New directions for hash function designs (was: More problems with hash functions)

[Thierry Moreau]

Hal Finney wrote:

>Another of the Crypto talks that was relevant to hash function security
>was by Antoine Joux, discoverer of the SHA-0 collision that required
>2^51 work.  Joux showed how most modern hash functions depart from the
>ideal of a random function.
>
>The problem is with the iterative nature of most hash functions, which
>are structured like this (view with a fixed with font):
>
>
>IV  --->  COMP   --->   COMP   --->   COMP   --->  Output
>           ^             ^             ^
>           |             |             |
>           |             |             |
>         Block 1       Block 2       Block 3
>
>The idea is that there is a compression function COMP, which takes
>two inputs: a state vector, which is the size of the eventual hash
>output; and an input block.  The hash input is padded and divided
>into fixed-size blocks, and they are run through the hash function
>via the pattern above.
>
>This pattern applies to pretty much all the hashes in common use,
>including MDx, RIPEMDx, and SHA-x.
>  
>
Just for the record, the Frogbit algorithm is a compression function with a 1-bit block size and a variable size state information.

(http://www.connotech.com/frogbit.htm)

The Helix cipher shares with the Frogbit algorithm the use of dual key stream usages between which the plaintext stream is inserted in the computations. However, the Helix authors do not recommend their construct for a hash function.

(http://www.schneier.com/paper-helix.pdf)

Perhaps ideas combined from these two approaches can help define other constructs for hash functions. Obviously, if the sate information is to end up in a standard size at the end of the plaintext processing, the additional state information has to be "folded", which means additional processing costs, of discarded.



-- 

- Thierry Moreau

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