International Association
for Cryptologic Research

Recently, Chaum et al. (ACNS'21) introduced $\mathcal{S}_{leeve}$, which describes an extra security layer for signature schemes, i.e., ECDSA. This distinctive feature is a new key generation mechanism, allowing users to generate a ''back up key'' securely nested inside the secret key of a signature scheme. Using this novel construction, the ''back up key'', which is secret, can be used to generate a ''proof of ownership'', i.e., only the rightful owner of this secret key can generate such a proof. This design offers a quantum secure fallback, i.e., a brand new quantum resistant signature, ready to be used, nested in the ECDSA secret key. In this work, we rely on the original $\mathcal{S}_{leeve}$ definition to generalize the construction to a modular design based on Tweakable Hash Functions, thus yielding a cleaner design of the primitive. Furthermore, we provide a thorough security analysis taking into account the security of the ECDSA signature scheme, which is lacking in the original work. Finally, we provide an analysis based on formal methods using Verifpal assuring the security guarantees our construction provides.

We give black-box, round-optimal protocol compilers from semi-honest security to malicious security in the Random Oracle Model (ROM) and in the 1-out-of-2 oblivious transfer (OT) correlations model. We use our compilers to obtain the following black-box constructions of general-purpose protocols for secure computation tolerating static, malicious corruptions of all-but-one participants: \begin{itemize} \item A two-round, two-party protocol in the random oracle model, making black-box use of a two-round semi-honest secure protocol. Prior to our work, such a result was not known even for special functionalities such as OT. As an application, we get efficient constructions of two-round malicious OT/OLE in the random oracle model based on a black-box use of two-round semi-honest OT/OLE. \item A three-round multiparty protocol in the random oracle model, making a black-box use of two-round semi-honest OT. This protocol matches a known round complexity lower bound due to Applebaum et al. (ITCS 2020) and is based on a minimal cryptographic primitive.

\item A two-round multiparty protocol in the OT correlations model, making a black-box use of a semi-malicious protocol. This improves over a similar protocol of the authors (Crypto 2021) by eliminating an adaptive security requirement and replacing nonstandard multiparty OT correlations by standard ones. As an application, we get 2-round protocols for arithmetic branching programs that make a black-box use of the underlying field. \end{itemize}

As a contribution of independent interest, we provide a new variant of the IPS compiler (Ishai, Prabhakaran and Sahai, Crypto 2008) in the two-round setting, where we relax requirements on the IPS ``inner protocol'' by strengthening the ``outer protocol''.

Cryptanalysis is to infer the secret key of cryptography algorithm. There are brute-force attack, differential attack, linear attack, and chosen plaintext attack. With the development of artificial intelligence, deep learning-based cryptanalysis has been actively studied. There are works in which known-plaintext attacks against lightweight block ciphers, such as S-DES, have been performed. In this paper, we propose a cryptanalysis method based on the-state-of-art deep learning technologies (e.g. residual connections and gated linear units) for lightweight block ciphers (e.g. S-DES and S-AES). The number of parameters required for training is significantly reduced by 93.16~\% and the average of bit accuracy probability increased by about 5.3~\%, compared with previous work.

We revisit the problem of finding $B$-block-long collisions in Merkle-Damgård Hash Functions in the auxiliary-input random oracle model, in which an attacker gets a piece of $S$-bit advice about the random oracle and makes $T$ oracle queries. Akshima, Cash, Drucker and Wee (CRYPTO 2020), based on the work of Coretti, Dodis, Guo and Steinberger (EUROCRYPT 2018), showed a simple attack for $2\leq B\leq T$ (with respect to a random salt). The attack achieves advantage ${\Omega}(STB/2^n+T^2/2^n)$ where $n$ is the output length of the random oracle. They conjectured that this attack is optimal. However, this so-called STB conjecture was only proved for $B\approx T$ and $B=2$.

Very recently, Ghoshal and Komargodski (CRYPTO 22) confirmed STB conjecture for all constant values of $B$, and provided an ${O}(S^4TB^2/2^n+T^2/2^n)$ bound for all choices of $B$.

