From: pmetzger@snark.shearson.com (Perry E. Metzger)
Subject: Facinating facts: 30 bit serial number, possibly fixed S1 and S2
In-Reply-To: denning@guvax.acc.georgetown.edu's message of 19 Apr 93 18:23:27 -0400
Reply-To: pmetzger@lehman.com
Organization: Lehman Brothers
Lines: 102


denning@guvax.acc.georgetown.edu (Vidkun Abraham Lauritz Quisling) 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
      U, an 80-bit secret key that unlocks all messages encrypted with the chip

Hmmm. A thirty bit serial number. And, we are told, the unit key U is
derived deterministically from this serial number. That means that
there are only one billion possible unit keys.

   To generate the unit key for a serial number N, the 30-bit value N is
   first padded with a fixed 34-bit block to produce a 64-bit block N1.
   S1 and S2 are then used as keys to triple-encrypt N1, producing a
   64-bit block R1:

	   R1 = E[D[E[N1; S1]; S2]; S1] .

   Similarly, N is padded with two other 34-bit blocks to produce N2 and
   N3, and two additional 64-bit blocks R2 and R3 are computed:  

	   R2 = E[D[E[N2; S1]; S2]; S1] 
	   R3 = E[D[E[N3; S1]; S2]; S1] .

   R1, R2, and R3 are then concatenated together, giving 192 bits. The
   first 80 bits are assigned to U1 and the second 80 bits to U2.  The
   rest are discarded.  The unit key U is the XOR of U1 and U2.  U1 and U2
   are the key parts that are separately escrowed with the two escrow
   agencies.

Hmmm. We must assume that generating the unit key U from the serial
number N rather than generating it from a randomly selected U1 and U2
is an intentional way of assuring a "fail safe" for the government --
U is completedly determined given S1, S2 and N. If S1 and S2 do not
change they constitute effective "master keys" (along with F), the
theft of which (or the possession of which by various authorities)
completely obviates the security of the system. However, more
interestingly, we know, for a fact that if S1 and S2 are fixed no
matter what the keyspace for U is no more than 2^30. Why not pick U1
and U2 at random? Why this interesting restriction of they key space
if it NOT to provide an additional back door?

I find it disturbing that at the very best my security is dependant on
approximately 30 bytes worth of information that could be written on
the back of a napkin.

Even if S1 and S2 change periodically, the rationale behind this
restriction in the size of the keyspace seems strange if one is
assuming that the goal is security -- and makes perfect sense if the
goal is an illusion of security.

If S1 and S2 do not change, even if they remain secret I wonder if
they can somehow be back-derived given enough unit key/serial number
pairs. We are assured that this cannot happen -- but no one
understands how Skipjack works outside of government officials and,
soon, foreign intelligence services that gain the information via
espionage. Presumably we will eventually have the information as well
-- reverse engineering gets more and more advanced every year -- but
by the time we know it may be too late.

   As a sequence of values for U1, U2, and U are generated, they are
   written onto three separate floppy disks.  The first disk contains a
   file for each serial number that contains the corresponding key part
   U1.  The second disk is similar but contains the U2 values.  The third
   disk contains the unit keys U.  Agent 1 takes the first disk and agent
   2 takes the second disk.  The third disk is used to program the chips.
   After the chips are programmed, all information is discarded from the
   vault and the agents leave.  The laptop may be destroyed for additional
   assurance that no information is left behind.

None of this makes me feel the least bit secure. The silly notion of
"destroying the laptop" appears to be yet another bizarre distraction.
We all know that you can't read data from DRAM that has been turned
off for more than a few moments. On the other hand, what we don't know
is why there is a need to generate the unit keys from S1 and S2 in the
first place other than to weaken the system. We don't know if the
agents in question would resist a million in cash a piece for their
information -- its probably worth hundreds of million, so you can make
the bribe arbitrarily hard to resist. And to tell you the truth, doing
this in a "vault" rather than in Joe Random Tempest-shielded Room
with a laptop computer seems like melodrama designed to make
high-school dropouts from Peoria impressed -- but it does very little
for most of the rest of us.

   The protocol may be changed slightly so that four people are in the
   room instead of two.  The first two would provide the seeds S1 and S2,
   and the second two (the escrow agents) would take the disks back to
   the escrow agencies.

What would this provide? Lets say the escrow agencies are the ACLU and
the NRA and their agents personally take back the disks and are always
honest. Who cares? The NSA must be laughing out loud, because they
have the algorithm to regenerate U given N and likely don't need to
steal they keys as they effectively already have them.

--
Perry Metzger		pmetzger@shearson.com
--
Laissez faire, laissez passer. Le monde va de lui meme.
