Re: Crack for DOS

[frantz@netcom.com (Bill Frantz)]

Bill Stewart wrote:
> Blowfish is new, so perhaps there are flaws that 10-20 years of analysis
> will find.  But it's pretty good - it's small, fast in software,
> and has an annoyingly slow key-schedule which makes it difficult to
> use brute-force crackers on.  It's got variable key lengths.

This comment about "annoyingly slow" key setup made me think of pipeline
hardware to set up the keys.  Now with blowfish, key setup requires running
the Blowfish algorithm 521 times in setting up the key, so the pipeline
will require 521 stages if you want a new key for every "real" decryption. 
Ignoring chip size restrictions, this pipeline will increase the chip real
estate by 521 which will have approximately the same economic effect on the
cost of cracks as extending the key by 9 bits.

For CBC cypher systems, brute force attacks will require at least two
decryptions (one for the initialization vector, and one for the first block
of the message).  however, these decryptions can take place in parallel
(for the first block of the message) so they do not change the above
argument.  However if the design calls for more than the first block, then
the pipeline can run slower and becomes a smaller portion of the cost of
the system.

Of course if you are using Blowfish with anywhere near its full 448 bit
key, brute force attacks are simply not practical.


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