Re: NSA's Venona Intercepts

[Jim Gillogly <jim@acm.org>]

smith@sctc.com (Rick Smith) writes:
>One question that I haven't found answered in my perusals of the site is a
>definitive statement of the cryptographic technology used by the Soviets. I
>was re-reading Kahn's 1967 chapter on Soviet crypto and he claimed that
>they relied primarily on one time pads. In fact, he was pretty specific
>about them using OTPs for exactly the type of traffic appearing in the
>Venona archive. But when I look at the partial decrypts in the Venona
>archive I don't understand how you'd get such partial decrypts from OTPs.

>The intercepts seem to indicate the use of ciphers with some codewords
>weakly layerd on top. Some intercepts show translations based on the
>phonetic properties of the extracted Russian plaintext. So I don't think
>the "unrecovered codegroups" are caused by a classic code that substitutes
>tokens for word meanings. But you're not going to crack only part of a OTP
>ciphertext -- presumably you'd need a compromised key tape, and that would
>either decrypt everything or nothing.

>So they were either really using rotor machines or they were using
>something else. Any other ideas? Other references?

I too am waiting eagerly for them to show more of the real details of
decryption; but from what we know so far, the partial decrypts seem quite
compatible with what NSA says they broke: an underlying code system
superencrypted with a OTP which occasionally becomes a 2TP.  For example,
suppose you have two chunks of ciphertext, and you've determined using the
kappa test that they have a partial overlap (the easy part of the
process).  You superimpose them as follows:

	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
		bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb

and then work on the parts where the a's and b's overlap.  If you know the
underlying code book (through a decade of previous hard work, or, if you're
really lucky, finding a partially-burned copy of a codebook in a castle in
Germany), you can with considerable sweat determine the code groups in
both messages in the overlapped part, recover that part of the OTP, and
then drag the recovered OTP through the rest of your traffic looking for
more matches.

If you don't have a complete code book you may not know what all of the
code words mean; for example, I've seen no evidence that ALES really does
stand for ALGER HISS.  In addition, if you have no more overlaps with the
part at the beginning of message "a" or the end of message "b" above, you
have no way to determine anything about those parts of the OTPs other than
the length of the bits you can't read.

These two sources of difficulty seem to me to explain the "unrecovered
codegroups" you noted: for the long stretches, they didn't have
overlapping messages to give an entry into the OTP; for individual code
groups, they didn't have enough context to break that part of the code.

The phonetic stuff you mentioned doesn't cause me heartburn -- a code will
include syllables or letters, so that concepts that don't have their own
code group can get assembled out of constituent parts.  Still seems
consistent to me.

	Jim Gillogly
	4 Halimath S.R. 1996, 17:08