###### Seminar

Our seminar is held each Wednesday at 3 PM. If you want to participate in the seminar or you need instructions on getting access to the building, please contact Stefan Dziembowski.

The detailed schedule of meetings is available below. You can add it to your own calendar by using the following link.

###### Recent talks

26/07/2023 Tomasz Lizurej - Individual Cryptography

We initiate a formal study of individual cryptography. Informally speaking, an algorithm Alg is individual if, in every implementation of Alg, there always exists an individual user with full knowledge of the cryptographic data S used by Alg. In particular, it should be infeasible to design implementations of this algorithm that would hide S by distributing it between a group of parties using an MPC protocol or outsourcing it to a trusted execution environment.

19/07/2023 Robert Śmiech - Quantum Money; Definition and Implementation

14/06/2023 Jan Dubiński - Realistic Membership Inference Attacks against Large Diffusion Models &

Active Defences Against Encoder Stealing

Generative diffusion models, including Stable Diffusion and Midjourney, are trained on billions of internet-sourced images, raising significant concerns about the potential unauthorized use of copyright-protected images. We examine whether it is possible to determine if a specific image was used in the training set. Our focus is on Stable Diffusion, and we address the challenge of designing a fair evaluation framework to answer this membership question.

Machine Learning as a Service (MLaaS) APIs provide ready-to-use and high-utility encoders that generate vector representations for given inputs. Since these encoders are very costly to train, they become lucrative targets for model stealing attacks during which an adversary leverages query access to the API to replicate the encoder locally at a fraction of the original training costs. We propose the first active defense that prevents stealing while the attack is happening without degrading representation quality for legitimate API users.

7/06/2023 Grzegorz Fabiański - Introduction to Why3, Part 2.

See https://why3.lri.fr/ for more details.

31/05/2023 Vincenzo Botta - Extendable Threshold Ring Signatures with Enhanced Anonymity

The paper proposes strong anonymity definitions for Extendable Threshold Ring Signatures (ETRS) and a construction of a new ETRS that satisfies such definitions. Interestingly, while satisfying stronger anonymity properties, the new ETRS asymptotically improves on the two ETRS presented in prior work [PKC 2022] in terms of both time complexity and signature size. The new ETRS relies on extendable non-interactive witness-indistinguishable proof of knowledge (ENIWI PoK), a novel technical tool that we formalize and construct, and that may be of independent interest. We build our constructions from pairing groups under the SXDH assumption.

24/05/2023 Grzegorz Fabiański - Introduction to Why3

See https://why3.lri.fr/ for more details.

12/05/2023 Shahriar Ebrahimi - Non-Interactive Remote Attestation

Shahriar will introduce the Remote Attestation (RA), explain how TPM-based RA works, and talk about this solution's limitations. Further a study of recent works about RAs will be given, as well as a new method proposed.

5/05/2023 Robert Śmiech - Pairings Friendly Elliptic Curves

21/04/2023 Shahriar Ebrahimi - Challenges of Practically Implementing LWE-Based Cryptosystems

The talk is more towards engineering rather than theory. Shahriar will talk about his PhD thesis, which was about efficient implementations of PQC based on LWE problem. Shahriar will start by overviewing the LWE problem and its variants. Then, he will discuss ideas and methods behind the papers published during his PhD.

14/04/2023 Paweł Kędzior - Introduction to Universally Composable Security

See https://eprint.iacr.org/2000/067.

31/03/2023 Marcin Mielniczuk - PLONK (Continued)

17/03/2023 Marcin Mielniczuk - PLONK: Permutations over Lagrange-bases for Oecumenical Noninteractive arguments of Knowledge

(...) We present a universal SNARK construction with fully succinct verification, and significantly lower prover running time (roughly 7.5-20 less group exponentiations than [MBKM] in the fully succinct verifier mode depending on circuit structure). (...)

