"S1" encryption system (was: this looked like it might be interesting)

[Matt Blaze]

Hal writes:
>I suppose the unstated implication is that this might be Skipjack.
>
>I have looked at the program a bit and have a few observations:
>
....
>The encryption function itself is a modified Feistel type cipher, with
>the blocks broken into 8 pieces and xor'd with functions involving F,
...

Someone sent me (to my bell labs address) a copy of this this afternoon via
an anon server in the netherlands.  It looks like others got it as well, and
it appears to have been posted to the cypherpunks list, though it hasn't
yet shown up here from the list (my mail seems to be slow today).  Did
anyone else have a copy mailed directly to them?

I don't quite know what to make of it.  A couple of random quick first-order
observations:

	The code appears to have been translated from some other
	language by someone not skilled in C.  Hal noted the
	lack of "for" loops where they are obviously called for,
	and at least two odd bits of code that appear to be bugs,
	at least one of which one would suspect would cause it to
	fail to interoperate with correct implementations (if we
	are to assume the "correct" cipher uses the entire key schedule).
	Also note the awkward assignement to the F and G tables.

	S1 could suggest Skipjack, but it is also a pretty generic name
	for a cryptosystem.

	I thought Skipjack (like most other NSA cryptosystems) is SECRET,
	not TOP SECRET, but on the other hand this appears to be part of
	some kind of "secondary analysis" package, whatever that is, so
	if this is really spook stuff, the TOP SECRET designation could
	be reasonable.

	The cipher is similar in some ways to one designed by Bruce
	Schneier and I last year (MacGuffin, described in
	ftp://research.att.com/dist/mab/mcg.ps ).  In particular, note
	that in each of the 32 rounds, 16 bits are operated on by 48
	(or 40, depending on the effect of the G function).

	There is at least one novel feature - the G function used to
	select which F's (Sboxes) to use.  I've not seen this before.

	The cipher appears to be designed for software implementation
	(byte oriented, etc.).  The software, on the the other hand,
	goes to some trouble to emulate a hardware interface, as if it
	were written to be dropped in to some pre-existing code or
	library.

	The F outputs are not uniformly distributed.  In fact, some outputs
	appear far more often than others (I base this on running "grep|wc",
	not on any real analysis.)

	What a strange key schedule.

	The "family" XOR business at the begining and end suggests
	RSA's DESX.  The lanuage in the comments suggests that it's there
	to allow for non-interoperable "families" of users.  GOST
	has similar features, though GOST couples this more closely to
	the cipher's internal structure.

	As far as I know, no one has EVER leaked TOP SECRET material
	cryptosystem in this way, so I'm very skeptical.  But there's
	always a first time.

I don't know what to believe.  If this is a real, classified cryptosystem,
it would be a very unusual first.  On the other hand, if this is a hoax,
whoever did it appears to have gone to some trouble, and has included some
interesting design features.  A third possibility, if we are to believe
the spook markings, is that it is a re-implementation of someone else's
cryptosystem, created for the purpose of cryptanlysis.

All in all, I remain very skeptical.  It smells like a hoax to me, but
I'm willing to look at it with an open mind.

-matt