TAP: A Novel Tunneling Approach for Anonymity in Structured P2P Systems

TAP: A Novel Tunneling Approach for Anonymity in Structured P2P Systems Yingwu Zhu and Yiming Hu University of Cincinnati

Outline • Motivation and Preliminaries • Design of TAP • Evaluation • Conclusions • Future Work

Static Mixes-based Anonymous Systems • Use a small, fixed core set of mixes to form an anonymous tunnel (e.g, anonymous remailer, onion routing) • Limitations • Corrupt entry mixes reveal traffic source • Colluding entry and exit mixes reveal traffic source and destination • Difficult to counter traffic analysis attacks (e.g., cover traffic is expensive and hurts performance) • Capacity problem (small # of mixes and potentially large # of users) • Law enforcement could be a hurdle for deployment

P2P-based Anonymous Systems • An anonymous tunnel is formed by a randomly chosen set of P2P nodes (e.g., Crowds, Tarzan) • Each peer node is a potential mix • Overcome the limitations of static mixes-based anonymous systems • Drawback • A functionality problem: anonymous tunnels are unstable due to node joins and departures in P2P systems

Why TAP? • P2P based anonymous systems pose a functionality problem for tunnels due to dynamism of P2P systems • TAP • A P2P based system, fault-tolerant to node failures • Avoids the functionality problem while providing anonymity • Supports applications in the face of node failures • Long-standing remote login sessions • Anonymous email systems, etc.

Design of TAP • Goal: to strike a balance point between functionality and anonymity in dynamic P2P systems • Two infrastructures TAP relies on: • P2P (secure) routing infrastructure (a message could be securely routed to a destination node in the case that a fraction of nodes are malicious) • P2P replication mechanism (k replicas for each data item are stored on k different nodes)

Design of TAP • Basic idea • Decouple anonymous tunnels from fixed nodes • A tunnel is formed by a sequence of tunnel hops, each of which is specified by a hopId(hop identifier) instead of IP address • A tunnel hop is an abstract for a hop node (whose nodeId is numerically closest to its hopId), a tunnel therefore is fault-tolerant to hop node failures by relying on P2P replication mechanism • Use a mix-style layered encryption

<h1, k1, H(PW1)> <h2, k2, H(PW2)> <h3, k3, H(PW3)> <h1, k1, H(PW1)> D I <h3, k3, H(PW3)> P1 P3 P2 <h3, k3, H(PW3)> TAP’s tunneling mechanism h1,{h2,{h3,{D,m}k3}k2}k1 {h2,{h3,{D,m}k3}k2}k1 {h3,{D,m}k3}k2 I: initiator node D: destination node Pi:tunnel hop node, whose nodeId is numerically closest to hopIdhi Tunnel hop anchor {M}K: encryption of message M withsymmetric key K m {D,m}k3

Tunnel Hop Anchor (THA) • A tunnel hop is “anchored” in the system through THA • In the form of <hopId, K, H(PW)> • hopId: hop identifier, acts as a DHT key for THA’s storage and retrieval • K: symmetric key for encryption/decryption • H(PW): hash of a password PW, to secure the THA • Stored on k nodes whose nodeIds are numerically closest tohopId (P2P replication)

TAP’s Tunnel • Step1: generate a set of THAs • THAs are node-specific, avoiding colliding with other nodes’ THAs • But do not reveal the node’s identity • Step2: anonymously deploy the generated THAs • Use a bootstrapping anonymous tunnel • Step3: form a tunnel using the deployed THAs • Select a sequence of deployed THAs • Selected THAs should be scattered in the identifier space as far as possible • Step4: send messages through the formed tunnel to achieve anonymity

Anonymous File Retrieval • An initiator node Iwants to anonymously retrieve a file f with fid as its fileId (file identifier) • Create a forward tunnel Tf consist of 3 hops with hopIds of h1, h2, and h3 respectively • Create a reply tunnel Tr consist of 3 hops with hopIds of h4, h5, and h6, Tr={h4,{h5,{h6,{bid,fakeOnion}k6}k5}k4}, where bid is an identifier falling into I’s responsible region • Create a message M={h1,{h2,{h3,{fid,K’,Tr}k3}k2}k1}, where K’ is a temporary public key • Send out M through Tf

Anonymous File Retrieval • The destination node D which is responsible for the file f • Encrypt f with a symmetric key k: {f}k • Encrypt k with K’: {k}K’ • Send out {f}k + {k}K’ through the reply tunnel Tr • The Initiator I • Receive the message {f}k + {k}K’ from the reply tunnel Tr • Decrypt the file f

Tunnel Performance Enhancement • Consider a message M which routes through a tunnel of 3 hops with hopIds of h1, h2 and h3: M ={h1,{h2,{h3,{D,m}k3}k2}k1} • Each tunnel hop involves logN hops (N is the number of nodes in the system) due to P2P routing algorithm • Enhancement: embedding IP address of tunnel hop nodes into M • M ={h1,IP1,{h2,IP2,{h3,IP3,{D,m}k3}k2}k1}

Evaluation • Fault-tolerant to node failures • Impact of colluding malicious nodes • Impact of P2P system dynamism • Tunneling performance

Fault-tolerant to Node Failures For a 10,000 node P2P system with 5,000 tunnels (each tunnel’s length is 5):(1) TAP’s tunnels are more fault-tolerant to node failures than current tunneling techniques;(2) A higher replication factor k makes TAP’s tunnels more fault-tolerant to node failures

Colluding Malicious Nodes For a 10,000 node P2P system with 5,000 tunnels (each tunnel’s length is 5 and the replication factor k is 3):(1) There is no significant corrupted tunnels even when the fraction of malicious nodes is large (=0.3)(2) The fraction of corrupted tunnels increases as the replication factor k increases (not shown here)(3) The fraction of corrupted tunnels decreases with the increasing tunnel length (not shown here)

Impact of P2P Dynamism For a 10,000 node P2P system with 5,000 tunnels initially (each tunnel’s length is 5, the replication factor k is 5, and the fraction of malicious nodes is fixed at 0.1):(1) During each time unit, 100 benign nodes leave and then another 100 nodes join(2) un-refreshed: keeps the 5,000 tunnels unchanged(3) refreshed: a new set of 5,000 tunnels are created to replace the old set of tunnels after each time unit --- TAP should reform tunnels periodically to deal with P2P dynamism in the face of malicious nodes

Tunneling Performance Transfer a 2Mb file in a P2P system ranging from 100 to 10000 nodes:(1) overt: rely on P2P routing without any anonymous tunneling mechanism(2) TAP_basic: using TAP’ basic tunneling mechanism(3) TAP_opt: using TAP’s enhanced scheme(4) l: tunnel length--- TAP’s basic tunneling introduces big overhead in file transferring--- a longer tunnel length introduces bigger overhead--- TAP’s enhanced scheme reduces overhead significantly

Conclusions • Leveraging P2P secure routing and replication mechanism, TAP is fault-tolerant to node failures • By carefully choosing tunnel lengthl and replication factor k, TAP strikes a balance between functionality and anonymity • TAP’s enhanced scheme improves its performance significantly • TAP users should reform their tunnels periodically against colluding malicious nodes in very dynamic P2P systems

Future Work • TAP lacks the ability to control future hops along a tunnel, and it trades this ability for functionality • If we can control future hops like Tarzan, TAP may provide stronger anonymity • TAP needs a mechanism to detect corrupted tunnels

TAP: A Novel Tunneling Approach for Anonymity in Structured P2P Systems