MYK-78

[Arthur Abraham <a2@well.sf.ca.us>]

I've been stalking Mykotronx with phone and smail since right
after the announcement, and finally got through the guy who
kept telling me that I'd undertand if I just knew a little
more crypto, to the guy who really know what was going on and
wanted to tell me.  This is what I found out: 

Mykotronx MYK-78 has been identified as the Privacy "Clipper"
chip.  The "Clipper" name comes from Washington, and the guys
at Mykotronx know about the Intergraph chip.

The data sheets, as those of you who have read them know, are
confusing, incomplete and internally inconsistent.  This is
evident even if you do not consider that they are to implement
the social protocol described by Dorothy Denning (her
19-Apr-93 paper, as published in Cypherpunks).

After some discussions with Mykotronx, I was able to convince
them of the truth of the last paragraph and to have them
explain just what the chip was designed to do.  I would also
like to emphasize that these discussions revealed that the
poor quality of the documentation does not result from any
attempt to obscure the operation of the chip, they were very
forth coming and eager to discuss its operation.  The
deficiencies result more from the nature of a military
contractor's relationship to its one customer: the customer
understands how to use the chip so there's no pressure to get
it described carefully.  Going public was a bit of a surprise
to them, in fact the announcement was made during their
application engineer's vacation.  I am sure there is an
interesting story in this timing, but the people I was talking
to didn't seem to know it. 

On to the chip: 

You don't just hook up a clear-text bit stream to one end and
get a Denning-stream out the other.  It needs a bit of care
and feeding.

At startup it requires a Random Seed (8 bytes/64-bits) and a
crypto-variable CV (10 bytes/80-bits) for its DES-type
algorithm.  This is Denning's "skipjack" algorithm and, like
DES, is a symmetric key block cypher, which performs in all
the DES modes:

64-Bit Electronic Code Book (ECB)
64-Bit Cypher Block Chaining (CBC)
8/16/32/64 Bit Cypher Feedback
64-Bit Output Feedback (OFB)

In the last three modes the encryption of each block is
dependent on the previous blocks.  (If you care to know more
about DES modes, see FIPS-PUB 81 which is cited in the data
sheets.)  

One other thing about Skipjack: Denning describes it as having
"32 rounds of scrambling" and this is supported by the data
sheet's timing charts, which note 64 clocks cycles to complete
an encryption.  Since this would operate on an 8-byte/64-bit
block, with the 15MHz internal clock we appear to have roughly
a 10M-bit/1.3MB transfer rate in encryption/decryption.  This
is fast enough for the average telephone, or several
telephones, or maybe a stereo CD.  It's probably just average
performance for 1 micron technology and some units clock up to
30MHz (they expect 0.8 micron eventually, with improved
performance).

Back to the Crypto-Variable, CV.  The CV is the session key,
is selected off-chip, and must always be accompanied by a 3
byte/24-bit checkword.  Where do you get the check word?...
you ask the chip! If you load a CV with a bad checkword, the
chip sets its ERROR line -- oh, sadness.  But then you can
read out a good checkword, and subsiquently reload the same CV
with the good checkword (happy now?).  The checkword is
actually just the first three bytes from an application of
Skipjack to the CV.

Do all this and the chip is loaded and ready for plaintext. 
You could just give it an Encryption command, and start
pulling cyphertext out the other side, but who would
understand it?  First you have to get the key information out
of the chip and send it to the chip on the other side of the
link.
 
Skipjack is DES-like so to run a decryption mode on the other
chip we're going to have to send it the session key, CV, and
the Initial Vector, IV, which is the starting state of the
stream for the non-ECB modes of operation.  We selected CV
ourselves, and learned its checkword during the startup
experience, but where's IV?

Well, we generate it using "a feature not found in current DES
chips" (data sheet, 1-3).  And quite a feature it is, too.  We
use this command, Generate IV, and it makes all 8
bytes/64-bits of the IV, based on the Random Seed... But
That's Not ALL!

You issue the Generate IV command three (3) times to get the
full 24 byte/192-bit LEEF block.  LEEF = Law Enforcement
Exploitation Field.  (I wrote this down very carefully to be
sure I had it right.)  

...Actually, you issue a Read Data command after each Generate
IV command, but I won't bore you with details.  The first 8
bytes/64-bits are called L1 or LEEF-1, the second 8
bytes/64-bits are L2 or LEEF-2, and then here is the IV we've
all been waiting for, in its full 8 byte/64-bit glory.  You
probably noticed that LEEF is 24 bytes/192-bits long, and has
the structure [L1,L2,IV].  Mykotronx is not supposed to tell
us the structure of L1,L2.

The interesting thing is that [CV,checkword,L1,L2,IV] is a
self-checking unit.  The receiving chip checks it as it is
loaded.  If something is wrong, the chip sets its ERROR line. 
If CV is fermished, you have to get all the way to IV before
you're rasberried.  In transmitting this we are advised to
encrypt CV because it is, after all, the session key.  

OK, so we are encrypting and the other chip is decrypting. 
Suppose something happens and the other chip wants to talk to
us, so that it encrypts and we decrypt.  It has all it needs
to encrypt and we have all we need to decrypt, but one more
thing has to be done.  We need to save the state of the
chaining cypher so we can resume it at the same place in the
chain when we return to encrypting.  Use the Save State
command, which pops out 8 bytes/64-bits of Saved State, SS, or
the current contents of the Skipjack encryption register.  To
make this a bit clearer, if we pulled the Saved State right
after Generate IV, we'd find SS = IV.

The chip's serial number is 4-bytes/32-bits long, not the 3.75
bytes/30-bits Denning reported, but don't worry, _you'll_
never see it.  It and the family key are written in over pins
Vpp1 and Vpp2, which are then burned out.  All chips are
currently planned to have the same family key, but if you
happen to meet a chip with a different family key and it sends
you [CV,checkword,L1,L2,IV], you could understand it.  

That's the main part of what's missing from the data sheets. 
The rest works pretty much as described, and is at a level of
detail too fine to interest anyone except a compulsive
hardware wonk.  Oh, one more thing, on page 1-4 where the
Configuration Register is shown with two "Arm CV" bits, the
one at position D5 should be "Arm IV".

-a2.

ps: I will be at a meeting the rest of the week, so please
don't expect me to respond to requests for clarification until
I return.  Sorry.

-a2.