In this work, we prove an ${O}((STB/2^n)\cdot\max\{1,ST^2/2^n\}+ T^2/2^n)$ bound for every $2< B < T$. Our bound confirms the STB conjecture for $ST^2\leq 2^n$, and is optimal up to a factor of $S$ for $ST^2>2^n$ (note as $T^2$ is always at most $2^n$, otherwise finding a collision is trivial by the birthday attack). Our result subsumes all previous upper bounds for all ranges of parameters except for $B={O}(1)$ and $ST^2>2^n$.

We obtain our results by adopting and refining the technique of Chung, Guo, Liu, and Qian (FOCS 2020). Our approach yields more modular proofs and sheds light on how to bypass the limitations of prior techniques.

Along the way, we obtain a considerably simpler and illuminating proof for $B=2$, recovering the main result of Akshima, Cash, Drucker and Wee.

Unclonable encryption, first introduced by Broadbent and Lord (TQC'20), is a one-time encryption scheme with the following security guarantee: any non-local adversary (A, B, C) cannot simultaneously distinguish encryptions of two equal length messages. This notion is termed as unclonable indistinguishability. Prior works focused on achieving a weaker notion of unclonable encryption, where we required that any non-local adversary (A, B, C) cannot simultaneously recover the entire message m. Seemingly innocuous, understanding the feasibility of encryption schemes satisfying unclonable indistinguishability (even for 1-bit messages) has remained elusive.

We make progress towards establishing the feasibility of unclonable encryption.

- We show that encryption schemes satisfying unclonable indistinguishability exist unconditionally in the quantum random oracle model.

- Towards understanding the necessity of oracles, we present a negative result stipulating that a large class of encryption schemes cannot satisfy unclonable indistinguishability.

- Finally, we also establish the feasibility of another closely related primitive: copy-protection for single-bit output point functions. Prior works only established the feasibility of copy-protection for multi-bit output point functions or they achieved constant security error for single-bit output point functions.

Oblivious RAM (ORAM) is a machinery that protects any RAM from leaking information about its secret input by observing only the access pattern. It is known that every ORAM must incur a logarithmic overhead compared to the non-oblivious RAM. In fact, even the seemingly weaker notion of differential obliviousness, which intuitively ``protects'' a single access by guaranteeing that the observed access pattern for every two ``neighboring'' logical access sequences satisfy $(\epsilon,\delta)$-differential privacy, is subject to a logarithmic lower bound.

In this work, we show that any Turing machine computation can be generically compiled into a differentially oblivious one with only doubly logarithmic overhead. More precisely, given a Turing machine that makes $N$ transitions, the compiled Turing machine makes $O(N \cdot \log\log N)$ transitions in total and the physical head movements sequence satisfies $(\epsilon,\delta)$-differential privacy (for a constant $\epsilon$ and a negligible $\delta$). We additionally show that $\Omega(\log\log N)$ overhead is necessary in a natural range of parameters (and in the balls and bins model).

As a corollary, we show that there exist natural data structures such as stack and queues (supporting online operations) on $N$ elements for which there is a differentially oblivious implementation on a Turing machine incurring amortized $O(\log\log N)$ overhead per operation, while it is known that any oblivious implementation must consume $\Omega(\log N)$ operations unconditionally even on a RAM. Therefore, we obtain the first \emph{unconditional} separation between obliviousness and differential obliviousness in the most natural setting of parameters where $\epsilon$ is a constant and $\delta$ is negligible. Before this work, such a separation was only known in the balls and bins model. Note that the lower bound applies in the RAM model while our upper bound is in the Turing machine model, making our separation stronger.

Physical characteristics of electronic devices, leaking secret and sensitive information to an adversary with physical access, pose a long-known threat to cryptographic hardware implementations. Among a variety of proposed countermeasures against such Side-Channel Analysis attacks, masking has emerged as a promising, but often costly, candidate. Furthermore, the manual realization of masked implementations has proven error-prone and often introduces flaws, possibly resulting in insecure circuits. In the context of automatic masking, a new line of research emerged, aiming to replace each physical gate with a secure gadget that fulfills well-defined properties, guaranteeing security when interconnected to a large circuit. Unfortunately, those gadgets introduce a significant amount of additional overhead into the design, in terms of area, latency, and randomness requirements. In this work, we present a novel approach to reduce the demands for randomness in such gadget-composed circuits by reusing randomness across gadgets while maintaining security in the probing adversary model. To this end, we embedded the corresponding optimization passes into an Electronic Design Automation toolchain, able to construct, optimize, and implement masked circuits, starting from an unprotected design. As such, our security-aware optimization offers an additional building block for existing or new Electronic Design Automation frameworks, where security is considered a first-class design constraint

