Mix networks, originally proposed in 1981, provide a way for Internet users to send messages--such as email, blog posts, or tweets--without automatically revealing their identities or their locations. In this talk, we will describe Sphinx, a cryptographic message format used to relay anonymized messages within a mix network. It is the first scheme to support a full set of security features: compactness, efficiency, provable security, indistinguishable replies, hiding the path length and relay position, as well as providing unlinkability for each leg of the message’s journey over the network. We will compare Sphinx to other mix formats, and will also briefly outline Sphinx's security reduction proof.