
Is the U used in the law enforcement field from the phone which placed the
call, from the unit whose 'start secure session' button was pressed first,
or does each phone transmit its own law enforcement field?  Even assuming
one of the first two choices, the FBI is going to get a fresh N,U for its
own database about every other phone call, eventually accumulating keys for
all the phones used to connect to the line they are monitoring, not just
the 'suspects' key.  (Assuming the ever-thrifty FBI doesn't forget each key
after its wiretap permission has expired.)  Not quite a pyramid, but not
bad, either.