From: caronni@nessie.cs.id.ethz.ch (Germano Caronni)
Subject: Some Questions (was: REVISED SUMMARY)
Organization: Swiss Federal Institute of Technology (ETH), Zurich, CH
Lines: 90



> Here is a revised version of my summary that corrects some errors and
> provides some additional information and explanation.

Thank you very much. After reading the text some distinct questions
arised to me, which I guess will also be asked by other people. Perhaps
would it be interesting to find an answer to these questions ?


>                      THE CLIPPER CHIP: A TECHNICAL SUMMARY
>    N, a 30-bit serial number (this length is subject to change)

shorter or longer ?

> 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.

First question: When will the LawEnforcmentField be transmitted, and how
does the remote Clipper Chip handle it? Is it transmitted periodically
in the stream of encrypted blocks, or just at the beginning ? Does the
phone at the other side discard those packets via a protocol whatsoever,
or tries it to turn them into voice-output ? (Which would not be disturbing)


> At the beginning of a session, a trusted agent from each of the two key
> escrow agencies enters the vault.  Agent 1 enters a secret, random
> 80-bit value S1 into the laptop and agent 2 enters a secret, random
> 80-bit value S2. These random values serve as seeds to generate unit
> keys for a sequence of serial numbers.  Thus, the unit keys are a
> function of 160 secret, random bits, where each agent knows only 80.

Second question: Why!?!? Why is such a strange procedure used, and not
a real RNG ? This turns those S1,S2 in a kind of bottleneck for system-
security.


> When law enforcement has been authorized to tap an encrypted line, they
> will first take the warrant to the service provider in order to get
> access to the communications line.  Let us assume that the tap is in
> place and that they have determined that the line is encrypted with the
> Clipper Chip.  The law enforcement field is first decrypted with the
> family key F, giving E[K; U] + N.  Documentation certifying that a tap
> has been authorized for the party associated with serial number N is
> then sent (e.g., via secure FAX) to each of the key escrow agents, who
> return (e.g., also via secure FAX) U1 and U2.  U1 and U2 are XORed
> together to produce the unit key U, and E[K; U] is decrypted to get the
> session key K.  Finally the message stream is decrypted.  All this will
> be accomplished through a special black box decoder.

So no (technical) provision will be taken to place a 'timeout' on these
warrants? This would be a unique possibility to realize such a technical
restriction, by letting the escrow-agencies perform the decoding of the
session key. Just take modem-lines instead of secure fax. Is this such
a bad idea ?


> A successor to the Clipper Chip, called "Capstone" by the government
> and "MYK-80" by Mykotronx, has already been developed.  It will include
> the Skipjack algorithm, the Digital Signature Standard (DSS), the
> Secure Hash Algorithm (SHA), a method of key exchange, a fast
> exponentiator, and a randomizer.  A prototoype will be available for
> testing on April 22, and the chips are expected to be ready for
> delivery in June or July.

Wow! (How does the randomizer work?) Are the SHA (and Key exchange) secret,
or publicly known ? Key-Exchange is DH, I guess ?

It seems that those who are opposed to this chip shall have a tough time,
your government realy means to act. :-(

Friendly greetings,
	   Germano Caronni

-- 
Instruments register only through things they're designed to register.
Space still contains infinite unknowns.
                                                              PGP-Key-ID:341027
Germano Caronni caronni@nessie.cs.id.ethz.ch   FD560CCF586F3DA747EA3C94DD01720F
