International Association
for Cryptologic Research

Distributed samplers, introduced by Abram, Scholl and Yakoubov (Eurocrypt ’22), are a one-round, multi-party protocol for securely sampling from any distribution. We give new lower and upper bounds for constructing distributed samplers in challenging scenarios. First, we consider the feasibility of distributed samplers with a malicious adversary in the standard model; the only previous construction in this setting relies on a random oracle. We show that for any UC-secure construction in the standard model, even with a CRS, the output of the sampling protocol must have low entropy. This essentially implies that this type of construction is useless in applications. Secondly, we study the question of building distributed samplers in the party-dynamic setting, where parties can join in an ad-hoc manner, and the total number of parties is unbounded. Here, we obtain positive results. First, we build a special type of unbounded universal sampler, which after a trusted setup, allows sampling from any distributed with unbounded size. Our construction is in the shared randomness model, where the parties have access to a shared random string, and uses indistinguishability obfuscation and somewhere statistically binding hashing. Next, using our unbounded universal sampler, we construct distributed universal samplers in the party-dynamic setting. Our first construction satisfies one-time selective security in the shared randomness model. Our second construction is reusable and secure against a malicious adversary in the random oracle model. Finally, we show how to use party-dynamic, distributed universal samplers to produce ideal, correlated randomness in the party-dynamic setting, in a single round of interaction.

Hofheinz et al. (TCC 2017) proposed several key encapsulation mechanism (KEM) variants of Fujisaki-Okamoto (\textsf{FO}) transformation, including $\textsf{FO}^{\slashed{\bot}}$, $\textsf{FO}_m^{\slashed{\bot}}$, $\textsf{QFO}_m^{\slashed{\bot}}$, $\textsf{FO}^{\bot}$, $\textsf{FO}_m^\bot$ and $\textsf{QFO}_m^\bot$, and they are widely used in the post-quantum cryptography standardization launched by NIST. These transformations are divided into two types, the implicit and explicit rejection type, including $\{\textsf{FO}^{\slashed{\bot}}, \textsf{FO}_m^{\slashed{\bot}}, \textsf{QFO}_m^{\slashed{\bot}}\}$ and $\textsf{FO}^{\bot}, \textsf{FO}_m^\bot, \textsf{QFO}_m^\bot$, respectively. The decapsulation algorithm of the implicit (resp. explicit) rejection type returns a pseudorandom value (resp. an abort symbol $\bot$) for an invalid ciphertext.

For the implicit rejection type, the \textsf{IND-CCA} security reduction of $\textsf{FO}^{\slashed{\bot}}$ in the quantum random oracle model (QROM) can avoid the quadratic security loss, as shown by Kuchta et al. (EUROCRYPT 2020). However, for the explicit rejection type, the best known \textsf{IND-CCA} security reduction in the QROM presented by Ho"velmanns et al. (ASIACRYPT 2022) for $\textsf{FO}_m^\bot$ still suffers from a quadratic security loss. Moreover, it is not clear until now whether the implicit rejection type is more secure than the explicit rejection type.

In this paper, a QROM security reduction of $\textsf{FO}_m^\bot$ without incurring a quadratic security loss is provided. Furthermore, our reduction achieves \textsf{IND-qCCA} security, which is stronger than the \textsf{IND-CCA} security. To achieve our result, two steps are taken: The first step is to prove that the \textsf{IND-qCCA} security of $\textsf{FO}_m^\bot$ can be tightly reduced to the \textsf{IND-CPA} security of $\textsf{FO}_m^\bot$ by using the online extraction technique proposed by Don et al. (EUROCRYPT 2022). The second step is to prove that the \textsf{IND-CPA} security of $\textsf{FO}_m^\bot$ can be reduced to the \textsf{IND-CPA} security of the underlying public key encryption (PKE) scheme without incurring quadratic security loss by using the Measure-Rewind-Measure One-Way to Hiding Lemma (EUROCRYPT 2020).

In addition, we prove that (at least from a theoretic point of view), security is independent of whether the rejection type is explicit ($\textsf{FO}_m^\bot$) or implicit ($\textsf{FO}_m^{\slashed{\bot}}$) if the underlying PKE scheme is weakly $\gamma$-spread.

In Internet security protocols including TLS 1.3, KEMTLS, MLS and Noise, HMAC is being assumed to be a dual-PRF, meaning a PRF not only when keyed conventionally (through its first input), but also when "swapped" and keyed (unconventionally) through its second (message) input. We give the first in-depth analysis of the dual-PRF assumption on HMAC.

