/cheaper/more secure than
other possible protocols.
