A.P. Lawrence

2005/03/18 SHA-1

SHA is the Secure Hash Algorithm, an encryption method that is used in all sorts of things from ssh to vpn's. Hashing is simple enough to understand, but a hash used for cryptography has to have specific characteristics: it needs to be collision free (so that a particular key can't decrypt more than one message) and it needs to be one-way, so that you can't easily recreate the original message from the hashed version. The very first SHA apparently had problems in one of those areas, so it was replaced by SHA-1.

I happened across this recently: http://www.schneier.com/blog/archives/2005/02/cryptanalysis_o.html

But.. yesterday's unimaginable resources are tomorrow's desktops. There's also the somewhat disturbing note that says:

Hash functions are the least-well-understood cryptographic primitive,
and hashing techniques are much less developed than encryption
techniques. Regularly there are surprising cryptographic results
in hashing. I have a paper, written with John Kelsey, that describes
an algorithm to find second preimages with SHA-1 -- a technique
that generalizes to almost all other hash functions -- in 2¹⁰⁶ 
calculations: much less than the 2¹⁶⁰ calculations for brute force.
This attack is completely theoretical and not even remotely practical,
but it demonstrates that we still have a lot to learn about hashing.

Which might mean that some mathematician may have some sudden surprising insight that makes hashes as transparent as rot-13 cyphers.

As always, security remains a journey and not a destrination.

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