  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.
