Hello Marc, Thanks for the work. It looks very interesting, I'm happy to see people looking into evolving FEPs. I don't think for us make sense to replace obfs4 for now, as we see censors blocking all FEPs. But I think we still have spaces where they will be useful and we should keep in mind your work for future needs. Quoting Marc Himmelberger via tor-dev (2025-07-31 20:12:14)
Hi,
I have just concluded my Master's Thesis at ETH Zurich titled "Implementing and Evaluating
Quantum-Safe Fully Encrypted Protocols" and I believe the content is relevant for this mailing list.
My thesis consists mainly of the implementation of the "Drivel" protocol [1]. Two of the authors of [1], Felix Günther (mail@felixguenther.info) and Shannon Veitch (shannon.veitch@inf.ethz.ch), supervised this thesis and are also available for questions regarding the protocol design in this thread.
Drivel is an evolution of obfs4 [2] that:
-
allows for key encapsulation mechanisms, particularly the ones proposed as part of the NIST standardization [3], -
has stronger obfuscation, providing more guarantees than obfs4 in the scenario that censors should get ahold of bridge information such as NodeID and long-term public key, -
contains a thin crypto-agility layer, allowing the replacement of cryptographic algorithms as needed, -
provides more configuration options to bridge operators regarding performance trade-offs by allowing for runtime-configuration of the cryptographic algorithms, and -
allows for post-quantum traditional hybrid instantiations. Note:; my thesis covers the “pure” post-quantum case (i.e., non-hybrid). Due to the crypto agility, hybrids are relatively easy to add by implementing an appropriate PQ/T hybrid combiner, such as OEINC [1].
Additionally, there are proposed changes to Drivel itself, evaluations of the implementation's performance, and some security proofs in the thesis. The main purpose of my proposed changes is to add more flexibility in traffic shaping (minimize “quiet periods” previously discussed on this list [4]) and to optimize performance.
The thesis PDF is available under: https://doi.org/10.3929/ethz-b-000746588
The chapters "Implementing Drivel" and "Experimental Evaluation" should be most relevant to this list.
I encourage everyone interested in pluggable transports to have a look if this sounds intriguing.
The implementation is done as a fork of the lyrebird repository [5] and is available under: https://github.com/marc-himmelberger/lyrebird-drivel
The Tor Project is more than welcome to use my work for future changes to lyrebird, although I do believe some changes to drivel and some optimizations to my code may be beneficial for performance reasons before the pluggable transport is used in production.
In case of any feedback or questions, do not hesitate to reach out.
Best regards,
Marc Himmelberger
[1] https://eprint.iacr.org/2025/408.pdf
[2] https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/lyreb...
[3] https://csrc.nist.gov/projects/post-quantum-cryptography
[4] https://archive.torproject.org/websites/lists.torproject.org/pipermail/tor-d...
[5] https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/lyreb...
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