Public-key cryptography (PKC), including conventional cryptosystems (e.g., RSA, ECC) and post-quantum cryptography, involves computation-intensive workloads. With noticing the extraordinary computing power of AI accelerators, in this paper, we further explore the feasibility to introduce AI accelerators into high-performance cryptographic computing. Since AI accelerators are dedicated to machine learning or neural networks, the biggest challenge is how to transform cryptographic workloads into their operations, while ensuring the correctness of the results and bringing convincing performance gains.

After investigating and analysing the workload of the commercial off-the-shelf AI accelerators, we utilize NVIDIA's AI accelerator, Tensor Core, to accelerate the polynomial multiplication, usually the most time-consuming part in lattice-based cryptography. A series of measures are taken, such as accommodating the matrix-multiply-and-add mode of Tensor Core and making a trade-off between precision and performance, to leverage Tensor Core as a high-performance NTT box performing NTT/INTT through CUDA C++ WMMA API. Meanwhile, we take CRYSTALS-Kyber, one of the NIST PQC 3rd round candidates, as a case study on RTX 3080 with the Ampere Tensor Core. The empirical results show that the defined NTT of polynomial vector ($n=256,k=4$) with our NTT box obtains a speedup of around 6.47x that of the state-of-the-art implementation on the same platform.

Many public-key cryptographic protocols, notably non-interactive key exchange (NIKE), require incoming public keys to be validated to mitigate some adaptive attacks. In CSIDH, an isogeny-based post-quantum NIKE, a key is deemed legitimate if the given Montgomery coefficient specifies a supersingular elliptic curve over the prime field. In this work, we survey the current supersingularity tests used for CSIDH key validation, and implement and measure two new alternative algorithms. Our implementation shows that we can determine supersingularity substantially faster, and using less memory, than the state-of-the-art.

Modular polynomial multiplication is a core and costly operation of ideal lattice-based schemes. In the context of embedded devices, previous works transform the polynomial multiplication to an integer one using Kronecker substitution. Then thanks to this transformation, existing coprocessors which handle large-integer operations can be re-purposed to speed-up lattice-based cryptography. In a nutshell, the Kronecker substitution transforms by evaluation the polynomials to integers, multiplies it with an integer multiplication and gets back to a polynomial result using a radix conversion. The previous work focused on optimization of the integer multiplication using coprocessor instructions. In this work, we pursue the seminal research by optimizing the evaluation, radix conversion and the modular reductions modulo q with today's RSA/ECC coprocessor. In particular we show that with a RSA/ECC coprocessor that can compute addition/subtraction, (modular) multiplication, shift and logical AND on integers, we can compute the whole modular polynomial multiplication using coprocessor instructions. The efficiency of our modular polynomial multiplication depends on the component specification and on the cryptosystem parameters set. Hence, we assess our algorithm on a chip for several lattice-based schemes, which are finalists of the NIST standardization. Moreover, we compare our modular polynomial multiplication with other polynomial multiplication techniques.

Frequently, users on the web need to show that they are, for example, not a robot, old enough to access an age restricted video, or eligible to download an ebook from their local public library without being tracked. Anonymous credentials were developed to address these concerns. However, existing schemes do not handle the realities of deployment or the complexities of real world identity. Instead, they make (often incorrect) assumptions, e.g., that the local department of motor vehicles will issue sophisticated cryptographic tokens to show users are over 18. In reality, there are multiple trust sources for a given identity attribute, their credentials have distinctively different formats, and many, if not all, issuers are unwilling to adopt new protocols.

We present and build $\texttt{zk-creds}$, a protocol that uses general-purpose zero-knowledge proofs to 1) remove the need for credential issuers to hold signing keys: credentials can be issued via a transparency log, Byzantine system, or even a blockchain; 2) convert existing identity documents into anonymous credentials without modifying documents or coordinating with their issuing authority; 3) allow for flexible, composable, and complex identity statements over multiple credentials. Concretely, identity assertions using $\texttt{zk-creds}$ take less than 300ms in a real-world scenario of using a passport to anonymously access age-restricted videos.