10/03/2023 Mikołaj Leonarski - Intrusion-Resilient Secret Sharing

We introduce a new primitive called intrusion-resilient secret sharing (IRSS), whose security proof exploits the fact that there exist functions which can be efficiently computed interactively using low communication complexity in k, but not in k-1 rounds. IRSS is a means of sharing a secret message amongst a set of players which comes with a very strong security guarantee. The shares in an IRSS are made artificially large so that it is hard to retrieve them completely, and the reconstruction procedure is interactive requiring the players to exchange k short messages. The adversaries considered can attack the scheme in rounds, where in each round the adversary chooses some player to corrupt and some function, and retrieves the output of that function applied to the share of the corrupted player. This model captures for example computers connected to a network which can occasionally he infected by malicious software like viruses, which can compute any function on the infected machine, but cannot sent out a huge amount of data. (...)

17/02/2023 Tomasz Lizurej - Efficiently Testable Circuits

We put forward the notion of ``efficiently testable circuits'' and provide circuit compilers that transform any circuit into an efficiently testable one. Informally, a circuit is testable if one can detect tampering with the circuit by evaluating it on a small number of inputs from some test set.

Our technical contribution is a compiler that transforms any circuit into a testable circuit for which we can detect arbitrary tampering with all wires by traversing a small testing set. The notion of a testable circuit is weaker or incomparable to existing notions of tamper-resilience, which aim to detect or even correct for errors introduced by tampering during every query, but our new notion is interesting in several settings, and we achieve security against much more general tampering classes -- like tampering with all wires -- with very modest overhead.

3/02/2023 Stefan Dziembowski - Lower Bounds for Off-Chain Protocols: Exploring the Limits of Plasma

(...) In this work we initiate the study of the inherent limitations of Plasma protocols. More concretely, we show that in every Plasma system the adversary can either (a) force the honest parties to communicate a lot with the blockchain, even though they did not intend to (this is traditionally called \emph{mass exit}); or (b) an honest party that wants to leave the system needs to quickly communicate large amounts of data to the blockchain. What makes these attacks particularly hard to handle in real life is that these attacks do not have so-called \emph{uniquely attributable faults}, i.e.~the smart contract cannot determine which party is malicious, and hence cannot force it to pay the fees for the blockchain interaction. An important implication of our result is that the benefits of two of the most prominent Plasma types, called \emph{Plasma Cash} and \emph{Fungible Plasma}, cannot be achieved simultaneously.

27/01/2023 Michał Zając - On Subversion-Resistant SNARKs

While NIZK arguments in the CRS model are widely studied, the question of what happens when the CRS was subverted has received little attention. In ASIACRYPT 2016, Bellare, Fuchsbauer, and Scafuro showed the first negative and positive results in the case of NIZK, proving also that it is impossible to achieve subversion soundness and (even non-subversion) zero-knowledge at the same time. On the positive side, they constructed an involved sound and subversion-zero-knowledge (Sub-ZK) non-succinct NIZK argument for NP. We consider the practically very relevant case of zk-SNARKs. We make Groth's zk-SNARK for \textsc{Circuit-SAT} from EUROCRYPT 2016 computationally knowledge-sound and perfectly composable Sub-ZK with minimal changes. We only require the CRS trapdoor to be extractable and the CRS to be publicly verifiable. To achieve the latter, we add some new elements to the CRS and construct an efficient CRS verification algorithm. We also provide a definitional framework for knowledge-sound and Sub-ZK SNARKs.

20/01/2023 Vincenzo Botta - On Redaction in Blockchain

A major issue for many applications of blockchain technology is the tension between immutability and compliance to regulations. For instance, the GDPR in the EU requires to guarantee, under some circumstances, the right to be forgotten. This could imply that at some point one might be forced to delete some data from a locally stored blockchain, therefore irreparably hurting the security and transparency of such decentralized platforms. Motivated by such data protection and consistency issues, in this work we design and implement a mechanism for securely deleting data from Bitcoin blockchain. We use zero-knowledge proofs to allow any node to delete some data from Bitcoin transactions, still preserving the public verifiability of the correctness of the spent and spendable coins. Moreover, we specifically use STARK proofs to exploit the transparency that they provide. Our solution, unlike previous approaches, avoids the complications of asking nodes to reach consensus on the content to delete. In particular, our design allows every node to delete some specific data without coordinating this decision with others. In our implementation, data removal can be performed (resp., verified) in minutes (resp., seconds) on a standard laptop rather than in days as required in previous designs based on consensus.