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Communications of the ACM

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Metadata-Private Communication for the 99%

metadata, illustration

Credit: Mirexon

Online privacy is a prominent topic in the news and receives growing attention from the public. This motivated messaging services such as WhatsApp, Signal, and Telegram to deploy end-to-end encryption, which hides the content of messages from anyone who listens on the communication. While encryption is widely deployed, it does not hide metadata: anyone capable of tapping the network links can learn who is communicating with whom, at what times, and study their traffic volumes. Metadata reveals a lot about the underlying content. Public announcements by ex-government officials as well as the leaked Snowden documents have made it clear that intelligence organizations have a substantial interest in metadata even for encrypted communication since it often obviates the need for the actual content.13,24,28

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The most popular system for hiding metadata—named Tor7—routes user traffic through a series of relay servers, as illustrated in Figure 1. In this fashion, the first relay only sees traffic to and from Alice, but it does not observe the other end of the conversation. Similarly, the last relay sees Bob's traffic, but does not observe the user at the other end. So even if just one of the relays is honest, that is, keeps secret which incoming message maps to what outgoing message that it forwards, the connection between Alice and Bob remains hidden. One of the key reasons for Tor's popularity is its performance, which can support mobile and desktop users and a variety of applications, such as messaging, Web surfing, and VoIP calls. However, Tor is vulnerable to attackers that can observe traffic going in and out of the honest relays. By tapping relays, attackers can correlate messages that a relay receives to those that it sends, and follow a message from its source to destination. In fact, it is sufficient to tap the first and last Tor relays to correlate traffic and break its privacy guarantees, as shown in Figure 2. Indeed, the Snowden documents revealed the U.S.'s National Security Agency (NSA) NSA and the U.K.'s Government Communications Headquarters (GCHQ) attempted to break the privacy provided by Tor in this manner (as well as by "contributing" their relays to the system).24


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