> (*) There are tradeoffs between the amount of RAM and the number of
> computations.  At one extreme is having 65536 keys in RAM; then no
> computations are required for any of 'em.  At the other extreme is having
> one key in RAM and having to do up to 65536 computations on it.  For a
> 16F84 implementation using 40-bit keys and a 2^20 domain, having 5 keys in
> memory is probably reasonable [25 bytes]; if the number of keys' worth of
> memory were increased to 10, then only 24 calls to the hash function would
> be needed per key.

I think I may have mis-spoken here: the algorithm I was thinking of needs
twice as much memory as I'd indicated.  Managing 1048575 (2^20-1) keys
would require 50 bytes of storage for the algorithm, not 25.  Even so,
I think it should still be possible on a 16F84.