The re-encryption mix-net (RMN) is a basic cryptographic tool that is widely used in the privacy protection domain and requires anonymity support; for example, it is used in electronic voting, web browsing, and location systems. To protect information about the relationship between senders and messages, a number of mix servers in RMNs shuffle and forward a list of input ciphertexts in a cascading manner. The output of the last mix server is decrypted to yield the set of original messages. The main downside of this approach is that the mixing process requires a number of rounds that is linear in the number of mix servers. This implies that a long round delay would cause network latency, which can dominate local computational latencies. To minimize the effect of network latency, RMN protocols with constant round complexity are more desirable. In this work, we propose a new RMN protocol that runs in $O(1)$ rounds in the number of mix servers and that UC-realizes a hybrid model with access to some functionalities for secure communication and zero-knowledge proof (ZKP). Interestingly, because our protocol does not require a ZKP protocol for a verifiable shuffle, we also achieve a considerable efficiency gain in terms of computation cost. Our main tools are secret sharing and an ElGamal encryption that is extended in the sense that it works on a multiplicative group under field extension. Importantly, this extended ElGamal encryption scheme acquires a new capability: it can efficiently decompose a decrypted message into unique values. We provide a detailed report on the theoretical performance and security analysis of this method.

The current state of the art in watermarked public-key encryption schemes under standard cryptographic assumptions suggests that extracting the embedded message requires either linear time in the number of marked keys or the a-priori knowledge of the marked key employed in the decoder.

We present the first scheme that obviates these restrictions in the secret-key marking model, i.e., the setting where extraction is performed using a private extraction key. Our construction offers constant time extraction complexity with constant size keys and ciphertexts and is secure under standard assumptions, namely the Decisional Composite Residuosity Assumption [Eurocrypt '99] and the Decisional Diffie Hellman in prime order subgroups of  square higher order residues.

Contact discovery allows users of mobile messengers to conveniently connect with people in their address book. In this work, we demonstrate that severe privacy issues exist in currently deployed contact discovery methods and propose suitable mitigations.

Our study of three popular messengers (WhatsApp, Signal, and Telegram) shows that large-scale crawling attacks are (still) possible. Using an accurate database of mobile phone number prefixes and very few resources, we queried 10% of US mobile phone numbers for WhatsApp and 100% for Signal. For Telegram we find that its API exposes a wide range of sensitive information, even about numbers not registered with the service. We present interesting (cross-messenger) usage statistics, which also reveal that very few users change the default privacy settings.

Furthermore, we demonstrate that currently deployed hashing-based contact discovery protocols are severely broken by comparing three methods for efficient hash reversal. Most notably, we show that with the password cracking tool "JTR" we can iterate through the entire world-wide mobile phone number space in <150s on a consumer-grade GPU. We also propose a significantly improved rainbow table construction for non-uniformly distributed input domains that is of independent interest.

Regarding mitigations, we most notably propose two novel rate-limiting schemes: our incremental contact discovery for services without server-side contact storage strictly improves over Signal's current approach while being compatible with private set intersection, whereas our differential scheme allows even stricter rate limits at the overhead for service providers to store a small constant-size state that does not reveal any contact information.

The public key encryption (PKE) protocol in lattice-based cryptography (LBC) can be modeled as a noisy point-to-point communication system, where the communication channel is similar to the additive white Gaussian noise (AWGN) channel. To improve the error correction performance, this paper investigates lattice-based PKE from the perspective of lattice codes. We propose an efficient labeling function that converts between binary information bits and lattice codewords. The proposed labeling is feasible for a wide range of lattices, including Construction-A and Construction-D lattices. Based on Barnes-Wall lattices, a few improved parameter sets with either higher security or smaller ciphertext size are proposed for FrodoPKE.

Event date: 12 December to 14 December 2022
Submission deadline: 23 August 2022
Notification: 28 October 2022

Technology Innovation Institute (TII) is a publicly funded research institute, based in Abu Dhabi, United Arab Emirates. It is home to a diverse community of leading scientists, engineers, mathematicians, and researchers from across the globe, transforming problems and roadblocks into pioneering research and technology prototypes that help move society ahead.

