The Clipper chip contains an encryption algorithm called Skipjack (see
Question 3.6.5), whose details have not been made public. Each
chip also contains a unique 80-bit unit key U, which is escrowed in two
parts at two escrow agencies; both parts must be known in order to recover
the key. Also present is a serial number and an 80-bit ``family key'' F;
the latter is common to all Clipper chips. The chip is manufactured so that
it cannot be reverse engineered; this means that the Skipjack algorithm and
the keys cannot be read off the chip.
When two devices wish to communicate, they first agree on an
80-bit``session key'' K. The method by which they choose this key is
left up to the implementer's discretion; a public-key method such as
RSA or Diffie-Hellman seems a likely choice. The message is encrypted
withthe key K and sent; note that the key K is not escrowed. In
addition to the encrypted message, another piece of data, called the
law-enforcement access field (LEAF), is created and sent. It includes
the session key K encrypted with the unit key U, then concatenated with
the serial number of the sender and an authentication string, and then,
finally, all encrypted with the family key. The exact details of the
law-enforcement field are classified.
The receiver decrypts the law-enforcement field, checks the
authentication string, and decrypts the message with the key K.
Now suppose a law-enforcement agency wishes to tap the line. It uses
the family key to decrypt the law-enforcement field; the agency now
knows the serial number and has an encrypted version of the session
key. It presents an authorization warrant to the two escrow agencies
along with the serial number. The escrow agencies give the two parts of
the unit key to the law-enforcement agency, which then decrypts to
obtain the session key K. Now the agency can use K to decrypt the
actual message.
Further details on the Clipper chip operation, such as the generation of the unit key, are sketched by Denning.