From: Karl Lui Barrus <klbarrus@owlnet.rice.edu> interesting property of DES is that of complimentation. That is, if
Thats complementation. T is the complement of T'. T is not complimenting T' on a new pair of shoes.
An upcoming book on cryptanalysis details this attack; the author calculates success rates given various estimates for hardware (speed and number) and frequency of key shifting.
Whose book? There is a recent paper on brute force DES breaking: ripem.msu.edu:/pub/crypt/docs/des_key_search.ps, which discusses pipelined DES chips capable of doing 50 Million DES iterations per second. (I believe the rate is conservative, and can be increased by 50 percent.) The point being that any concept of time to attack DES found in print is in danger of being inaccurate shortly after publication. Also DES runs 1,000 - 2,000 times faster in hardware than software implementations (although someone sent mail saying they had a CRAY MP software version). NSA tried to get DES decertified for unclassified government data in 1987-88. I would guess they either saw the handwriting on the wall or have a key breaking machine. The machine in the paper cited above could break a key in 3 1/2 hours for $1 million in hardware costs, or a few minutes for $10 million worth of hardware. There are several criteria involved in cost/protection. NSA is still discouraging the spread of DES. Enough people using DES everywhere would make it harder to pick who they target, or require lots of key breaking machines to search for keys to lots of traffic. Assuming that an adversary had a machine capable of doing a search in 3 1/2 minutes that he could dedicate to finding your key, how often would you have to change keys to make it not worth while? If he was very interested in your traffic, it becomes a question of volume - can he decrypt all the traffic you generate in 24 hours in a 24 hour interval? A dedicated target search with unlimited money would make DES totally insecure. If on the other hand they have to wade through lots of encrypted traffic (Because there is no Law Enforcement Access Field ala clipper) from various parties, and it all needs to be decrypted, the cost/benefit ratio goes to hell. The real world falls somewhere in between. Hardware key attack machines can be defeated by using variants of the cryptographic algorithm they attack. Most variations of DES have been shown to be cryptographically weaker (Biham-Shamir, "Differential Cryptanalysis of the Digital Encryption Standard", (Chap 4), Springer-Verlag 1993). The Biham-Shamir book does point to at least one method of modifying DES that doesn't weaken it, incidently closing the complementation property. A lot of people have been using triple DES for years. There are several other schemes, or algorithms that are supposed to be harder to break for that matter. DES has had the lions share of non-government attention in the last several decades.