UBC Theses and Dissertations
Twistor : anonymous message transfer through Twitter Alavi, Marjan Sadat
Anonymous communications is a long sought goal of dissidents, privacy advocates, and a host of other user communities. While a large number of systems have been proposed, those systems generally require large-scale communication infrastructure to be built in order to achieve a non-trivial amount of anonymity. However, the very nature of anonymity has meant that a business rationale for building such infrastructure is lacking. Motivated to design a practical receiver anonymity network with large anonymity set sizes, we present Twistor: a new system for receiver-anonymous communications which leverages the Twitter social graph as the underlying anonymizing layer. To send a twist, a Twistor client first checks for reachability of its intended recipient, using local graph information maintained by interactions with the Twistor server. It then encrypts the message under the recipient's public key and posts the ciphertext to the corresponding user's timeline. Larger ciphertexts are encoded into an image, so as to conform with Twitter's 140 UTF-8 character limit. Twistor clients are listening for Twistor posts and decrypt and repost when those they follow publish a new twist, except for when a TTL indicator is 0. Given self-reducibility properties of ElGamal, even if the adversary, e.g., Twitter, monitors all Twistor posts and re-posts that cascade from an initial post, the anonymity of the receiver and the confidentiality of the plaintext are reducible to the hardness of Decisional Diffie-Hellman problem. Twistor derives its increase in size of the receiver anonymity set from asymmetric social connections combined with the publish-subscribe communication model in Twitter. Our aim is to achieve receiver anonymity set sizes on the order of hundreds of thousands. In this thesis we describe our built system, the cost to the underlying infrastructure and the tradeoffs between those costs and the size of the anonymity set.
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Attribution-NonCommercial-NoDerivs 2.5 Canada