Re: migration paradigm (was: Is PGP broken?)

[David Honig]

At 10:23 AM 12/8/00 -0800, Bram Cohen wrote:
>On Tue, 5 Dec 2000, David Honig wrote:
>
>> Is there a reason not to use AES block cipher in a hashing mode
>> if you need a secure digest of some data? 
>
>Hashing modes of block ciphers require a re-key for every block, and hence
>are really, really slow.
>
>-Bram Cohen

Except for blowfish, most ciphers (IDEA, DES, Rijndael) can take a new key
at the same time as a block of plaintext.

Oops, was thinking hardware.  Never mind.

Seems though you could take a block cipher, key it with the first 
N bits of your message, then feed in blocks chained together, and output
the last block.  This has the hash property that changing any input 
changes the hash.  Because we have used a cipher correctly, we get
a secure hash --ie, its crypto-hard to spoof.  

What is the point (ie how does security gain) of rekeying in a block cipher
used as a hash?

Thanks,
dh