The increasing amount of personal and sensitive information disseminated over the Internet prompts commensurately growing privacy concerns. Digital data often lingers indefinitely and users lose its control. This motivates the desire to restrict content availability to an expiration time set by the data owner. This paper presents and formalizes the notion of Ephemeral Publishing (EphPub), to prevent the access to expired content. We propose an efficient and robust protocol that builds on the Domain Name System (DNS) and its caching mechanism. With EphPub, sensitive content is published encrypted and the key material is distributed, in a steganographic manner, to randomly selected and independent resolvers. The availability of content is then limited by the evanescence of DNS cache entries. The EphPub protocol is transparent to existing applications, and does not rely on trusted hardware, centralized servers, or user proactive actions. We analyze its robustness and show that it incurs a negligible overhead on the DNS infrastructure. We also perform a large-scale study of the caching behavior of 900K open DNS resolvers. Finally, we propose Firefox and Thunderbird extensions that provide ephemeral publishing capabilities, as well as a command-line tool to create ephemeral files.
Observe that the main flaw of Vanish design is that DHTs are assumed to be resistant to crawling. On the contrary, monitoring all DNS resolvers is realistically infeasible. Further, EphPub provides several improvements in terms of robustness and usability. First, it allows users to select the expiration time: by choosing domain names matching desired lifetime period, users assign their content a more precise expiration time (while in Vanish this is assigned by the specific DHT implementation and due to network churn, unless using a so-called refreshing proxy'' that re-pushes key shares). Also, as we show next, given its simple nature, EphPub does not require users to install any additional software (e.g., DHT client). Thus, it can be easily ported on any architecture and deployed even on mobile phones. Indeed, researchers are already advocating efficient solutions to guarantee retroactive privacy, for instance, in location sharing applications over smartphones @cite_6 .
@cite_6
2140655729
Researchers at the University of Washington recently proposed Vanish [20], 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). DHTs expunge data older than a certain age; after this happens to the key shares, 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.