Protecting Free Expression Online Peer-to-Peer Networking Peer-to-Peer with Freenet Freenet uses a decentralized P2P architecture to create an uncensorable and secure global information storage system. Ian Clarke he growth of censorship and ero- version of the software is currently and Scott G. Miller sion of privacy on the Internet available under open source at http:// Uprizer Tincreasingly threatens freedom of www.freenetproject.org/. expression in the digital age. Personal In simulations of up to 200,000 nodes, Theodore W.Hong information flows are becoming subject Freenet has proved scalable and fault tol- Imperial College of Science, to pervasive monitoring and surveillance, erant. It operates as a self-organizing P2P Technology, and Medicine and various state and corporate actors are network that pools unused disk space trying to block access to controversial across potentially hundreds of thousands Oskar Sandberg information and even destroy certain of desktop computers to create a collab- and Brandon Wiley materials altogether. Recent incidents orative virtual file system. To increase Freenet Project Inc. such as the publication of Monica Lewin- network robustness and eliminate single sky’s deleted personal e-mails in a U.S. points of failure, Freenet employs a com- congressional report further point to an pletely decentralized architecture. Given unprecedented level of intrusion into pri- that the P2P environment is inherently vate life.1 These trends cause concern not untrustworthy and unreliable, we must only to whistleblowers and political dis- assume that participants could operate sidents, but to anyone disturbed by the maliciously or fail without warning at thought of others reading their e-mail or any time. Therefore, Freenet implements following their Web activities. strategies to protect data integrity and Fortunately, concurrent advances in prevent privacy leaks in the former the power of personal computers have instance, and provide for graceful degra- made it possible to develop peer-to-peer dation and redundant data availability in technologies to respond to these chal- the latter. The system is also designed to lenges. Our project, Freenet, is a distrib- adapt to usage patterns, automatically uted information storage system replicating and deleting files to make the designed to address information priva- most effective use of available storage in cy and survivability concerns.2 A beta response to demand. 40 JANUARY • FEBRUARY 2002 http://computer.org/internet/ 1089-7801/02/$17.00 ©2002 IEEE IEEE INTERNET COMPUTING Freenet Design Motivation Maintaining privacy for creating and retrieving As documented by Human Rights Watch files means little without also protecting the files (http://www.hrw.org/advocacy/internet/) and the themselves — in particular, keeping their holders Global Internet Liberty Campaign (http://www. hidden from attack. We have thus made it hard to gilc.org/), governments around the world have discover exactly which computers store which undertaken efforts to force Internet service files. Together with redundant replication of data, providers to block access to content deemed holder privacy makes it extremely difficult for unsuitable or subversive, or to make them liable censors to block or destroy files on the network. for such material hosted on their servers. The Elec- Freenet does not, however, explicitly try to tronic Privacy Information Center (http://www. guarantee permanent data storage. Because disk epic.org/) has also raised privacy and civil liber- space is finite, a tradeoff exists between publish- ties questions about developments like the Feder- ing new documents and preserving old ones. Many al Bureau of Investigation’s Carnivore electronic systems solve this problem by requiring payment monitoring system and the European Union’s new (in disk space or money, for example), but we “Convention on Cybercrime,” which gives author- would rather encourage publishing than keep out ities broad powers to intercept and record digital authors who can’t run peer nodes themselves or communications. are too poor to pay for storage. To keep junk doc- Though seemingly separate, the prevention of uments from filling all available space or overwrit censorship and the maintenance of privacy are both ing existing data, we fundamental to free expression in a potentially hos- implement a proba- tile world. Preserving the availability of controver- bilistic storage policy. We must assume that sial information is only half the problem; individu- We hope, however, that als can often be subject to adverse personal Freenet will attract suf- participants could consequences for writing or reading such informa- ficient resources from tion and might need to conceal their activity in order participants to preserve operate maliciously to protect themselves. Indeed, the U.S. Supreme most files indefinitely. Court, among others, has long recognized the impor- or fail without warning. tant role of anonymous speech in political dissent. Freenet A common objection to mechanisms for