Re: Secure key exchange with hashing and the birthday paradox

"Luc The Perverse" <sll_noSpamlicious_z_XXX_m@xxxxxxxxxx> wrote in message
news:4dkse3xtrg.ln2@xxxxxxxxxxxxxxxxxxxx
What is your take on this? This seems crazy to me.

http://burtleburtle.net/bob/crypto/exchange.html

I can tell you it is completely infeasible, and the feasible versions are
considered weak. Take for example the 64-bt example given, I'll even assume
the hash is 64-bits (we also have to assume it is actually onto in this case
as well). The 2^32 64-bit values will take 32GB, so each person I
communicate with has taken 32GB of download space, over an OC144 this isn't
that much, but over a 16Mbps line that's almost a full hour, and 16Mbps is
considered a really fast connection. What's worse is that a 64-bit secret is
considered insecure. If we move up to an 80-bit secret you're now looking as
a 10TB download to establish a connection, not to mention that fact that
someone has to actually store this, and you have to store your own. So for
borderline security you need 10TB of storage space, for 128-bit ciphers
you've got 256PB of data coming your way. I don't want to generate, store,
download, manipulate, or in any way deal with an unordered dataset of that
size (the related key is unordered).

This is all assuming a hash that is onto but not reversible, once we add the
overhead necessary to avoid collisions without having a computable onto
function we're looking at doubling all those numbers.

Basically, I don't see the usefulness of such a protocol, even though it
will in theory work.
Joe


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