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

The article says that SHA-1 encryption has been broken.. but don't panic yet, because it takes massive computing power to do so.

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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