secure Architecture communication is that criminals might use them Freenet participants each run a node that provides to evade law enforcement. Freenet is not particu- the network some storage space. To add a new file, larly attractive for such purposes, as it is designed a user sends the network an insert message con- to broadcast content to the world — not so useful taining the file and its assigned location-indepen- for secret criminal plots. In any case, however, dent globally unique identifier (GUID), which anonymous electronic communication is simply a causes the file to be stored on some set of nodes. tool, like payphones or postal mail, to be used for During a file’s lifetime, it might migrate to or be good or bad. A terrorist might use it to plan an replicated on other nodes. To retrieve a file, a user attack, or an informant could use it to turn the ter- sends out a request message containing the GUID rorist in to the authorities. Most importantly, the key. When the request reaches one of the nodes freedom to communicate is a fundamental value where the file is stored, that node passes the data in a democratic society. There is no way to deny it back to the request’s originator. to the “bad guys” without also denying freedom to the “good guys” — civil rights activists, minority GUID Keys religious groups, or ordinary citizens who simply Freenet GUID keys are calculated using SHA-1 wish to keep their affairs private. secure hashes. The network employs two main In designing Freenet, we focused on types of keys: content-hash keys, used for prima- ry data storage, and signed-subspace keys, intend- I privacy for information producers, consumers, ed for higher-level human use. The two are anal- and holders; ogous to inodes and filenames in a conventional I resistance to information censorship; file system. I high availability and reliability through decen- tralization; and Content-hash keys. The content-hash key (CHK) is I efficient, scalable, and adaptive storage and the low-level data-storage key and is generated by routing. hashing the contents of the file to be stored. This IEEE INTERNET COMPUTING http://computer.org/internet/ JANUARY • FEBRUARY 2002 41 Peer-to-Peer Networking Related Work in P2P The best-known systems similar to Freenet such a service.Free Haven is an Eternity-like tage to these systems is that they can pro- are Napster (http://www.napster.com/) and anonymous P2P publication system that uses vide strong guarantees that data will be Gnutella (http://gnutella.wego.com/), which trust mechanisms and file trading to enforce located within certain time bounds (gener- both implement large-scale pooling of disk server accountability and user anonymity.2 ally logarithmic) if it exists.Thus, they can space among individual users.The major Unfortunately,it can take a very long time — provide better handling of issues like stor- difference is that whereas Freenet provides even days — to retrieve files from it. age management. a file-storage service, these systems pro- The main disadvantage of these systems vide a file-sharing service.That is, partici- Security Issues relative to Freenet is that they are more dif- pants make files available to others but do Several recently developed P2P file-storage ficult to secure against attack. It is easier for not push files to other nodes for storage. systems focus on efficient data location a malicious node to manipulate its identity This architecture means that data is not rather than privacy and security against to gain responsibility for a particular piece persistent in the network; rather, files are malicious participants. Systems such as of data and suppress it. Links and routing available only when their originators (or OceanStore,3 Cooperative File System are also more visible and deterministically subsequent requesters) are online.Anoth- (CFS),4 and PAST5 are all based on routing structured, making it easier to trace mes- er difference is that neither system models in which each node is assigned a sages and harder to route around malicious attempts to provide anonymity.Gnutella is fixed identity and maintains some knowl- nodes that sabotage requests (for example, also extremely inefficient, broadcasting edge of nodes whose identities vary in by pretending data could not be found). thousands of messages per request. specified ways from its own.These systems PAST,as currently constituted, also requires Freenet more closely resembles the Eter- deterministically place data on nodes that users to trust external smart cards. nity service, which was described in a pro- most closely match the data’s globally posal for a highly survivable network for per- unique identifier (GUID).A user can thus Privacy Issues manently and anonymously archiving locate data by progressively visiting nodes Systems focusing on privacy for informa- information.1 However,the proposal lacked whose identities match more and more tion consumers include browser proxy ser- specifics on how to efficiently implement bits of the desired GUID.The main advan- continued on p.