Serge Vaudenay

Biographie

Serge Vaudenay entered at the Ecole Normale Supérieure in 1989 with a major in mathematics. He earned his agrégation (secondary teaching degree) in mathematics in 1992, then a PhD in Computer Science at the University of Paris 7 - Denis Diderot in 1995. He subsequently became a senior research fellow at the CNRS, prior to being granted his habilitation à diriger des recherches (a postdoctoral degree authorizing the recipient to supervise doctoral students). In 1999, he was appointed as a Professor at the EPFL, where he created the Security and Cryptography Laboratory.

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https://people.epfl.ch/serge.vaudenay?lang=fr

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This course introduces the basics of cryptography. We review several types of cryptographic primitives, when it is safe to use them and how to select the appropriate security parameters. We detail how they work and sketch how they can be implemented.

This course reviews some failure cases in public-key cryptography. It introduces some cryptanalysis techniques. It also presents fundamentals in cryptography such as interactive proofs. Finally, it presents some techniques to validate the security of cryptographic primitives.

This seminar introduces the participants to the current trends, problems, and methods in the area of communication security.

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A Gapless Post-quantum Hash Proof System in the Hamming Metric

Bénédikt Minh Dang Tran, Serge Vaudenay

A hash proof system (HPS) is a form of implicit proof of membership to a language. Out of the very few existing post-quantum HPS, most are based on languages of ciphertexts of code-based or lattice-based cryptosystems and inherently suffer from a gap caused by the possibility for an ill-formed ciphertext to decrypt to a valid plaintext. Since this gap is inconvenient when proving the security in the universal composability framework by Canetti et al., Bettaieb et al. proposed the first gapless post-quantum HPS based on the Rank Quasi-Cyclic (RQC) cryptosystem in the rank metric while conjecturing the existence of a similar HPS in the usual Hamming metric. We solve this conjecture by designing a gapless post-quantum HPS based on the Hamming Quasi-Cyclic (HQC) cryptosystem which, in contrast to RQC, is a NIST post-quantum cryptography standardization alternate candidate. We describe a novel proof of validity for HQC ciphertexts, thereby closing the adversarial gap and present a witness encryption scheme secure in the standard model and a password-based authenticated key exchange protocol secure in the Bellare–Pointcheval–Rogaway (BPR) model.

2023

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Optimal Symmetric Ratcheting for Secure Communication

Andrea Felice Caforio, Daniel Patrick Collins, Serge Vaudenay, Hailun Yan

To mitigate state exposure threats to long-lived instant messaging sessions, ratcheting was introduced, which is used in practice in protocols like Signal. However, existing ratcheting protocols generally come with a high cost. Recently, Caforio et al. proposed pragmatic constructions, which compose a weakly secure 'light' protocol and a strongly secure 'heavy' protocol, in order to achieve so-called ratcheting on-demand. The light protocol they proposed has still a high complexity. In this paper, we propose the lightest possible protocol we could imagine, which essentially encrypts and then hashes the secret key. We prove it secure in the standard model by introducing a new security notion, which relates symmetric encryption with key updates by hashing. Our protocol composes well with the generic transformation techniques by Caforio et al. to offer high security and performance at the same time. In a second step, we propose another protocol based on a newly defined integrated primitive, extending standard one-time authenticated encryption with an additional output block used as a secret key for the next message. We instantiate this primitive firstly from any authenticated encryption with associated data, and then we propose an efficient instantiation using advanced encryption standard (AES) encryption to update the key and AES-Galois/Counter mode of operation to encrypt and decrypt messages.

OXFORD UNIV PRESS2022

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SwissCovid in the Perspective of Its Goals

Serge Vaudenay, Martin Vuagnoux

SwissCovid is the Swiss digital contact tracing app, which was deployed to help fighting against the COVID-19 pandemic. After a year of activity, it is high time to evaluate how effective it has been in its mission. At the highest peak, about 22% of the Swiss population was actively using SwissCovid. The activity of SwissCovid follows the curve on the number of COVID-19 cases. However, performances are rather poor. About 1% of the cases may have been discovered by SwissCovid and much less than 2% of SwissCovid alerts may have been useful, while SwissCovid generates 5% of the quarantines. The measure of the proximity of each encounter and its duration are also imprecise. It further comes with security and privacy issues: adversaries can inject false alerts for SwissCovid users and users can be tracked. On top of that, SwissCovid contributes to strengthen the monopoly of Apple and Google and to make users and their data captive of these giants. It also digs the digital divide. Contrarily to the original plan, the implementation is not open source and the law was twisted to fit the constraints by Apple and Google. Therefore, SwissCovid did not meet its goals.

2022

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