[messaging] Reliable Security Estimates for Key Stretching

Joseph Bonneau jbonneau at cs.stanford.edu
Wed Jun 3 00:40:01 PDT 2015

On Tue, Jun 2, 2015 at 9:56 AM, Daniel Kahn Gillmor <dkg at fifthhorseman.net>
> So it looks like it's saying "if we slow down the user by a factor of X,
> then we slow down an attacker by the same factor."
> The assumption here appears to be that there is no speedup for an
> attacker that is better than brute force.  If an attacker has some
> clever precomputation, or a way to reuse intermediate results
> efficiently, it seems like the slowdown for the attacker may be a
> smaller factor than the slowdown for the user, unfortunately.

If you're salting the hash (basically, if you're doing password-based
decryption of a private key file instead of password-based re-derivation of
the private key), then there should be no effective precomputation as long
as the hash is cryptographically secure.

If you can't salt, then rainbow tables are a possibility. But iterating the
same hash function still makes rainbow tables linearly more expensive to

As dkg said, this is just a linear numbers game, but I believe that's the
worst case scenario. If you take N times longer to hash on the client side,
you should always be able to slow the attacker down by at least a factor of

Using a memory-hard function, or some other function designed to be
difficult to implement in hardware, can slow some attackers down by a
greater factor if it prevents them from building hardware as cheaply. As
Taylor said there isn't a definitive paper on this yet as far as I know,
though there are quite a few papers arising from the password-hashing
competition with different design approaches, threat models, terminology,
etc. Hopefully we'll settle on a model eventually which can give simpler
"bits of security" estimates as Nadim asked for.
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