International Association
for Cryptologic Research

In May 2020, Zoom Video Communications, Inc. (Zoom) announced a multi-step plan to comprehensively support end-to-end encrypted (E2EE) group video calls and subsequently rolled out basic E2EE support to customers in October 2020. In this work we provide the first formal security analysis of Zoom's E2EE protocol, and also lay foundation to the general problem of E2EE group video communication.

We observe that the vast security literature analyzing asynchronous messaging does not translate well to synchronous video calls. Namely, while strong forms of forward secrecy and post compromise security are less important for (typically short-lived) video calls, various liveness properties become crucial. For example, mandating that participants quickly learn of updates to the meeting roster and key, media streams being displayed are recent, and banned participants promptly lose any access to the meeting. Our main results are as follows:

1. Propose a new notion of leader-based continuous group key agreement with liveness, which accurately captures the E2EE properties specific to the synchronous communication scenario. 2. Prove security of the core of Zoom's E2EE meetings protocol in the above well-defined model. 3. Propose ways to strengthen Zoom's liveness properties by simple modifications to the original protocol, which subsequently influenced updates implemented in production.

Motivated by the violation of two fundamental assumptions in secure communication - receiver-privacy and sender-freedom - by a certain entity referred to as ``the dictator'', Persiano et al. introduced the concept of Anamorphic Encryption (AME) for public key cryptosystems (EUROCRYPT 2022). Specifically, they presented receiver/sender-AME, directly tailored to scenarios where receiver privacy and sender freedom assumptions are compromised, respectively. In receiver-AME, entities share a double key to communicate in anamorphic fashion, raising concerns about the online distribution of the double key without detection by the dictator. The sender-AME with no shared secret is a potential candidate for key distribution. However, the only such known schemes (i.e., LWE and Dual LWE encryptions) suffer from an intrinsic limitation and cannot achieve reliable distribution.

Here, we reformulate the sender-AME, present the notion of $\ell$-sender-AME and formalize the properties of (strong) security and robustness. Robustness refers to guaranteed delivery of duplicate messages to the intended receiver, ensuring that decrypting normal ciphertexts in an anamorphic way or decrypting anamorphic ciphertexts with an incorrect duplicate secret key results in an explicit abort signal. We first present a simple construction for pseudo-random and robust public key encryption that shares the similar idea of public-key stegosystem by von Ahn and Hopper (EUROCRYPT 2004). Then, inspired by Chen et al.'s malicious algorithm-substitution attack (ASA) on key encapsulation mechanisms (KEM) (ASIACRYPT 2020), we give a generic construction for hybrid PKE with special KEM that encompasses well-known schemes, including ElGamal and Cramer-Shoup cryptosystems.

The constructions of $\ell$-sender-AME motivate us to explore the relations between AME, ASA on PKE, and public-key stegosystem. The results show that a strongly secure $\ell$-sender-AME is such a strong primitive that implies reformulated receiver-AME, public-key stegosystem, and generalized ASA on PKE. By expanding the scope of sender-anamorphic encryption and establishing its robustness, as well as exploring the connections among existing notions, we advance secure communication protocols under challenging conditions.

Event date: 25 May to 26 May 2024
Submission deadline: 16 February 2024
Notification: 30 March 2024

We are excited to announce an internship opportunity at Microsoft Research focused broadly around cryptography, mathematical modeling, and system design for online trust, safety, and reliability in the age of AI. Our interns will have the opportunity to work on real-world problems alongside other interns, researchers, and engineers.

We are looking for creative thinkers, doers, and problem solvers, who are unafraid to venture outside their comfort zone to find solutions to difficult and messy real-world problems. If you are unsure whether you have appropriate background and match this description, we highly recommend applying anyway!

Apply now at: https://jobs.careers.microsoft.com/global/en/job/1639824

Closing date for applications:

Contact: Kim Laine (kim.laine@microsoft.com)

There is an open call for a Postdoc position in the Cyber Security and Applied Cryptograhy research group at the Institute of Computer Science, University of St.Gallen, led by Prof. Katerina Mitrokotsa.

Our research interests are centered around information security and applied cryptography, with the larger goal of safeguarding communications and providing strong privacy guarantees. We are active in several areas, a subset of which include:

• Verifiable computation
• Secure, private and distributed aggregation
• Secure multi-party computation
• Privacy-preserving biometric authentication
• Anonymous credentials
• Distributed and privacy-preserving authentication

Candidates should have a strong background in applied cryptography and provable security, are able to work independently and also collaborate in a team. Applicants must hold a Ph.D., with contributions in the relevant research topics and have publications in good venues.

