Outfox: a Packet Format for a Layered Mixnet

TL;DR

Outfox introduces a quantum-resistant, efficient packet format for mixnets that maintains strong anonymity guarantees while reducing computational and communication costs compared to traditional schemes.

Contribution

We propose Outfox, a simplified, post-quantum secure variant of Sphinx, with formal security proof and improved efficiency for layered mixnet architectures.

Findings

Outfox achieves comparable anonymity to Sphinx with lower costs.

It is secure under the UC framework against quantum adversaries.

Outfox is adaptable to various post-quantum cryptographic primitives.

Abstract

Anonymous communication relies on encrypted packet formats that resist traffic analysis and ensure unlinkability. Sphinx, the current standard for mixnets, provides strong anonymity but relies on classical public-key cryptography, making it vulnerable to quantum attacks. In this paper, we present Outfox, a simplified variant of Sphinx tailored for mixnets with fixed-length routes and designed for post-quantum security. Outfox reduces both computational and communication costs. We formally define Outfox and prove its security in the Universal Composability (UC) framework. Our evaluation shows that Outfox retains strong anonymity guarantees while offering improved efficiency and adaptability to quantum-resistant cryptographic primitives.