Social Networking for Anonymous Communication Systems: A Survey Rodolphe Marques Andre´ Zuquete´ Instituto de Telecomunicac¸oes˜ Universidade de Aveiro/IEETA Aveiro, Portugal Aveiro, Portugal [email protected] [email protected] Abstract relay node. This leaves a door open to malicious behavior in the network, where an attacker can run one or several Anonymous communication systems have been around for relay nodes in the network in an attempt to undermine the sometime, providing anonymity, enhanced privacy, and cen- anonymity that these networks provide to their clients. sorship circumvention. A lot has been done, since Chaum’s Attacks on anonymous communication systems have been seminal paper on mix networks, in preventing attacks able thoroughly studied, most of them being passive attacks to undermine the anonymity provided by these systems. through traffic analysis. The most common attacks are the This, however, is a difficult goal to achieve due to the de- predecessor attack [1], the intersection attacks [2], timing centralized nature of these systems. In the end it boils down attacks [3], and Sybil attacks [4]. These attacks can be to finding a subset of trusted nodes to be placed in critical easily performed by controlling some specific relay nodes in positions of the communication path. But the question re- the circuits created by the clients. Timing attacks are more mains: ”How to know if a given node can be trusted?”. In critical in low latency anonymity networks and are extremely this paper we present a survey of a new research area which difficult to detect do to their passive nature. goal is to exploit trust in social links to solve some of the shortcomings of anonymous communication systems. Recent One way to thwart these attacks would be to gain some research shows that by using social networking features it information about the nodes in the network, in order to is possible to prevent traffic analysis attacks and even detect establish trust relationships with a subset of nodes to relay Sybil attacks. clients’ traffic. This model would substitute the actual model in which nodes are chosen randomly. Index Terms For the purpose of this survey we define a social network as a network of people that share a common goal and are Social Network, Security, Anonymity, Sybil Attack, Repu- connected through bi-directional trust relationships. tation In this survey we study current proposals that try to exploit trust relationships of social links. The end goal varies, but the 1. Introduction main idea is to find out if it is possible to create anonymity sets which are trusted, and offer protection against traffic In recent years a new research area emerged with the goal analysis. of exploiting social networking features in order to solve Anonymity sets are sets of entities that have the same some of the short comings of anonymous communication probability of being the confused with the real client of an systems. anonymization network. The larger the anonymity set, the Anonymous communication systems are usually built as highest the degree of anonymity provided by the set. Choos- peer-to-peer overlay networks, that rely on a community of ing relay nodes from a trusted anonymity set, as opposed to volunteers that are responsible for maintaining and running randomly chosen nodes, may result in smaller anonymity the nodes that constitute the network. Users then make use sets but, as we will show, this does not necessarily mean of this networks to remain anonymous, by creating a circuit, less anonymity. or multi-hop path, through a set of selected relay nodes in the network. This paper is organized as follows: in section 2 we give Volunteers are an important part of these networks, since a brief overview of anonymous communication systems, in the degree of anonymity provided by such networks depends section 3 we present an overview of the most common on the number of nodes and their behavior in the network. attacks. In section 4 we present a survey of the work pre- On the other end, the open nature of these networks allow sented this far on how to exploit social networks to improve anyone willing to share some of its bandwidth to run a anonymity, followed by the conclusions in section 5. 2. Overview of Anonymous Communication crowd to issue requests to Web servers in a way that prevents Systems Web servers and other crowd members from determining who initiated those requests, by randomly choosing another In 1981 Chaum proposed a method to protect the identities member of the crowd to which it redirects its requests. This of communicating parties, called mix networks [5]. The random request redirection is then performed several times basic functionality of a mix network is to provide sender within the crowd, until a crowd member delivers that request and receiver anonymity by using proxy relaying servers. to the target Web server. Each message is encrypted in each proxy using public key Freenet [8] can be seen as a social network where users cryptography, resulting in a layered encryption. can anonymously share files, browse and publish Web sites Onion routing [6] is based on the mix networks proposed accessible only through Freenet, and chat on forums. Freenet by Chaum. It provides connections that are strongly resistant is decentralized, in order to make it less vulnerable to to both eavesdropping and traffic analysis. Onion Routing attacks, and it allows an operation mode where users only operates by dynamically building anonymous connections connect to their friends. Freenet stores documents and allows within a network of real-time Chaum mixes [5]. them to be retrieved later by an associated key. The system An onion is a recursively layered data structure that has no central servers and is not subject to the control of specifies properties of the connection at each point along the any individual or organization. Freenet is implemented as a route. Each onion router along the route uses its public key to peer-to-peer network of nodes that query one another to store decrypt onions that it receives. This operation exposes the and retrieve files, which are named by location-independent cryptographic control information, the identity of the next keys. onion router, and the embedded onion. The onion router pads Anonymizer [9] is a tool that attempts to make activity the embedded onion to maintain a fixed size, and sends it on the Internet untraceable by accessing the Internet on the to the next onion router. After the connection is established, users’ behalf, hiding computer identifiers of source users. data can be sent in both directions. Data from the initiator Anonymizer acts as a trusted intermediary between its users is repeatedly pre-encrypted using the algorithms and keys and the Web sites they are viewing, hiding the user IP that were specified in the onion. As data moves through address, making it almost impossible for third parties to track the anonymous connection, each onion router removes one the sites that the user visits and build profiles of the user layer of encryption as defined by the cryptographic control activities on the Internet. It is also able to protect personal information in the onion defining the route, so the data information by redirecting users’ traffic through its secure arrives as plain text at the recipient servers. Tor [7] is a popular software that uses onion routing. The Tor network is a distributed relay network that relays 3. Common Attacks in Anonymous communi- arbitrary TCP streams over a network of relay nodes called cation systems Onion Routers. Each onion router maintains a long term identity key used to sign TLS certificates and maintain a TLS The most common attacks know to anonymous commu- connection to every other Onion Router. Each Tor client runs nication systems are: a local software called Onion Proxy which is responsible • Predecessor attack: In the predecessor attack, the at- for fetching router lists, the current network state, establish tacker tracks an identifiable stream of communications circuits across the network, and handle connections from the over a number of rounds (path reformation). In each user applications. Clients choose a path through the network round, the attacker simply logs any node that sends a and build a circuit, in which each node in the path knows its message that is part of the tracked stream. The attack predecessor and successor, but no other nodes in the circuit. does not always require analysis of timing or size Traffic flows down the circuit in fixed-size cells, which are of packets, but instead exploits the process of path unwrapped by a symmetric key at each node and relayed initialization. downstream. • Intersection attack: An attacker having information Crowds [1] prevents a Web server from learning any po- about which users are active at any given time can, tentially identifying information about its clients, including through repeated observations, determine which users the clients’ IP address or domain name. Crowds works by communicate with each other. This attack is based on collecting Web clients into a geographically diverse group the assumption that users typically communicate with called a “crowd” that performs Web transactions on behalf a relatively small number of parties. of its members. A user joins the crowd by starting a process • Timing attack: In a timing attack, the attacker studies in his local machine, called a “jondo”, during which it the timing of messages moving through the system is informed of the other current crowd members and the to find correlations. It may be used by two or more other crowd members are informed of the new jondo’s attackers to determine that they are in the same commu- membership.