The starting date for the position is flexible and come with a very competitive salary. The selection process runs until the suitable candidate has been found. The University of St.Gallen conducts excellent research with international implications. The city of St.Gallen is located one hour from Zurich and offers a high quality of life.

Please apply by 10th December 2023 through the job portal (via link).

Closing date for applications:

Contact:
Please apply through the job portal (via link).

Eriane Breu (Administrative matters)
Prof. Katerina Mitrokotsa (Research related questions)

More information: https://jobs.unisg.ch/offene-stellen/postdoc-fellow-in-cryptography-information-security-m-f-d-m-w-d/831c6e8a-e191-48ec-92d5-320b2822a9ab

The candidate will be working on algorithmic and mathematical aspects of lattice-based or isogeny-based cryptography, in the context of ERC project AGATHA CRYPTY.
The candidate will be joining the Number Theory team of ENS de Lyon. They will benefit from the resources of AGATHA CRYPTY (travelling, equipment, organizing events, hiring interns...).
The candidate should hold a PhD degree in Mathematics or Computer Science. They should have a strong record related to any of the following research topics: number theory, computational number theory, lattice-based cryptography, isogeny-based cryptography.

Closing date for applications:

Contact: Benjamin Wesolowski. Candidates should apply at: https://emploi.cnrs.fr/Offres/CDD/UMR5669-BENWES-002/Default.aspx?lang=EN

More information: https://emploi.cnrs.fr/Offres/CDD/UMR5669-BENWES-002/Default.aspx?lang=EN

Teaching. The expected profile is a teacher-researcher capable of teaching the fundamentals of Computer Science (in the core curriculum of an engineering cycle in computer science), as well as specialized subjects close to his/her research themes (in the engineering cycle majors, in apprenticeship training, in the school's international Master's degree and/or Bachelor's degree in cybersecurity). The teaching load is approximately that of a university lecturer.

Research. We are recruiting to consolidate our teams and research areas in the following areas:

• Software and architecture security:
  • Detection of security attacks (learning);
  • Malware and reverse engineering;
  • Cryptography;
• Systems:
  • Operating systems and kernels;
  • Cloud computing and virtualization;
  • Embedded system.

The teacher-researcher's profile will fit into one of these three fields: mathematical computer science, fundamental computer science or applied computer science, a geeky and versatile profile being very welcome.

The "research" component is expected to account for around 45% of time, including participation in national and international research activities (review of articles, etc.). Research will be carried out within a local team and with external collaborators as appropriate. A dynamic approach to setting up and participating in collaborative projects and/or industrial contracts will be highly appreciated

Closing date for applications:

Contact: thierry.geraud@epita.fr

More information: https://www.lre.epita.fr/

With Carbyne Stack (https://carbynestack.io), Bosch is developing an open source cloud platform for computing on encrypted data using Secure Multi-party Computation (MPC). Potential use cases include Privacy-Preserving Machine Learning and Privacy-Preserving Data Analytics. For such large computations on big data, active secure MPC becomes quite expensive. Bosch Research is therefore trying to reduce the computational and communication costs of MPC by optimizing the underlying cryptographic primitives and protocols.

Thus, we are looking for a highly motivated PhD candidate with a strong background and/or interest in applied cryptography. The successful candidate will:

• become a part of the team and advance research on MPC
• develop novel approaches to improve the practical efficiency of actively secure MPC protocols
• design efficient MPC protocols for diverse use-cases
• integrate the results into our Cabyne Stack open source MPC platform
• publish and present the results in top-tier journals and at conferences

The candidates should meet the following requirements:

• Education: Hold an M.Sc. degree (or equivalent) with excellent grades in IT security, computer science, mathematics, or a related field
• Experience and Knowledge: Strong background in (applied) cryptography with a particular focus on cryptographic protocols/MPC, including security models and basic security proof techniques. Good software development/programming skills and the motivation to integrate scientific results into Carbyne Stack.
• Personality and Working Practice: Self-motivated and enthusiastic, independent, reliable, creative, and able to work in an international team with diverse background
• Language: Fluent English language skills

Please submit your application via: https://smrtr.io/hmG3C

Closing date for applications:

Contact: Informal inquiries can be made to Christoph Bösch (christoph.boesch (at) de.bosch.com). Formal applications must be submitted through: https://smrtr.io/hmG3C

We are looking for a bright and motivated PhD student to work in the topics of information security and cryptography.

