Comments: Idle speculation - I wonder if the NSA knew this all along?

(Greg writes on cryptography in answer to another question:)

If you look at Phil Hawkes' paper , you will see that the SHA-2s are very different algorithms, and my own opinion is that the data-expansion part of the algorithm is *seriously* beefed up. My guess is that the NSA were already worried about this kind of attack (whether they'd found it or not). We don't have a good analysis of the data-expansion part, but I'm pretty sure that it'll defeat the Wang attacks.

Greg.

Posted by Greg R at February 17, 2005 05:11 PM

I wanted to second Greg's point about SHA256, but with a caveat: Since we haven't seen Wang's techniques, it's hard to have a huge amount of confidence that she won't be able to find something despite rather different structure of SHA256. But the message expansion is much more complex, and introduces some carry bits into the mix. Similarly, the internal mixing of bits among different bit positions within the "encryption" function in MDx, RIPE-MDx, and SHA0/1 is mostly done by carry bits and the small number of rotations. SHA256 introduces a much stronger internal (linear) mixing operation that's going on by rotating a word three times by different amounts and XORing the result together.

The point is that it seems much more straightforward to apply the same techniques to MD5 and SHA1, than to apply the same techniques to SHA1 and SHA256. But then, a week ago, I thought SHA1 would probably be okay for the time being--what do I know?

--John

Posted by John Kelsey at February 18, 2005 10:00 AM
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