Reusable anonymous return channels

Mix networks are used to deliver messages anonymously to recipients, but do not straightforwardly allow the recipient of an anonymous message to reply to its sender. Yet the ability to reply one or more times, and to further reply to replies, is essential to a complete anonymous conversation. We propose a protocol that allows a sender of anonymous messages to establish a reusable anonymous return channel. This channel enables any recipient of one of these anonymous messages to send back one or more anonymous replies. Recipients who reply to different messages can not test whether two return channels are the same, and there-fore can not learn whether they are replying to the same person. Yet the fact that multiple recipients may send multiple replies through the same return channel helps defend against the counting attacks that defeated earlier proposals for return channels. In these attacks, an adversary traces the origin of a message by sending a specific number of replies and observing who collects the same number of messages. Our scheme resists these attacks because the replies sent by an attacker are mixed with other replies submitted by other recipients through the same return channel. Moreover, our protocol straightforwardly allows for replies to replies, etc. Our protocol is based upon a re-encryption mix network, and requires four times the amount of computation and communication of a basic mixnet.

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