The student is expected to work on topics that include security and privacy issues in authentication. More precisely, the student will be working on investigating efficient and privacy-preserving authentication that provides: i) provable security guarantees, and ii) rigorous privacy guarantees.

Key Responsibilities:

• Perform exciting and challenging research in the domain of information security and cryptography.
• Support and assist in teaching computer security and cryptography courses.

Profile:

• The PhD student is expected to have a MSc degree or equivalent, and strong background in cryptography, network security and mathematics.
• Experience in one or more domains such as cryptography, design of protocols, secure multi-party computation and differential privacy is beneficial.
• Excellent programming skills.
• Excellent written and verbal communication skills in English

The Chair of Cyber Security, https://cybersecurity.unisg.ch/, led by Prof. Katerina Mitrokotsa, is a part of the Institute of Computer Science (ICS) at the University of St.Gallen. Our research interests are centered around information security and applied cryptography, with the larger goal of safeguarding communications and providing strong privacy guarantees. We are currently active in multiple areas including the design of provably secure cryptographic protocols and cryptographic primitives that can be employed for reliable authentication, outsourcing computations in cloud-assisted settings, network security problems as well as secure and privacy-preserving machine learning. As a doctoral student you will be a part of the Doctoral School of Computer Science (DCS), https://dcs.unisg.ch.

The starting date for the position is flexible and come with a very competitive salary. The selection process runs until the suitable candidate has been found.

Please apply by 10th December 2023 through the job portal (via link).

Closing date for applications:

Contact:
Please apply through the job portal (via link).
Eriane Breu (Administrative matters)
Prof. Katerina Mitrokotsa (Research related questions)

More information: https://jobs.unisg.ch/offene-stellen/funded-phd-student-in-applied-cryptography-privacy-preserving-biometric-authentication-m-f-d-m-w-d/6ce1d454-47ca-4710-a9f2-33429243b4ac

Password-Authenticated Key Exchange (PAKE) allows two parties to establish a common high-entropy secret from a possibly low-entropy pre-shared secret such as a password. In this work, we provide the first PAKE protocol with subversion resilience in the framework of universal composability (UC), where the latter roughly means that UC security still holds even if one of the two parties is malicious and the honest party's code has been subverted (in an undetectable manner).

We achieve this result by sanitizing the PAKE protocol from oblivious transfer (OT) due to Canetti et al. (PKC'12) via cryptographic reverse firewalls in the UC framework (Chakraborty et al., EUROCRYPT'22). This requires new techniques, which help us uncover new cryptographic primitives with sanitation-friendly properties along the way (such as OT, dual-mode cryptosystems, and signature schemes).

As an additional contribution, we delve deeper in the backbone of communication required in the subversion-resilient UC framework, extending it to the unauthenticated setting, in line with the work of Barak et al. (CRYPTO'05).

Most existing MPC protocols consider the homogeneous setting, where all the MPC players are assumed to have identical communication and computation resources. In practice, the weakest player often becomes the bottleneck of the entire MPC protocol execution. In this work, we initiate the study of so-called load-balanced MPC in the heterogeneous computing. A load-balanced MPC protocol can adjust the workload of each player accordingly to maximize the overall resource utilization. In particular, we propose new notions called composite circuit and composite garbling scheme, and construct two efficient server-aided protocols with malicious security and semi-honest security, respectively. Our maliciously secure protocol is over 400$\times$ faster than the authenticated garbling protocol (CCS'17); our semi-honest protocol is up to 173$\times$ faster than the optimized BMR protocol (CCS'16).

By leveraging the no-cloning principle of quantum mechanics, unclonable cryptography enables us to achieve novel cryptographic protocols that are otherwise impossible classically. Two most notable examples of unclonable cryptography are quantum copy-protection and unclonable encryption. Despite receiving a lot of attention in recent years, two important open questions still remain: copy- protection for point functions in the plain model, which is usually considered as feasibility demonstration, and unclonable encryption with unclonable indistinguishability security in the plain model. In this work, by relying on previous works of Coladangelo, Liu, Liu, and Zhandry (Crypto’21) and Culf and Vidick (Quantum’22), we establish a new monogamy-of-entanglement property for subspace coset states, which allows us to obtain the following new results: • We show that copy-protection of point functions exists in the plain model, with different challenge distributions (including arguably the most natural ones). • We show, for the first time, that unclonable encryption with unclonable indistinguishability security exists in the plain model.