For the swap case, we note that security does not hold in general, but completely characterize when it does; we show that HMAC is swap-PRF secure if and only if keys are restricted to sets satisfying a condition called feasibility, that we give, and that holds in applications. The sufficiency is shown by proof and the necessity by attacks. For the conventional PRF case, we fill a gap in the literature by proving PRF security of HMAC for keys of arbitrary length.

Our proofs are in the standard model, make assumptions only on the compression function underlying the hash function, and give good bounds in the multi-user setting. The positive results are strengthened through achieving a new notion of variable key-length PRF security that guarantees security even if different users use keys of different lengths, as happens in practice.

A distributed sampler is a way for several mutually distrusting parties to non-interactively generate a common reference string (CRS) that all parties trust. Previous work constructs distributed samplers in the random oracle model, or in the standard model with very limited security guarantees. This is no accident, as standard model distributed samplers with full security were shown impossible. In this work, we provide new definitions for distributed samplers which we show achieve meaningful security guarantees in the standard model. In particular, our notion implies that the hardness of a wide range of security games is preserved when the CRS is replaced with a distributed sampler. We also show how to realize our notion of distributed samplers. A core technical tool enabling our construction is a new notion of single-message zero knowledge.

Group actions are fundamental mathematical tools, with a long history of use in cryptography. Indeed, the action of finite groups at the basis of the discrete logarithm problem is behind a very large portion of modern cryptographic systems. With the advent of post-quantum cryptography, however, the method for building protocols shifted towards a different paradigm, centered on the difficulty of discerning 'noisy' objects, as is the case for lattices, codes, and multivariate systems. This method yields promising results for 'core' primitives such as encryption or signature, but can be less than ideal in the case when more advanced functionalities are required. In this work, we show that isomorphism problems which stem from cryptographic group actions, can be viable building blocks for threshold signature schemes. In particular, we construct a full $N$-out-of-$N$ threshold signature scheme, and discuss the efficiency issues arising from extending it to the generic $T$-out-of-$N$ case. To give a practical outlook on our constructions, we instantiate them with the LESS and MEDS frameworks, which are two flavors of code-based cryptographic group actions. Finally, we highlight some ideas that would allow for a more efficient and compact $(T,N)$ threshold variant of LESS, whose security relies on new hardness assumptions.

Pairings are useful tools in isogeny-based cryptography and have been used in SIDH/SIKE and other protocols. As a general technique, pairings can be used to move problems about points on curves to elements in finite fields. However, until now, their applicability was limited to curves over fields with primes of a specific shape and pairings seemed too costly for the type of primes that are nowadays often used in isogeny-based cryptography. We remove this roadblock by optimizing pairings for highly-composite degrees such as those encountered in CSIDH and SQISign. This makes the general technique viable again: We apply our low-cost pairing to problems of general interest, such as supersingularity verification and finding full-torsion points, and show that we can outperform current methods, in some cases up to four times faster than the state-of-the-art. Furthermore, we analyze how parings can be used to improve deterministic and dummy-free CSIDH. Finally, we provide a constant-time implementation (in Rust) that shows the practicality of these algorithms.

A zero-knowledge proof is a cryptographic protocol where a prover can convince a verifier that a statement is true, without revealing any further information except for the truth of the statement. More precisely, if $x$ is a statement from an NP language verified by an efficient machine $M$, then a zero-knowledge proof aims to prove to the verifier that there exists a witness $w$ such that $M(x,w)=1$, without revealing any further information about $w$. The proof is a proof of knowledge, if the prover additionally convinces the verifier that it knows the witness $w$, rather than just of its existence.

This article is a survey of recent developments in building practical systems for zero-knowledge proofs of knowledge using vector oblivious linear evaluation (VOLE), a tool from secure two-party computation.

A large number of works prove lower bounds on space-time trade-offs in preprocessing attacks, i.e., trade-offs between the size of the advice and the time needed to break a scheme given such advice. We contend that the question of how much {\em time} is needed to produce this advice is equally important, and often highly non-trivial. However, this question has received significantly less attention. In this paper, we present lower bounds on the complexity of preprocessing attacks that depend on both offline and online time. As in the case of space-time trade-offs, we focus in particular on settings with ideal primitives, where both the offline and online time-complexities are approximated by the number of queries to the given primitive. We give generic results that highlight the benefits of salting to generically increase the offline costs of preprocessing attacks. The majority of our paper presents several results focusing on {\em salted} hash functions. In particular, we provide a fairly involved analysis of the pre-image-and collision-resistance security of the (two-block) Merkle-Damgård construction in our model.

