length-extension and Merkle-Damgard hashes

Travis H. travis+ml-cryptography at subspacefield.org
Sun Jan 28 14:33:30 EST 2007

So I was reading this:

It seems to me the length-extension attack (given one collision, it's
easy to create others) is not the only one, though it's obviously a
big concern to those who rely on it.

This attack thanks to Schneier:

If the ideal hash function is a random mapping, Merkle-Damgard hashes
which don't use a finalization function have the following property:

If h(m0||m1||...mk) = H, then h(m0||m1||...mk||x) = h(H||x) where the
elements of m are the same size as the block size of the hash, and x
is an arbitrary string.  Note that encoding the length at the end
permits an attack for some x, but I think this is difficult or
impossible if the length is prepended.

The driving force behind innovation is sublimation.
-><- <URL:http://www.subspacefield.org/~travis/>
For a good time on my UBE blacklist, email john at subspacefield.org.
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