The Learning with Errors (LWE) problem has been widely utilized as a foundation for numerous cryptographic tools over the years. In this study, we focus on an algebraic variant of the LWE problem called Group ring LWE (GR-LWE). We select group rings (or their direct summands) that underlie specific families of finite groups constructed by taking the semi-direct product of two cyclic groups. Unlike the Ring-LWE problem described in \cite{lyubashevsky2010ideal}, the multiplication operation in the group rings considered here is non-commutative. As an extension of Ring-LWE, it maintains computational hardness and can be potentially applied in many cryptographic scenarios. In this paper, we present two polynomial-time quantum reductions. Firstly, we provide a quantum reduction from the worst-case shortest independent vectors problem (SIVP) in ideal lattices with polynomial approximate factor to the search version of GR-LWE. This reduction requires that the underlying group ring possesses certain mild properties; Secondly, we present another quantum reduction for two types of group rings, where the worst-case SIVP problem is directly reduced to the (average-case) decision GR-LWE problem. The pseudorandomness of GR-LWE samples guaranteed by this reduction can be consequently leveraged to construct semantically secure public-key cryptosystems.

In recent years, quantum computers and Shor’s quantum algorithm have been able to effectively solve NP (Non-deterministic Polynomial-time) problems such as prime factorization and discrete logarithm problems, posing a threat to current mainstream asymmetric cryptography, including RSA and Elliptic Curve Cryptography (ECC). As a result, the National Institute of Standards and Technology (NIST) in the United States call for Post-Quantum Cryptography (PQC) methods that include lattice-based cryptography methods, code-based cryptography methods, multivariate cryptography methods, and hash-based cryptography methods for resisting quantum computing attacks. Therefore, this study proposes a PQC neural network (PQC-NN) that maps a code-based PQC method to a neural network structure and enhances the security of ciphertexts with non-linear activation functions, random perturbation of ciphertexts, and uniform distribution of ciphertexts. The main innovations of this study include: (1) constructing a neural network structure that complies with code-based PQC, where the weight sets between the input layer and the ciphertext layer can be used as a public key for encryption, and the weight sets between the ciphertext layer and the output layer can be used as a private key for decryption; (2) adding random perturbations to the ciphertext layer, which can be removed during the decryption phase to restore the original plaintext; (3) constraining the output values of the ciphertext layer to follow a uniform distribution with a significant similarity by adding the cumulative distribution function (CDF) values of the chi-square distribution to the loss function, ensuring that the neural network produces sufficiently uniform distribution for the output values of the ciphertext layer. In practical experiments, this study uses cellular network signals as a case study to demonstrate that encryption and decryption can be performed by the proposed PQC neural network with the uniform distribution of ciphertexts. In the future, the proposed PQC neural network could be applied to various applications.

VOX has been submitted to the NIST Round 1 Additional Signature of the Post-Quantum Signature Competition in June 2023. VOX is a strengthened variant of UOV which uses the Quotient-Ring (QR) setting to reduce the public-key size. At the end of August 2023, Furue and Ikamatsu posted on the NIST mailing-list a post, indicating that the parameters of VOX can be attacked efficiently using the rectangular attack in the QR setting.

In this note, we explain the attack in the specific case of VOX, we detail the complexity, and show that as Furue and Ikematsu indicated, the attack can be completely avoided by adding one more constraint on the parameter selection. Finally, we show that this constraint does not increase the sizes of the public keys or signature.

This note presents attacks on the lightweight hash function TS-Hash proposed by Tsaban, including a polynomial-time preimage attack for short messages (at most n/2 bits), high-probability differentials, a general subexponential-time preimage attack, and linearization techniques.

Multi-signatures allow for compressing many signatures for the same message that were generated under independent keys into one small aggregated signature. This primitive is particularly useful for proof-of-stake blockchains, like Ethereum, where the same block is signed by many signers, who vouch for the block's validity. Being able to compress all signatures for the same block into a short string significantly reduces the on-chain storage costs, which is an important efficiency metric for blockchains.

