1. Responder Anonymity and Anonymous Peer-to-Peer File Sharing. by Vincent Scarlata, Brian Levine and Clay Shields Presentation by Saravanan

2. Introduction Peer-to-Peer file sharing:  Enable users to share files locally without needing an external server to store all the files. Examples: Napster, Gnutella etc. Issues with Peer-to-Peer file sharing: Initiator and Responder anonymity

3. Overview of Napster and Gnutella Napster – A centralized system which answers users queries directly. Lacks any significant security and has single point of failure. Gnutella – Fully distributed, file sharing system Users query their neighbors recursively.

4. Query process causes significant traffic. Lacks anonymity. Anonymity: –Initiator anonymity. –Responder anonymity. –Mutual anonymity.

5. Anonymity Normal TCP/IP provides no privacy for network users. Anonymous protocols trade bandwidth for anonymity e.g. Onion Routing, Hordes etc. Onion Routing: Initiator randomly chooses a path of proxies and encrypts message multiple times using public keys of each proxy.

6. Issues with providing Anonymity Reforming path between initiator and responder over time gives attacker information to determine the initiator of the connection. Responder anonymity protocols should avoid setting up path between initiator and responder ahead of time. Responders tend to be Servers which stay up for a long time resulting in degradation of anonymity.

7. Proxy for Responder Anonymity Based on unicast routing. Responder sets up a connection to a proxy using initiator anonymity. Proxy maintains an alias for the responder. Initiator contacts proxy over an anonymous channel specifying the alias of the responder.

8. Multicast Responder Anonymity Responder chooses a responder alias(m,id), m is a multicast address and advertises it on a public site (e.g. IRC channels, newsgroups, etc) Initiator sends its message to multicast group specifying the responder alias. Responder can then contact initiator anonymously by using initiator anonymity protocol.

9. Mutual Anonymity- APFS A Coordinator acts as a bootstrapping point. APFS removes a single central server problem by using peers as servers for a short time. Client gets list of other clients that are willing to form anonymous route from coordinator. Peers that want to act as query server send anonymous message to coordinator containing server identifier, a Nonce and current Tail node of the server. The message is encrypted using coordinator’s public key.

10. APFS contd.. Servers wait for other peers to anonymously send lists of files to share. When a sever receives a query from a peer for a file, for each match it sends Filename, Anonymous ID of the peer sharing the file and the tail node for the peer. The user should contact the tail node to contact the sharing peer.

11. APFS contd.. To stop participating in a session, a server sends a message with its nonce incremented to the coordinator. The Nonce prevents the message from being easily forged by others which would have resulted in Denial of Service attack.

12. APFS:Anonymous Peers Peers start by anonymously querying the coordinator for available servers. The coordinator replies with list of servers including the server id and tail node for each server. Peer announces the servers about the user’s shared content. User’s periodically send updates of their file list to inform the server.

13. APFS Multicast Initial participant selects a multicast address for the session. Servers, Peers send message to multicast group instead of coordinator. Servers handle queries and return query results unicast.