Cryptography Research Centre

In our connected digital world, secure and reliable cryptography is the foundation of digital information security and data integrity. We address the world’s most pressing cryptographic questions. Our work covers post-quantum cryptography, lightweight cryptography, cloud encryption schemes, secure protocols, quantum cryptographic technologies and cryptanalysis.

Position: Senior MPC Researcher

Conduct research on state-of-the-art MPC protocols

Analyze project requirements and provide technical and functional recommendations

Design and implementation of building blocks to utilize privacy-preserving cryptographic techniques to cloud computing and machine learning applications

Propose new projects and research directions

Skills required for the job

MSc or PhD degree in Cryptography, Applied Cryptography, Information Theory, Mathematics or Computer Science

4+ years of work experience in the field

Knowledge of MPC protocols

Experience in C desired, C++, Rust or Go relevant as well. Solid software engineering skills, such as agile methodologies, versioning, and best practices

Quick learner, geared towards implementation. Eager to develop new skills and willing to take ownership of projects

Experience with MPC frameworks (e.g. Scale-Mamba, MP-SPDZ, Obliv-C) is a plus

Familiarity with HE and ZK, and other advance cryptographic primitives, is a plus

Closing date for applications:

Contact: Mehdi Messaoudi - Talent Acquisition Manager
Email: mehdi.messaoudi@tii.ae

The ZK and Blockchain Technology Laboratory at Edinburgh University has several open Ph.D. and postdoc positions to work on zero-knowledge proofs, succinct arguments, and multi-party computation protocols with applications to decentralization and privacy protection. You will work in a dynamic collaborative environment with a focus on conceptual and foundational problems of practical relevance. To express interest, send an email including a CV to markulf.kohlweiss@ed.ac.uk. Positions are open until filled, but we give priority to applications received by Aug 10. Female candidates are strongly encouraged to apply.

Closing date for applications:

Contact: Markulf Kohlweiss (markulf.kohlweiss@ed.ac.uk)

As for any cryptographic algorithm, the deployment of post-quantum CCA-secure public-key encryption schemes may come with the need to be protected against side-channel attacks. For existing post-quantum schemes that have not been developed with leakage in mind, recent results showed that the cost of these protections can make their implementations more expensive by orders of magnitude. In this paper, we describe a new design, coined \(\texttt{POLKA}\), that is specifically tailored for this purpose. It leverages various ingredients in order to enable efficient side-channel protected implementations such as: (i) the rigidity property (which intuitively means that de-randomized encryption and decryption are injective functions) to avoid the very leaky re-encryption step of the Fujisaki-Okamoto transform, (ii) the randomization of the decryption thanks to the incorporation of a dummy ciphertext, removing the adversary’s control of its intermediate computations and making these computations ephemeral, (iii) key-homomorphic computations that can be masked against side-channel attacks with overheads that scale linearly in the number of shares, (iv) hard physical learning problem to argue about the security of some critical unmasked operations. Furthermore, we use an explicit rejection mechanism (returning an error symbol for invalid ciphertexts) to avoid the additional leakage caused by implicit rejection. As a result, all the operations of \(\texttt{POLKA}\) can be protected against leakage in a much cheaper way than state-of-the-art designs, opening the way towards schemes that are both quantum-safe and leakage-resistant.

Given the high transaction confirmation latencies in public blockchains, cryptocurrencies such as Bitcoin, Ethereum, etc. are not yet suitable to support real-time services such as transactions on retail markets. There are several solutions to address this latency problem, with layer-2 solutions being the most promising ones. Existing layer-2 solutions, however, suffer from privacy and/or collateral issues when applied to retail environments where customer-merchant relationships are usually ephemeral.

In this paper, we propose Nirvana, that can be combined with existing cryptocurrencies to provide instant, anonymous and unlinkable payment guarantees. Nirvana does not require any trusted third party. It conceals the identities of honest participants, thus ensuring customer anonymity within the system while only relying on efficient Groth-Sahai proof systems. We introduce a novel randomness-reusable threshold encryption that mitigates double-spending by revealing the identities of malicious users. We formally prove how our scheme provides customer anonymity, unlinkability of transactions and payment guarantees to merchants. Our experiments demonstrate that Nirvana allows for fast (zero-confirmation) global payments in a retail setting with a delay of less than $1.7$ seconds.