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A Concept of an Anonymous Direct P2P Distribution Overlay System 22nd International Conference on Advanced Information Networking and Applications A Concept of an Anonymous Direct P2P Distribution Overlay System Igor Margasiński, Michał Pióro Institute of Telecommunications, Warsaw University of Technology {I.Margasinski, M.Pioro}@tele.pw.edu.pl Abstract an anonymous network composed of nodes called Mixes that forward anonymous messages. The strength The paper introduces a peer-to-peer system called of the solution consists in: (i) a specific operation of P2PRIV (peer-to-peer direct and anonymous nodes which “mixes” forwarded messages, and (ii) an distribution overlay). Basic novel features of P2PRIV asymmetric encryption of messages exchanged are: (i) a peer-to-peer parallel content exchange between them. The purpose of such mixing is to hide architecture, and (ii) separation of the anonymization the correlation between received and forwarded process from the transport function. These features messages. In general, received data units are padded to allow a considerable saving of service time while a constant size length, encrypted, delayed for a batch preserving high degree of anonymity. In the paper we aggregation and then sent (flushed) in a random order. evaluate anonymity measures of P2PRIV (using a Anonymous messages are sent usually via a chain of normalized entropy measurement model) as well as its Mixes to eliminate presence of a trusted party and also traffic measures (including service time and network to omit single point of failure imposed by a single Mix. dynamics), and compare anonymity and traffic In Mix-net, each message is encrypted recursively with performance of P2PRIV with a well known system public keys of Mixes from a forwarding path. Finally, a called CROWDS. message for each successive forward has different bit representation and place in a flow of other Mix-net 1. Introduction messages. This makes Mix-net communications practically untraceable and secured against The broadband Internet access has given a way for eavesdropping [6]. explosion of large scale distributed overlay networks. The development of mixing methods is primarily Peer-to-Peer (P2P) overlays have grown to one of the aimed at resolving a tradeoff between delay time leading Internet applications and constitute a significant part of the Internet traffic. From its imposed by batching and reordering of messages and inception, development of P2P communications goes the anonymity level. Regardless of potential hand in hand with demand for anonymity. Still, we discontinuities in incoming traffic, Mixes have to wait observe that highly secured P2P overlays of today for sufficient number of messages to achieve struggle to provide good traffic performance. A novel untraceable mixing. One solution to this problem is P2P system proposed in this paper introduces a parallel generation of fake messages (dummy traffic) ([4], [8], content exchange architecture separating the [13]). Dummies enhance anonymity and allow anonymization process from the transport function. particular Mixes to flush faster. However additional The anonymity and traffic performance measures of and “empty” traffic finally delays delivery of parallel the new system are presented and compared with user data. analogous measures obtained for a classical The concept of Mix-net has been used in a wide architecture. range of applications such as E-mail [9], Web browsing [3], ISDN [18]. Other solutions [5] seem to 1.1. Related work play a less important role or (as CROWDS [20]) utilize an idea of “blending into a crowd” by traffic The first concept of network anonymity was forwarding via a group of nodes before its delivery. introduced in the seminal paper of Chaum [6]. The Anonymity of CROWDS is based on a random walk Mix-net system proposed there has become a algorithm. A random set of CROWDS nodes forward foundation of modern anonymity solutions. Mix-net is 1550-445X/08 $25.00 © 2008 IEEE 590 DOI 10.1109/AINA.2008.117 anonymous messages without usage of mixing or with a file id to a randomly chosen peer. Then, the public key encryption techniques. P2P overlays as well selected peer flips an asymmetric coin to decide as other applications usually adapt Mix-net to assure whether to forward the token (with probability pf) to anonymity. Many variants of Mix-net such as Free the next random peer. This communication may be Haven [10], Tarzan [11] and MorphMix [19] were additionally secured and anonymized by Mix-net introduced for P2P. Other system, as Freenet [7] and mechanisms, as numerous but short control messages GNUNet [2], use heuristics with encryption to achieve of constant length, generated by cloning, can be anonymity. It should be noted that the basic common effectively exchanged by the Mix cascades [12]. mechanism used to achieve P2P anonymization is Step 2: Data connection – transport of the requested traffic forwarding by a set of middleman nodes. content. After a random interval of time and based on the content id received earlier, the copies of the