Outfox: a Packet Format for a Layered Mixnet

We propose Outfox, a packet format based on layered encryption that is suitable for mixnets in which all paths have the same length and where all mix nodes are associated with a single layer. Outfox is a variant of the packet format Sphinx that removes unnecessary padding and optimizes the computation cost of packet processing by halving the number of public key operations performed by mix nodes. Outfox uses a KEM scheme as a building block and is quantum-safe when instantiated with a quantum-safe KEM scheme. To analyze the security of Outfox, we describe an ideal functionality for a layered replyable mixnet that requires reply-request indistinguishability, and a construction based on Outfox that realizes our ideal functionality.