Secure computation has become a necessity in the modern world. Its applications are widespread: from allowing medical researchers to compute statistics over private patient data without violating HIPAA to helping large companies like Meta and Google avoid GDPR fines. A ubiquitous and popular choice for secure computation is Multi-party Computation (MPC). Most MPC protocols work over finite fields or rings, which means that encoding techniques are required to map rational-valued data into the algebraic structure being used. Leveraging an encoding technique introduced in "$\mathsf{PIE}$ : $p$-adic Encoding for High-Precision Arithmetic in Homomorphic Encryption", we present $\mathsf{Mercury}$ - a family of protocols for addition, multiplication, subtraction, and division of rational numbers. Notably, the output of our division protocol is exact (i.e., it does not use iterative methods). Our protocols offer significant improvements in both round complexity and communication complexity when compared with prior art, and are secure for a dishonest minority of semi-honest parties. We emphasize that the encoding technique our protocols are based on is composable, so it can be paired with any MPC protocol over a prime-order field.

A standard paradigm for building key exchange protocols with full forward secrecy (and explicit authentication) is to add key confirmation messages to an underlying protocol having only weak forward secrecy (and implicit authentication). Somewhat surprisingly, we show through an impossibility result that this simple trick must nevertheless incur a linear tightness loss in the number of parties for many natural protocols. This includes Krawczyk's HMQV protocol (CRYPTO 2005) and the protocol of Cohn-Gordon et al. (CRYPTO 2019).

Cohn-Gordon et al. gave a very efficient underlying protocol with weak forward secrecy having a linear security loss, and showed that this is optimal for certain reductions. However, they also claimed that full forward secrecy could be achieved by adding key confirmation messages, and without any additional loss. Our impossibility result disproves this claim, showing that their approach, in fact, has an overall quadratic loss.

Motivated by this predicament we seek to restore the original linear loss claim of Cohn-Gordon et al. by using a different proof strategy. Specifically, we start by lowering the goal for the underlying protocol with weak forward secrecy, to a selective security notion where the adversary must commit to a long-term key it cannot reveal. This allows a tight reduction rather than a linear loss reduction. Next, we show that the protocol can be upgraded to full forward secrecy using key confirmation messages with a linear tightness loss, even when starting from the weaker selective security notion. Thus, our approach yields an overall tightness loss for the fully forward-secret protocol that is only linear, as originally claimed. Finally, we confirm that the underlying protocol of Cohn-Gordon et al. can indeed be proven selectively secure, tightly.

Digital and paper-based authentication are the two predominant mechanisms that have been deployed in the real world to authenticate end-users. When verification of a digital credential is performed in person (e.g. the authentication that was often required to access facilities at the peak of the COVID global pandemic), the two mechanisms are often deployed together: the verifier checks government-issued ID to match the picture on the ID to the individual holding it, and then checks the digital credential to see that the personal details on it match those on the ID, and to discover additional attributes of the holder. This pattern is extremely common and very likely to remain in place for the foreseeable future. However, it poses an interesting problem: if the digital credential is privacy-preserving (e.g. based on BBS+ on CL signatures), but the holder is still forced to show an ID card or a passport to verify that the presented credential was indeed issued to the holder, what is the point of deploying privacy-preserving digital credential? In this paper we address this problem by redefining what an ID card should show, and force a minimal but mandatory involvement of the card in the digital interaction. Our approach permits verifiers to successfully authenticate holders and to determine that they are the rightful owners of the digital credential. At the same time, optimal privacy guarantees are preserved. We design our scheme, formally define and analyse its security in the Universal Composability (UC) framework, and implement the card component, showing the running time to be below 200ms irrespective of the number of certified attributes.

The $k$-nearest neighbors classifier is a simple machine learning algorithm with applications in image recognition, finance, medical diagnosis and so on. It involves a measurement which is compared against a database of preclassified vectors, so that the result depends on the $k$ vectors in the database that are closest to the measurement. In the client-server model, this classification process can be outsourced to an external party that offers machine learning as a service, where the measurement is sent in the form of a query. However, this raises privacy concerns if sensitive information is contained in the query.

We design a secure and non-interactive version of the $k$-nearest neighbors classifier, based on fully homomorphic encryption, which does not leak any information about the query to the server. Our algorithm is instantiated with the TFHE homomorphic encryption scheme, and the selection of the top-$k$ elements is done with a novel strategy based on a type of data-oblivious algorithm---sorting networks. Compared to prior work from PoPETs 2021, the asymptotic complexity is improved from $O(d^2)$ to $O(d \log^2 {k})$, where $d$ is the number of entries in the $k$-NN model. Experimental results show that the proposed protocol can be up to 16 times faster (not accounting for difference in CPU) than previous approaches for a moderately sized database.