In this work, we consider multi-signatures in the synchronized setting, where the signing algorithm takes an additional time parameter as input and it is only required that signatures for the same time step are aggregatable. The synchronized setting is simpler than the general multi-signature setting, but is sufficient for most blockchain related applications, as signers are naturally synchronized by the length of the chain.

We present Chipmunk, a concretely efficient lattice-based multi-signature scheme in the synchronized setting that allows for signing an a-priori bounded number of messages. Chipmunk allows for non-interactive aggregation of signatures and is secure against rogue-key attacks. The construction is plausibly secure against quantum adversaries as our security relies on the assumed hardness of the short integer solution problem.

We significantly improve upon the previously best known construction in this setting by Fleischhacker, Simkin, and Zhang (CCS 2022). Our aggregate signature size is $5.6 \times$ smaller and for $112$ bits of security our construction allows for compressing 8192 individual signatures into a multi-signature of size around $136$ KB. We provide a full implementation of Chipmunk and provide extensive benchmarks studying our construction's efficiency.

In their seminal work, Ishai, Kushilevitz, Ostrovsky, and Sahai (STOC`07) presented the MPC-in-the-Head paradigm, which shows how to design Zero-Knowledge Proofs (ZKPs) from secure Multi-Party Computation (MPC) protocols. This paradigm has since then revolutionized and modularized the design of efficient ZKP systems, with far-reaching applications beyond ZKPs. However, to the best of our knowledge, all previous instantiations relied on fully-secure MPC protocols, and have not been able to leverage the fact that the paradigm only imposes relatively weak privacy and correctness requirements on the underlying MPC.

In this work, we extend the MPC-in-the-Head paradigm to game-based cryptographic primitives supporting homomorphic computations (e.g., fully-homomorphic encryption, functional encryption, randomized encodings, homomorphic secret sharing, and more). Specifically, we present a simple yet generic compiler from these primitives to ZKPs which use the underlying primitive as a black box. We also generalize our paradigm to capture commit-and-prove protocols, and use it to devise tight black-box compilers from Interactive (Oracle) Proofs to ZKPs, assuming One-Way Functions (OWFs).

We use our paradigm to obtain several new ZKP constructions:

1. The first ZKPs for NP relations $\mathcal{R}$ computable in (polynomial-time uniform) $NC^1$, whose round complexity is bounded by a fixed constant (independent of the depth of $\mathcal{R}$'s verification circuit), with communication approaching witness length (specifically, $n\cdot poly\left(\kappa\right)$, where $n$ is the witness length, and $\kappa$ is a security parameter), assuming DCR. Alternatively, if we allow the round complexity to scale with the depth of the verification circuit, our ZKPs can make black-box use of OWFs.

2. Constant-round ZKPs for NP relations computable in bounded polynomial space, with $O\left(n\right)+o\left(m\right)\cdot poly\left(\kappa\right)$ communication assuming OWFs, where $m$ is the instance length. This gives a black-box alternative to a recent non-black-box construction of Nassar and Rothblum (CRYPTO`22).

3. ZKPs for NP relations computable by a logspace-uniform family of depth-$d\left(m\right)$ circuits, with $n\cdot poly\left(\kappa,d\left(m\right)\right)$ communication assuming OWFs. This gives a black-box alternative to a result of Goldwasser, Kalai and Rothblum (JACM).

We build the first unleveled fully homomorphic signature scheme in the standard model. Our scheme is not constrained by any a-priori bound on the depth of the functions that can be homomorphically evaluated, and relies on subexponentially-secure indistinguishability obfuscation, fully-homomorphic encryption and a non-interactive zero-knowledge (NIZK) proof system with composable zero-knowledge. Our scheme is also the first to satisfy the strong security notion of context-hiding for an unbounded number of levels, ensuring that signatures computed homomorphically do not leak the original messages from which they were computed. All building blocks are instantiable from falsifiable assumptions in the standard model, avoiding the need for knowledge assumptions. The main difficulty we overcome stems from the fact that bootstrapping, which is a crucial tool for obtaining unleveled fully homomorphic encryption (FHE), has no equivalent for homomorphic signatures, requiring us to use novel techniques.

This paper describes a way to protect medications against falsification, a long-standing problem in the world.

We combine several existing technologies to achieve the stated goal. The building-blocks used are inherent physical randomness generated during the packaging process, artificial vision, short digital signatures and QR-codes.