Propping up SHA-1 (or MD5)

[Ben Laurie]

Barney Wolff wrote:
> On Mon, Mar 21, 2005 at 11:56:44AM +0000, Ben Laurie wrote:
> 
>>Musing on these points, I wondered about the construction:
>>
>>H'(x)=H(H(x) || H(H(x) || x))
>>
>>which doesn't allow an attacker any choice, doesn't change APIs and 
>>doesn't change the length of the hash. Does this have any merit? Note 
>>that this is essentially an HMAC where the key is H(x). I omitted the 
>>padding because it seems to me that this actually makes HMAC weaker 
>>against the current attacks.
> 
> 
> I believe the fatal flaw here is not the crypto, but losing the ability
> to hash a stream without keeping all of it.  Both the hashes and HMAC
> have this sometimes-vital property.

This can be fixed quite easily:

H'(x)=H(H(x || H(x)) || H(x))

Cheers,

Ben.

-- 
http://www.apache-ssl.org/ben.html       http://www.thebunker.net/

"There is no limit to what a man can do or how far he can go if he
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