---------- From: Vin McLellan[SMTP:vin@shore.net] Reply To: Vin McLellan Sent: Monday, October 09, 2000 3:22 AM To: Ray Dillinger; cypherpunks@cyberpass.net Subject: Re: Musings on AES and DES
Ray Dillinger <bear@sonic.net> wrote:
<snip>
[As the DES,] Dataseal/Demon/Lucifer was pretty good. It may not have been the *most* secure algorithm of its time, but neither was it a transparent and useless "cipher" with obvious flaws other than the 56-bit keyspace. However, the important part of building up trust (or lack thereof) in the cipher came after it was chosen as the DES.
I suggest that you give insufficient weight to the importance of the NSA imprimatur on the DES.
The DES became the standard we know today -- for years, universally accepted in US commerce, banking, and trade -- largely because
the US National Security Agency (NSA) issued, upon the designation of the DES by NIST, a statement that the NSA's cryptanalysts knew of no attack on
the DES algorithm more effective than a brute force search of all possible
56-bit keys. [...]
DES was pretty much what they said it was (even down to that tweak in the S-boxes to block differential analysis, which the academic crypto researchers didn't discover for many years.) The NSA was/is really very good at what they did, and -- particularly in the US computer industry (which until 1960 had been pretty much guided by NSA R&D contracts) -- their cryptanalytic expertise was wholly unchallenged.
If you read the ostensible charter of the NSA, its duties include assisting in the securing of US civilian communications. While I expect this mainly means making sure that Boris & Natasha aren't tapping US internal comm links without permission, it can also be interpreted to make sure we aren't using snakeoil ciphers. Making DES not suck seems well within the NSA charter. In 1986, when the second recertification came up, I remember considerable consternation over the key-length reduction to 56 bits, and the unexplained tweaking of the S-boxes. There was serious discussion at the time that one or both of these changes were done to introduce backdoors. You'd probably have to find a usenet archive from the period to confirm this. I seem to recall reading somewhere that the extra (8?) bits in the original were shown not to add to the security of the cipher. Clearly 56 was too short - Diffie & Hellman published a paper to that effect in 1977. In the end, we now know that the tweaking prevented differential cryptanalysis, but not linear cryptanalysis. DCA had apparently been discovered internally at IBM (and presumably at NSA). LCA was not then known within IBM (whether it was known inside NSA is an interesting question :-) I would not be suprised if 30 or 50 years down the road, we find out that NSA did its level best to ensure that the AES selection process picked the best candidate. Equally, I would not be suprised to find that they already have some black cryptanalytic technique which can defeat it. On the balance I favor the former: the NSA is as aware as the rest of us of the huge cost (both financial and security) of embedding a broken cipher in the infrastructure of the nation. Peter Trei