arXiv subjects

Florentin Rochet

Publications and source records attributed to Florentin Rochet.

7 recordsLinked to original sources

arXiv2026

Sociotechnical Aspects of Tor Relay Rejection

In 2019, the Tor Project enforced an end-of-life (EoL) policy for Tor versions, leading to the rejection of outdated relays, amounting to a notable fraction of consensus weight. While this policy aids network maintenance, reduces backporting efforts, and shortens vulnerability exposure, its sociotechnical implications remain unstudied. A user study ($N=26$) reveals that relay operators, though not universally aware of the EoL policy, generally view it favorably. Operational practices vary, occasionally excluding newly installed relays from the network. Network simulations, grounded in historical data, assess the policy's immediate impact on Tor clients against common adversaries. Results indicate a marginal adversarial advantage, with network churn (i.e., relays entering and exiting) exerting a more pronounced effect on user anonymity. Security metrics are introduced to evaluate relay contributions against two adversary models, enabling ranking by individual utility and security. Analysis of four exclusion rounds shows that a minority of rejected relays typically account for over 50% of the security provided by all excluded relays. Recommendations for EoL policy implementation are proposed to mitigate potential drawbacks.

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arXiv2025

Contiguous Zero-Copy for Encrypted Transport Protocols

We propose in this paper to revisit the design of existing encrypted transport protocols to improve their efficiency. We call the methodology ``Reverso'' from reversing the order of field elements within a protocol specification. We detail how such a benign-looking change within the specifications may unlock contiguous zero-copy for encrypted protocols during data transport. To demonstrate our findings, we release \texttt{quiceh}, a QUIC implementation of QUIC VReverso, an extension of the QUIC V1 standard (RFC9000). Our methodology applied to the QUIC protocol reports $\approx 30\%$ of CPU efficiency improvement for processing packets at no added cost on the sender side and without relaxing any security guarantee from QUIC V1. We also implement a fork of Cloudflare's HTTP/3 module and client/server demonstrator using \texttt{quiceh} and show our optimizations to directly transfer to HTTP/3 as well, resulting in our new HTTP/3 to be $\approx 38\%$ more efficient than the baseline implementation using QUIC V1. We argue that Reverso applies to any modern encrypted protocol and its implementations and that similar efficiency improvement can also be unlocked for them, independently of the layer in which they operate. Indeed, this research shows that the ability to implement contiguous zero-copy on the receiver side inherently depends on the specified encrypted protocol wire image, and that we may need to reverse how we are used to write them.

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arXiv2024

Towards Flexible Anonymous Networks

Anonymous Communication designs such as Tor build their security on distributed trust over many volunteers running relays in diverse global locations. In practice, this distribution leads to a heterogeneous network in which many versions of the Tor software co-exist, each with differing sets of protocol features. Because of this heterogeneity, Tor developers employ forward-compatible protocol design as a strategy to maintain network extensibility. This strategy aims to guarantee that different versions of the Tor software interact without unrecoverable errors. In this work, we cast protocol tolerance that is enabled by forward-compatible protocol considerations as a fundamental security issue. We argue that, while being beneficial for the developers, protocol tolerance has resulted in a number of strong attacks against Tor in the past fifteen years. To address this issue, we propose Flexible Anonymous Network (FAN), a new software architecture for volunteer-based distributed networks that shifts the dependence away from protocol tolerance without losing the ability for developers to ensure the continuous evolution of their software. We i) instantiate an implementation, ii) evaluate its overheads and, iii) experiment with several of FAN's benefits to defend against a severe attack still applicable to Tor today.

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arXiv2023

VeraSel: Verifiable Random Selection for Mixnets Construction

The security and performance of Mixnets depends on the trustworthiness of the Mixnodes in the network. The challenge is to limit the adversary's influence on which Mixnodes operate in the network. A trusted party (such as the Mixnet operator) may ensure this, however, it is a single point of failure in the event of corruption or coercion. Therefore, we study the problem of how to select a subset of Mixnodes in a distributed way for Mixnet construction. We present VeraSel, a scheme that enables Mixnodes to be chosen according to their weights in a distributed, unbiased, and verifiable fashion using Verifiable Random Functions (VRFs). It is shown that VeraSel enables any party to learn and verify which nodes has been selected based on the commitments and proofs generated by each Mixnode with VRF.

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arXiv2022

Stopping Silent Sneaks: Defending against Malicious Mixes with Topological Engineering

Mixnets provide strong meta-data privacy and recent academic research and industrial projects have made strides in making them more secure, performance, and scalable. In this paper, we focus our work on stratified Mixnets -- a popular design with real-world adoption -- and identify that there still exist heretofore inadequately explored practical aspects such as: relay sampling and topology placement, network churn, and risks due to real-world usage patterns. We show that, due to the lack of incorporating these aspects, Mixnets of this type are far more susceptible to user deanonymization than expected. In order to reason and resolve these issues, we model Mixnets as a three-stage ``Sample-Placement-Forward'' pipeline, and using the results of our evaluation propose a novel Mixnet design, Bow-Tie. Bow-Tie mitigates user deanonymization through a novel adaption of Tor's guard design with an engineered guard layer and client guard-logic for stratified mixnets. We show that Bow-Tie has significantly higher user anonymity in the dynamic setting, where the Mixnet is used over a period of time, and is no worse in the static setting, where the user only sends a single message. We show the necessity of both the guard layer and client guard-logic in tandem as well as their individual effect when incorporated into other reference designs. Ultimately, Bow-Tie is a significant step towards addressing the gap between the design of Mixnets and practical deployment and wider adoption because it directly addresses real-world user and Mixnet operator concerns.

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arXiv2019

Flexible Anonymous Network

Internet technologies have been designed from guidelines like the robustness principle also known as Postel's law. Jon Postel's law is described as: "Be conservative in what you do, be liberal in what you accept from others." Fundamentally, it advises protocol designs to be tolerant with what they accept from the other peers. We propose to take a step back and wonder how the robustness principle could be revisited to support security requirements as well as unifying flexibility from specifications, protocol design and software implementations. Our goal would be to define a software architecture that offers the benefits of the robustness principle (i.e., efficient network services despite the presence of various software versions), while also guaranteeing that this robustness cannot be exploited by making sure that it is only used to support authentic evolution of the protocol specification.

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arXiv2016

Waterfiling: Balancing the Tor network with maximum diversity

We present the Waterfilling circuit selection method, which we designed in order to mitigate the risks of a successful end-to-end traffic correlation attack. Waterfilling proceeds by balancing the Tor network load as evenly as possible on endpoints of user paths. We simulate the use of Waterfilling thanks to the TorPS and Shadow tools. Applying several security metrics, we show that the adoption of Waterfilling considerably increases the number of nodes that an adversary needs to control in order to be able to mount a successful attack, while somewhat decreasing the minimum amount of bandwidth required to do so. Moreover, we evaluate Waterfilling into Shadow and show that it does not impact significantly the performance of the network. Furthermore, Waterfilling reduces the benefits that an attacker could obtain by hacking into a top bandwidth Tor relay, hence limiting the risks raised by such relays.

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