Changing preferences
David Shaw
dshaw at jabberwocky.com
Wed Sep 24 06:21:39 CEST 2008
On Sep 23, 2008, at 11:32 PM, Kevin Hilton wrote:
> Robert can probably give a better explanation that I, however with
> 3072 DSA signing keys, the SHA512 and SHA256 algorithms "functionally"
> produce the same length hash since the lower 256 bits are dropped as
> per the FIPS specification. I've often wondered the consequences of
> such an action -- whether this makes the chance of a collision higher
> or equal in comparing the SHA512 modified hash product to the SHA256
> hash product. Perhaps someone could elaborate on this.
In a perfect world, SHA512 truncated to 256 bits is exactly as strong
as SHA256. We don't, of course, live in a perfect world. However,
we're close enough in this case to treat the two as interchangeable in
a practical world. This is what NIST did when specifying the new DSA
algorithm in FIPS-186-3. They note that a 3072-bit DSA key needs a
256-bit hash, but that any hash larger than necessary can be truncated
to fit. OpenPGP follows that spec, and so GPG will happily chop
SHA512 to fit.
David
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