MLB> ... we happen to have a random K-bit integer N assigned to MLB> each member. How many independent experiments can MLB> these N help us do? Obviously we can do K, ... MS> Why not use a cryptographic hash function? I think Mike is on the right track; there is no reason to do any designing unless you need high efficiency. What you need is a stream cipher that evolves each N into an arbitrary number of bits. Hashing the concatenation of N and the experiment number and taking however many bits you want should do fine. An only slightly different proposal is to use a block cipher to construct a stream cipher with counter mode. Here is one proposal. Take a block cipher (I think any of the standard ones will do): use N as the key and the experiment number as the input and take as many bits as you like from the output. Do you need something faster that that? Whit