Paper 2025/860

sPAR: (Somewhat) Practical Anonymous Router

Abstract

Anonymous communication is one of the fundamental tools to achieve privacy for communication over the internet. Almost all existing design strategies (e.g., onion routing/Tor, mixnets) for anonymous communication rely on the existence of some honest server/router in the network infrastructure to provide anonymity. A seminal work by Shi and Wu (Eurocrypt 2021) proposes the first cryptographic design for a non-interactive anonymous router (NIAR) that can use a single untrusted server or router to permute a set of messages without revealing the permutation to the untrusted router. This work is a really important step towards showing the possibility of designing such protocol from standard cryptographic assumptions. However, the existing constructions are only of theoretical nature and still leaves many open questions towards realizing such systems in practice: (1) the cryptographic building blocks used in those designs are really difficult to implement in practice. (2) Their setup phase takes the permutation as an input to generate the encryption/decryption keys; which means that the messages from the same sender in different rounds will be at the same position in the output vector, unless the setup phase is run before every round with a new permutation. (3) It is not known how to realize such a setup procedure, that initializes a random permutation obliviously, without any trusted entities in the system. In this paper, we propose the first (somewhat) practical design, which we call sPAR, that solves the above problems. Our design also relies on a one-time setup phase, however the setup phase does not take any specific permutation as input. Instead, our design can reuse the same setup for many rounds and generates a fresh permutation for every round based on the random values locally generated by the clients. Our design is implementable and deployable in practice; and this presents a new direction for designing anonymous communication systems. Unlike some existing systems like Tor, sPAR does not scale to millions of users, however, we demonstrate with a proof-of-concept implementation that sPAR supports around one hundred users, and show that an optimized variant of our algorithm reduces the latency to a few seconds per message.