POW / rPOW

[Gregory Maxwell <gmaxwell@xxxxxxxxx>]

has any thought been given to the use of proof of work tokens
(hashcash) or reusable proof of work tokens (rpow) for message
admission control to the mixminion network?

I suspect rpow would have some privacy concerns, so I guess that in
each layer of routing a user could store a non-reusable pow inside the
message..  I'd suspect that there are some padding issues that would
need to be resolved to prevent such a system from affecting anonymity.

In any case, the use of POW tokens could help inhibit flooding attacks
the drain the anonymity mix. Exit gateways that transfer to normal
email could require significant POW tokens coded for the email
recipient that could help dissuade any attack based on output volume.

Also, nym servers that have a finite supply of reply blocks could
demand (user configured) large POW which must be provided by the
sender to inhibit DOS attacks.

I think it's important that such DOS attacks be addressed: even if the
mix still promises good anonymity properties in the face of such an
attack, the impact on the network may cause users to behave in a way
which compromises their anonymity.