Key Management for Onion Routing in a True Peer to Peer Setting.

Abstract

Onion routing is a technique for anonymous and privacy preserving communication at the base of popular Internet anonymity tools such as Tor. In onion routing, traffic is relayed by a number of intermediary nodes (called relays) before it reaches the intended destination. To guarantee privacy and prevent tampering, each packet is encrypted multiple times in a layered manner, using the public keys of the relays. Therefore, this mechanism makes two important assumptions: first, that the relays are able to communicate with each other; second, that the user knows the list of available relays and their respective public keys. Tor implements therefore a distributed directory listing the relays and their keys. When a user is not able to communicate with relays directly, he has to use special bridge servers to connect to the onion network. This construction, however, does not work in a fully peer to peer setting, where each peer only knows a limited number of other peers and may not be able to communicate with some of them due, for instance, to NAT or firewalls. In this paper we propose a key management scheme for onion routing that overcomes these problems. The proposed solution does not need a directory system and does not imply knowledge of all active relays, while it guarantees the secure distribution of public keys. We also present an alternative strategy for building circuit of relays based on bloom filters. The proposed construction overcomes some of the structural inefficiencies of the Tor design, and opens the way for implementing onion routing over a true peer to peer overlay network.