Defeating Vanish with Low-Cost Sybil Attacks Against Large DHTs

http://z.cs.utexas.edu/users/osa/unvanish/papers/vanish-broken.pdf

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Abstract

Researchers at the University of Washington recently proposed Vanish, a system for creating messages that automatically “self-destruct” after a period of time. Vanish works by encrypting each message with a random key and storing shares of the key in a large, public distributed hash table (DHT). Normally, DHTs expunge data older than a certain age. After they expire, the key is permanently lost, and the encrypted data is permanently unreadable. Vanish is an interesting approach to an important privacy problem, but, in its current form, it is insecure. In this paper, we defeat the deployed Vanish implementation, explain how the original paper’s security analysis is flawed, and draw lessons for future system designs.

We present two Sybil attacks against the current Vanish implementation, which stores its encryption keys in the million-node Vuze BitTorrent DHT. These attacks work by continuously crawling the DHT and saving each stored value before it ages out. They can efficiently recover keys for more than 99% of Vanish messages. We show that the dominant cost of these attacks is network data transfer, not memory usage as the Vanish authors expected, and that the total cost is two orders of magnitude less than they estimated. While we consider potential defenses, we conclude that public DHTs like Vuze probably cannot provide strong security for Vanish.