The Fifteen Year Struggle of Decentralizing Privacy-Enhancing
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The fifteen year struggle of decentralizing privacy-enhancing technology Rolf Jagerman, Wendo Sabee,´ Laurens Versluis, Martijn de Vos, Johan Pouwelse (course supervisor) F Abstract—Ever since the introduction of the internet, it has anonymous internet system, due to the initial been void of any privacy. The majority of internet traffic currently lack of users. To support a large number of is and always has been unencrypted. A number of anonymous users, such a network has to be decentralized. communication overlay networks exist whose aim it is to provide A lack of decentralization would otherwise re- privacy to its users. However, due to the nature of the internet, there is major difficulty in getting these networks to become sult in bottlenecks that place constraints on the both decentralized and anonymous. We list reasons for having number of users. Relying on server bandwidth anonymous networks, discern the problems in achieving decen- donations has proven to be a difficult to sustain tralization and sum up the biggest initiatives in the field and their strategy. current status. To do so, we use one exemplary network, the The most widely used anonymous commu- Tor network. We explain how Tor works, what vulnerabilities this nication system is Tor. In this technical report network currently has, and possible attacks that could be used to violate privacy and anonymity. The Tor network is used as a we will analyse Tor and its semi-centralized key comparison network in the main part of the report: a tabular nature. Tor struggles to keep up with the band- overview of the major anonymous networking technologies in width demands of its users. As the number of use today. users increases, the need to decentralize Tor be- comes more urgent. Decentralizing Tor isn’t an easy task: After fifteen years of decentralization 1 INTRODUCTION attempts, the network is still partially central- All feelings of privacy concerning browsing the ized. Only few decentralized alternatives to Tor internet, talking on the telephone or location exist, however they lack the user base to be tracking of cellphones are an illusion. In recent considered safe and useful. Examples include years the need for privacy-enhancing technol- Gnutella [32], Freenet [19] and Tapestry. arXiv:1404.4818v1 [cs.CY] 18 Apr 2014 ogy has become more apparent. Revelations by This technical report is structured as follow- Edward Snowden of government misconduct ing: In section 2 we will give an introduction and constitutional violations have sent shock and overview of Tor. Known vulnerabilities in waves through the internet. Tor are discussed in section 3. After that, we We failed to make an internet that is secure will talk about decentralization and its prob- and private. Although much research has been lems in section 4. The current state of decen- done on anonymous internet communication, tralized internet systems is discussed in sec- only few systems have been actually imple- tion 5. A comparison of existing decentralized mented and only one is actively being used. networks is made in section 6. Finally, we will One of the most important factors that impact conclude and discuss our findings in section 7. anonymity in a communication system is the number of users. A sufficiently large number of users are required for a system to make guaran- 2 INTRODUCTION TO TOR tees about its ability to protect the privacy of its The implementation of The Onion Router users. This makes it difficult to introduce a new (TOR) was first described in 1996 by the U.S. Navy Research Laboratory, as a means to pro- with its private key, and uses that secret to tect government communications from digital, decrypt the rest of the packet. The result is a as well as physical attacks, by hiding the loca- header with the address for node Y and the tion of the communicating party or parties [15]. payload encrypted with secrets for the follow- The idea behind onion routing traces back to ing nodes in the circuit, which is forwarded to 1981, where Chaum described it in his famous node Y . paper “Untraceable electronic mail, return ad- As a node receives a packet from the previ- dresses, and digital pseudonyms”[9]. ous node, it peels off another layer of encryp- In 2002, the Tor project discontinued their tion, much as how you can peel an onion layer old code base and re-implement the project for layer, and forwards it to the next node in the as Tor, the Second Generation Onion Router. circuit (as specified in the decrypted header). They introduced perfect forward secrecy, direc- When exit node Z decrypts the last layer, it tory servers, hidden services and more [10]. In forwards the payload outside the network to this section, we will explain the various com- the original destination that our trusted client ponents of Tor, the structure of the network, C tried to contact, acting as a traditional proxy. circuit creation and disadvantages of Tor. When exit node Z receives a response, this whole process is applied in reverse order, en- crypting the payload with its secret along the 2.1 Onion routing way, instead of decrypting. When our client C As described in the original design paper of receives the packet, it peels off all the encryp- The Onion Router [15], network traffic is for- tion layers to retrieve the unencrypted payload. warded through a circuit of nodes, where each With the second generation onion routing node only knows the previous and next node used in Tor, a modified algorithm is used to in the circuit. With a sufficiently long circuit derive the encryption keys, called telescoping of (independent) nodes, this means that two path-building, which also provides perfect for- communicating parties can remain oblivious of ward secrecy. This algorithm is described in each others physical location. section 2.4. Say we have a circuit consisting of four nodes: our trusted client node (C), an (entry or guard) relay node (X), a (middle) relay node 2.2 Directory servers (Y ) and an (exit) relay node (Z). In this case, The original Onion Router used an unsafe, de- there are three relay nodes, but this is not centralized node discovery mechanism called necessarily always the case as more middle in-band status updates. During such a status relay nodes can be added. A visualization of update each node broadcasts known nodes to this path can be seen in figure 1. Each node its neighbours. An attacker could exploit this has its own public key and a corresponding to isolate and limit the knowledge of a client, private key. When building the circuit, our forcing connections through malicious nodes. client generates a distinct secret for each of Another disadvantage is that in-band status these nodes. More information about the circuit updates take longer to propagate throughout setup, can be found in 2.4. the network and create a global consensus. The payload of each packet flowing through To mitigate these concerns, directory servers the circuit is first encrypted with the distinct se- were introduced to Tor during its reimple- cret for last node Z, then with the distinct secret mentation. These directory servers keep a re- for node Y , and last with the secret for node dundant central consensus about the network. X. With each layer of encryption, a header is They act as HTTP servers to which Tor nodes added with the address of the next node in the can publish signed information about them- circuit, plus the used distinct secret encrypted selves. Tor clients can in turn download this with corresponding nodes public key. information, as seen in figure 1. After node X receives this packet from our The information distributed by the directory trusted node C, it decrypts the attached secret servers is signed. The keys to verify these Directory Server Guard Node Internet Exit User Node Relay Node Fig. 1: The components of the Tor network. After downloading the node list from the Directory Server, the user creates a circuit through a guard node, a relay node and an exit node. This circuit is used to communicate (anonymously) with the internet. signatures are preloaded in the Tor software, • The entrance Tor router: this router is di- along with the list of directory servers. This rectly connected to an onion proxy and can implies trust by the Tor client in the directory observe the origin of a request through the servers. Tor network. The entrance router sends the packet to the middle Tor router. • The middle Tor router: this router is con- 2.3 Relay and exit nodes nected to the entrance router and the exit The Tor network consists of several compo- router. nents. The clients in the Tor network are known • The exit Tor router: this router is connected as onion proxies. The software to run an onion to the web server. Note that the exit Tor proxy is available for free on the Tor website router is the only router that can observe [37] and is easy for users to configure. The the final destination of the request. onion proxies are responsible for downloading The first router in a circuit is the entrance the directory information, establishing circuits router. The entrance router sends the data to across the network and handling connections one of the middle routers which forwards the from user applications. data to the exit router. The routing in the network is done by onion routers, also called relay nodes. The relay nodes relay the data from the onion proxy to 2.4 Circuit creation the web server across a circuit (circuits are Data on the Tor network travels over several re- described in 2.4).