From: tomw@orac.asd.sgi.com (Tom Weinstein)
Subject: Re: REVISED TECHNICAL SUMMARY OF CLIPPER CHIP
Organization: Silicon Graphics Inc.
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Distribution: world
Reply-To: tomw@asd.sgi.com
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In-reply-to: denning@guvax.acc.georgetown.edu's message of 21 Apr 93 19:26:15 -0400

In article <1993Apr21.192615.3465@guvax.acc.georgetown.edu>, denning@guvax.acc.georgetown.edu writes:

> Each chip includes the following components:

>    the Skipjack encryption algorithm
>    F, an 80-bit family key that is common to all chips
>    N, a 30-bit serial number (this length is subject to change)
>    U, an 80-bit secret key that unlocks all messages encrypted with the chip
[ ... ]

> To see how the chip is used, imagine that it is embedded in the AT&T
> telephone security device (as it will be).  Suppose I call someone and
> we both have such a device.  After pushing a button to start a secure
> conversation, my security device will negotiate an 80-bit session key K
> with the device at the other end.  This key negotiation takes place
> without the Clipper Chip.  In general, any method of key exchange can
> be used such as the Diffie-Hellman public-key distribution method.

> Once the session key K is established, the Clipper Chip is used to
> encrypt the conversation or message stream M (digitized voice).  The
> telephone security device feeds K and M into the chip to produce two
> values:

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

> which are transmitted over the telephone line.  The law enforcement
> field thus contains the session key K encrypted under the unit key U
> concatenated with the serial number N, all encrypted under the family
> key F.  The law enforcement field is decrypted by law enforcement after
> an authorized wiretap has been installed.

One very interesting thing I notice about this is that the only use of
the chip key is to encode the session key for the law enforcement field.
It is not used at all by the encryption algorithm.

It seems like it would be possible to create a device that would be
otherwise compatible, but would send out a bogus law enforcement field.
This might render the device unusable with "normal" devices if there's
some sort of validation involving the law enforcement field, but it
could certainly be used with other such modified devices.

Of course, this is irrelevant if the NSA has a backdoor in the
algorithm, but it does make it possible to defeat the key escrow system.

--
Wilt thou seal up avenues of ill?  Pay every      |   Tom Weinstein
debt, as though God wrote the bill. -- Emmerson   |   tomw@orac.esd.sgi.com