Recently, Biryukov et al. presented a new technique for key recovery in differential cryptanalysis, called meet-in-the-filter (MiF). In this work, we develop theoretical and practical aspects of the technique, which helps understanding and simplifies application. In particular, we show bounds on MiF complexity and conditions when the MiF-enhanced attack may reach them. We present a method based on trail counting which allows to estimate filtering strength of involved rounds and perform consequent complexity analysis with pen and paper, compared to the computer-aided approach of the original work. Furthermore, we show how MiF can be combined with plaintext structures for linear key schedules, allowing to increase the number of attacked rounds or to reduce the data complexity.

We illustrate our methods on block cipher families CHAM and KATAN and show best-to-date single-key differential attacks for these ciphers.

Hash-based signatures offer a conservative alternative to post-quantum signatures with arguably better-understood security than other post-quantum candidates.

As a core building block of hash-based signatures, the efficiency of one-time signature (OTS) largely dominates that of hash-based signatures. The WOTS$^{+}$ signature scheme (Africacrypt 2013) is the current state-of-the-art OTS adopted by the signature schemes standardized by NIST---XMSS, LMS and SPHINCS$^+$.

A natural question is whether there is (and how much) room left for improving one-time signatures (and thus standard hash-based signatures). In this paper, we show that WOTS$^{+}$ one-time signature, when adopting the constant-sum encoding scheme (Bos and Chaum, Crypto 1992), is size-optimal not only under Winternitz's OTS framework, but also among all tree-based OTS designs. Moreover, we point out a flaw in the DAG-based OTS design previously shown to be size-optimal at Asiacrypt 1996, which makes the constant-sum WOTS$^{+}$ the most size-efficient OTS to the best of our knowledge. Finally, we evaluate the performance of constant-sum WOTS$^{+}$ integrated into the SPHINCS$^+$ (CCS 2019) and XMSS (PQC 2011) signature schemes which exhibit certain degrees of improvement in both signing time and signature size.

Topology-Hiding Computation (THC) enables parties to securely compute a function on an incomplete network without revealing the network topology. It is known that secure computation on a complete network can be based on oblivious transfer (OT), even if a majority of the participating parties are corrupt. In contrast, THC in the dishonest majority setting is only known from assumptions that imply (additively) homomorphic encryption, such as Quadratic Residuosity, Decisional Diffie-Hellman, or Learning With Errors.

In this work we move towards closing the gap between MPC and THC by presenting a protocol for THC on general graphs secure against all-but-one semi-honest corruptions from constant-round constant-overhead secure two-party computation. Our protocol is therefore the first to achieve THC on arbitrary networks without relying on assumptions with rich algebraic structure. As a technical tool, we introduce the notion of locally simulatable MPC, which we believe to be of independent interest.

Updatable encryption is a useful primitive that enables key rotation for storing data on an untrusted storage provider without the leaking anything about the plaintext or the key. In this work, we make two contributions. Firstly, we extend updatable encryption to the public-key setting, providing its security model and three different efficient constructions. Using a public-key updatable encryption scheme, a user can receive messages directly in the cloud from multiple senders without revealing their secret key. Secondly, we add signatures on ciphertexts to guarantee plaintext integrity and authenticity. We call our new primitive \emph{Public-Key Signable Updatable Encryption} ($\mathsf{PSigUE}$). Our approach ensures that only legitimate ciphertexts are accepted by the server, and the adversary cannot compromise the message integrity in the database. We bypass the conflict between public integrity verification and the malleability that comes from the update functionality.

We provide three pairing-based constructions of public-key signable updatable encryption. The first scheme, $\mathsf{PSigUE}_1$, is built using a dual-mode zero-knowledge proof of knowledge system under an assumption closely related to the $k$-linear assumption. The second scheme, $\mathsf{PSigUE}_2$, provides unlinkability in addition to public authenticity. In the third scheme, $\mathsf{PSigUE}_\mathsf{T}$, we achieve the tight security with respect of number of epochs. The construction of $\mathsf{PSigUE}_\mathsf{T}$ is inspired by tag-based tightly-secure PKE schemes.

Salary: 35,308 to 40,745 GBP

The Surrey Centre for Cyber Security (SCCS) at the University of Surrey is seeking to recruit a full-time Research Fellow in Data Resilience, Security and Privacy. The post is available with the opportunity for hybrid working – some time on campus and some from home. We welcome applicants who wish to pursue the role through flexible working patterns.

