Security Implications of Using the Data Encryption Standard (DES)

Leichter, Jerry leichter_jerrold at
Mon Dec 25 08:53:23 EST 2006

| note that there have been (at least) two countermeasures to DES brute-force
| attacks ...  one is 3DES ... and the other ... mandated for some ATM networks,
| has been DUKPT. while DUKPT doesn't change the difficulty of brute-force
| attack on single key ... it creates a derived unique key per transaction and
| bounds the life-time use of that key to relatively small window (typically
| significantly less than what even existing brute-force attacks would take).
| The attractiveness of doing such a brute-force attack is further limited
| because the typical transaction value is much less than the cost of typical
| brute-force attack....
Bounds on brute-force attacks against DESX - DES with pre- and post-whitening
- were proved a number of years ago.  They can pretty easily move DES out
of the range of reasonable brute force attacks, especially if you change
the key reasonably often (but you can safely do thousands of blocks with
one key).

One can apply the same results to 3DES.  Curiously, as far as I know there
are to this day no stronger results on the strength of 3DES!

I find it interesting that no one seems to have actually made use of these
results in fielded systems.  Today, we can do 3DES at acceptable speeds in
most contexts - and one could argue that it gives better protection against
unknown attacks.  But it hasn't been so long since 3DES was really too
slow to be practical in many places, and straight DES was used instead,
despite the vulnerability to brute force.  DESX costs you two XOR's - very
cheap for what it buys you.

Question:  How does DUKPT generate its unique keys?  If it's using DES
on the previous key, or on a counter, or anything simple like that, at
best, it's making brute force a bit more expensive - one brute forces
a couple of transaction keys, then uses them to brute force the DUKPT
key stream.  (There are certainly ways to make this much harder, but I
wonder what they actually do.)
							-- Jerry

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