Nonesuch: a mix network with sender unobservability

Abstract

Oblivious submission to anonymity systems is a process by which a message may be submitted in such a way that neither the anonymity network nor a global passive adversary may determine that a valid message has been sent. We present Nonesuch: a mix network with steganographic submission and probabilistic identification and attenuation of cover traffic. In our system messages are submitted as stegotext hidden inside Usenet postings. The steganographic extraction mechanism is such that the the vast majority of the Usenet postings which do not contain keyed stegotext will produce meaningless output which serves as cover traffic, thus increasing the anonymity of the real messages. This cover traffic is subject to probabilistic attenuation in which nodes have only a small probability of distinguishing cover messages from "real" messages. This attenuation prevents cover traffic from travelling through the network in an infinite loop, while making it infeasible for an entrance node to distinguish senders.