The successful candidate will be expected to conduct research within the context of the Defence Data Research Centre https://ddrc.uk/, funded by DSTL, in which SCCS is a partner, alongside the Universities of Exeter and Liverpool, and the Digital Catapult. The Centre is focusing on problems related to the use of data for Artificial Intelligence applications, particularly around the challenges of bringing raw data to the state where it can be used. We consider these problems within a defence context, such as logistics support, object tracking and data wrangling. SCCS is focused on the area of data resilience, security and privacy, considering problems such as the trustworthiness and resilience of data and issues around anonymisation.

The post holder will benefit from the research environment provided by the Surrey Centre for Cyber Security, an Academic Centre of Excellence in Cyber Security Research recognised by the National Cyber Security Centre. The Centre’s broader research agenda is in the areas of trusted computing, data privacy, privacy preserving security, applied cryptography, and a range of cyber security topics.

Closing date for applications:

Contact: Steve Schneider.

s.schneider@surrey.ac.uk

More information: https://jobs.surrey.ac.uk/Vacancy.aspx?id=13336

We are looking for 1 post-doc researcher to work on the theory and practice of composition of secure hardware devices. The position is funded as part of the UKRI/EPSRC project "SECCOM: Securing composable Hardware Platforms" with funding from MoD/Dstl (UK's Defence Science and Technology Laboratory). Offers for the position will therefore be conditional on passing an identity check with Dstl.

The postdoc will be hosted by Bernardo Magri at the Systems and Software Security group at the CS department of the University of Manchester, located in the Northwest of England (https://www.cs.manchester.ac.uk/research/expertise/systems-and-software-security/).

The ideal candidate should have a PhD degree in Computer Science or related areas, and a proven record of publications in cryptography and/or security venues such as Crypto, Eurocrypt, Asiacrypt, TCC, PKC, CCS, S&P, USENIX, ACNS, ESORICS, etc. Experience with protocol composition frameworks (such as the UC framework) is a plus, but not required.

The position is at grade 7 (salary between £43k-53k/year depending on experience) and it lasts for 2 years. Positions can be filled from September 1st to December 1st, 2023, and will remain open until filled.

To apply, please send an email with the subject "SECCOM application" to bernardo.magri@manchester.ac.uk with:

Cover letter

CV

Short research statement

Names of at least 2 references

Closing date for applications:

Contact: Bernardo Magri

The Linear Equivalence Problem (LEP) asks to find a linear isometry between a given pair of linear codes; in the Hamming weight this is known as a monomial map. LEP has been used in cryptography to design the family of LESS signatures, which includes also some advanced schemes, such as as ring and identity-based signatures. All of these schemes are obtained applying the Fiat-Shamir transformation to a Sigma protocol, in which the prover's responses contain a description of how the monomial map acts on all code coordinates; such a description constitutes the vast majority of the signature size. In this paper, we propose a new formulation of LEP, which we refer to as Information-Set (IS)-LEP. Exploiting IS-LEP, it is enough for the prover to provide the description of the monomial action only on an information set, instead of all the coordinates. Thanks to this new formulation, we are able to drastically reduce signature sizes for all LESS signature schemes, without any relevant computational overhead. We prove that IS-LEP and LEP are completely equivalent (indeed, the same problem), which means that improvement comes with no additional security assumption, either.

Polynomial commitments schemes are a powerful tool that enables one party to commit to a polynomial $p$ of degree $d$, and prove that the committed function evaluates to a certain value $z$ at a specified point $u$, i.e. $p(u) = z$, without revealing any additional information about the polynomial. Recently, polynomial commitments have been extensively used as a cryptographic building block to transform polynomial interactive oracle proofs (PIOPs) into efficient succinct arguments.

In this paper, we propose a lattice-based polynomial commitment that achieves succinct proof size and verification time in the degree $d$ of the polynomial. Extractability of our scheme holds in the random oracle model under a natural ring version of the BASIS assumption introduced by Wee and Wu (EUROCRYPT 2023). Unlike recent constructions of polynomial commitments by Albrecht et al. (CRYPTO 2022), and by Wee and Wu, we do not require any expensive preprocessing steps, which makes our scheme particularly attractive as an ingredient of a PIOP compiler for succinct arguments.

We further instantiate our polynomial commitment, together with the Marlin PIOP (Eurocrypt 2020), to obtain a publicly-verifiable trusted-setup succinct argument for Rank-1 Constraint System (R1CS). Performance-wise, we achieve 26 MB proof size for $2^{20}$ constraints, which is 10X smaller than currently the only publicly-verifiable lattice-based SNARK proposed by Albrecht et al.