About message mixing: A measure that is used for situations like this is entropy. Indeed. This is exactly the mathematical measure for what I've called "privacy diffusion" in a remailer network. It is, namely a measure of of the uncertainty to a watcher of what ingoing message corresponds to what outgoing message. As soon as you begin to write down some of the equations for this value, several things become distinct possibilities: -- duplicate messages may decrease security -- retries may reduce security -- interactive protocols may reduce security -- there is such a thing as a needlessly lengthy remailer path -- noise messages might not be worth the bother -- multiple different routes may reduce security One thing becomes blaringly obvious: -- it's reordering that's mathematically significant; that's what goes directly into the equations. To consider different batching strategies, consider a remailer where the messages come in one per hour, at 1:00, 2:00, 3:00, etc. Since the particulars of the time don't matter for this analysis, I'd suggest using the terminology "message interval", since the entropy calculation is time-scale invariant. Hal's suggestion for rollover schemes is a good one. I'll be working on the math for it. Eric