?splints for broken hash functions

John Denker jsd at av8n.com
Sat Aug 28 23:37:42 EDT 2004

Jerry Leichter writes:
 >> However ... *any* on-line algorithm falls to a Joux-style attack.

Hal Finney wrote:

> ... hashes that support incremental hashing, as any useful hash surely must,

If you insist on being able to hash exceedingly long strings
(i.e. longer than your storage capacity) here is a modification
of the "splint" I proposed earlier.

Run two hash-machines in parallel.  The first one operates
normally.  The second one puts the first block of the string
into a buffer (bounded size!) and then proceeds to hash the
rest of the string, starting with the second block.  At the
end of the message, it appends the saved block to the tail of
the message and hashes it.

The two results are combined in some nonlinear way, perhaps
by appending the other machine's hash onto this machine's
input string.

The strength here comes from the idea the that you cannot
engineer a collision in the "saved" block in the second
machine until you have engineered the collision in the
first machine, and then if you change the saved block to
engineer a collision in the second machine you have to
redo everything you did in the first machine.

Of course it would be nice to have hash functions that were
strong on a block-by-block basis ... but still I think there
is value in destroying the prefix property without destroying
the online property, i.e. not destroying the ability to hash
strings that are too long to store.


   Small point: I'm not entirely convinced that *any* useful
   hash must have the online property.  That's not a defining
   property of hash functions according to most authorities.
   I've done lots of hashing in situations where I would have
   been happy to store the entire input-string.

   Still we can agree that the online property is *sometimes*
   nice, and I'm happy to be able to provide it.

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