content 1.2. Outline of the paper are directly downloaded by selected (cloned) peers from nodes which store data. One of these nodes is the The rest of the paper is organized as follows. Section initial requestor. Files can be looked up by the DHT 2 describes the system and its anonymization idea. algorithm. Like the cloning exchange, lookup Analytical evaluation of anonymity is presented in messages can be secured by Mix-net mechanisms. The Section 3 while traffic performance simulations are resulting data redundancy (the file is downloaded and described in Section 4. Conclusions and future work stored by each clone) improves content accessibility, are presented in Section 5. because the popularity of a content automatically increases the number of its copies. Notice, that in our 2. P2PRIV overview solution the anonymization process is separated from the content transport, in contrast to classical schemes. P2PRIV is an application layer solution for P2P overlay networks assuring a sender’s and a receiver’s 3. Anonymity anonymity. The basic novel idea of the solution consists in parallel content exchange instead of Below we will analyze the degree of anonymity of widespread cascade transmission between chaining P2PRIV using entropy measurement model ([14], nodes. Certainly, anonymity assured by P2PRIV [21]). The CROWDS system, which combines imposes traffic overheads, as in any other system of anonymity and performance with simplicity and this type, for example Mix-net. A motivation behind reputability, will be used as a reference. The the parallel architecture with direct content transport is anonymity analysis will cover receiver anonymity decrease of the service time while preserving high referred to as an unlinkability of the requested content degree of anonymity. and the requestor. The anonymous publication process, P2PRIV peers are symmetric and do not include any which can be performed in many different ways using privileged nodes (supernodes). The P2PRIV specific the state of the art methods, is not considered in the connection and content exchange architecture utilizes a proposed scheme and omitted in the analysis. We will classical concept of chaining with encryption not analyze sender anonymity as well. However, when anonymization, for example Mix-net. It also uses we assume that P2PRIV utilizes the DHT storage, structured lookup system, which can be based on DHT peers of P2PRIV are involved in the process of storing algorithms (distributed hash table, [1]). The cascade and sending data independently from decisions of they anonymization mechanism assures anonymity of an users. entire P2PRIV control messages exchange, including To achieve practical results it is important to assume the communications of distributed content location realistic capabilities of an adversary corresponding to process. We can distinguish two steps of P2PRIV the specific environment of the attack. P2PRIV is operation: primarily dedicated to public WANs, especially the Step 1: Cloning – exchange of management Internet. We assume that users do not establish private messages applied for anonymous random selection of a groups and that no additional trust or access control subset of peers referred to as a cloning cascade (CC). mechanisms are provided. Any person can become the Each such CC contains the requestor and its clones; system user and can utilize the system’s provided each peer can be potentially selected for such a clone. information at his own way. Initially, we assume that This step is similar to the network random walk the adversary obeys the protocol and conducts a mechanism of CROWDS. The requestor sends a token passive observation. Next we will consider active 591 1− S p attacks enabling attackers to change protocol operation = 1 1 p2 . (6) to disclose more information. Keeping in mind the S 2 large scale of public overlays, we will analyze local According to the information theory [22], entropy of attacks where the adversary can control only a part of P2PRIV for this scenario will be the system. We will study an impact of the number of = − N ()= − ()− ( ) collaborating nodes C on the degree of anonymity, and H PS ∑ pi log2 pi S1 p1log 2 p1 S 2 p2 log 2 p2 . (7) end with a look at a global attack. We assume that i=1 Using the entropy measurement model ([14], [21]) the collaborating nodes can collect all information the degree of the anonymity (normalized entropy) system is leaking and send this statistics via an provided by P2PRIV is independent channel to the adversary headquarter for a ≥ ∨ ≥ 0 p1 1 p2 1 summarizing analysis. C ()−1 log 2 p1 N p ≤ 0 − 2 3.1. Passive-static attacks log 2 (N C) − C ()−1 d = 1 log 2 p2 PS N (8) Static attackers cannot predict which nodes will be p ≤ 0 − 1 log 2 (N C) randomly selected to form the CC for a particular C ()−1 + − C ()−1 request anonymization. The adversary can distinguish log 2 p1 1 log 2 p2 N N p ∈ ()0,1 ∧ p ∈ ()0,1 , two sets of peers {S ,S } among all N nodes and assign − 1 2 1 2 log 2 (N C) their members probabilities of being the requestor where {p1,p2}.
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