International Association
for Cryptologic Research

While numerous physically unclonable functions (PUFs) were proposed in recent years, the conventional PUF-based authentication model is centralized by the data of challenge-response pairs (CRPs), particularly when $n$-party authentication is required. In this work, we propose a novel concept of clonable PUF (CPUF), wherein two or more PUFs having equivalent responses are manufactured to facilitate decentralized authentication. By design, cloning is only possible in the fabrication period and the responses are determined based on the variability induced during the fabrication. We establish the usage model and the circuit design of CPUFs. Numerical experiments using a circuit simulator show an ideal matching rate of responses between the CPUFs.

The ever-growing size of the Bitcoin UTXO state is a factor preventing nodes with limited storage capacity from validating transactions. Cryptographic accumulators, such as Merkle trees, offer a viable solution to the problem. Full nodes create a Merkle tree from the UTXO set, while stateless nodes merely store the root of the Merkle tree. When provided with a proof, stateless nodes can verify that a transaction's inputs belong to the UTXO set. In this work, we present a systematic study of Merkle tree based accumulators, with a focus on factors that reduce the proof size. Based on the observation that UTXOs typically have a short lifetime, we propose that recent UTXOs be co-located in the tree. When proofs for different transactions are batched, such a design reduces the per-transaction proof size. We provide details of our implementation of this idea, describing certain optimizations that further reduce the proof size in practice. On Bitcoin data before August 2019, we show that our design achieves a 4.6x reduction in proof size vis-a-vis UTREEXO [Dryja 2019], which is a different Merkle-tree based system designed to support stateless nodes.

We present a non-interactive publicly verifiable secret sharing scheme where a dealer can construct a Shamir secret sharing of a field element and confidentially yet verifiably distribute shares to multiple receivers. We also develop a non-interactive publicly verifiable resharing scheme where existing share holders of a Shamir secret sharing can create a new Shamir secret sharing of the same secret and distribute it to a set of receivers in a confidential, yet verifiable manner.

A public key may be associated with the secret being shared in the form of a group element raised to the secret field element. We use our verifiable secret sharing scheme to construct a non-interactive distributed key generation protocol that creates such a public key together with a secret sharing of the discrete logarithm. We also construct a non-interactive distributed resharing protocol that preserves the public key but creates a fresh secret sharing of the secret key and hands it to a set of receivers, which may or may not overlap with the original set of share holders.

Our protocols build on a new pairing-based CCA-secure public-key encryption scheme with forward secrecy. As a consequence our protocols can use static public keys for participants but still provide compromise protection. The scheme uses chunked encryption, which comes at a cost, but the cost is offset by a saving gained by our ciphertexts being comprised only of source group elements and no target group elements. A further efficiency saving is obtained in our protocols by extending our single-receiver encryption scheme to a multi-receiver encryption scheme, where the ciphertext is up to a factor 5 smaller than just having single-receiver ciphertexts.

The non-interactive key management protocols are deployed on the Internet Computer to facilitate the use of threshold BLS signatures. The protocols provide a simple interface to remotely create secret-shared keys to a set of receivers, to refresh the secret sharing whenever there is a change of key holders, and provide proactive security against mobile adversaries.

A verifiable shuffle of known values is a method for proving that a collection of commitments opens to a given collection of known messages, without revealing a correspondence between commitments and messages. We propose the first practical verifiable shuffle of known values for lattice-based commitments.

Shuffles of known values have many applications in cryptography, and in particular in electronic voting. We use our verifiable shuffle of known values to build a practical lattice-based cryptographic voting system that supports complex ballots. Our scheme is also the first construction from candidate post-quantum secure assumptions to defend against compromise of the voter's computer using return codes.

We implemented our protocol and present benchmarks of its computational runtime. The size of the verifiable shuffle is $17 \tau$ KB and takes time $33 \tau$ ms for $\tau$ voters. This is around $5$ times faster and at least $50$ % smaller per vote than the lattice-basedvoting scheme by del Pino et al. (ACM CCS 2017), which can only handle yes/no-elections.

In the era of cloud computing, massive quantities of data are encrypted and uploaded to the cloud to realize a variety of applications and services while protecting user confidentiality. Accordingly, the formulation of methods for efficiently searching encrypted data has become a critical problem. Public-key encryption with keyword search is an efficient solution that allows the data owner to generate encrypted keywords for a given document while also allowing the data user to generate the corresponding trapdoor for searching. Huang and Li proposed a public-key authenticated encryption with keyword search (PAEKS) scheme to resist keyword guessing attacks, where the data owner not only encrypts keywords but also authenticates them.However, existing PAEKS-related schemes carry a trade-off between efficiency, storage cost, and security.In this paper, we introduce a novel framework, called identity-certifying authority-aided identity-based searchable encryption, which has the advantage of reducing storage space while remaining the efficiency and security.We formally define the system model and desired security requirements to represent attacks in a real scenario. In addition, we propose a provably secure scheme based on the gap bilinear Diffie--Hellman assumption and experimentally evaluate our scheme in terms of its performance and theoretical features against its state-of-the-art counterparts.

In this article we look at the question of the security of Data Encryption Standard (DES) against non-linear polynomial invariant attacks. Is this sort of attack also possible for DES? We present a simple proof of concept attack on DES where a product of 5 polynomials is an invariant for 2 rounds of DES. Furthermore we present numerous additional examples of invariants with higher degrees. We analyse the success probability when the Boolean functions are chosen at random and compare to DES S-boxes. For more complex higher degree attacks the difficulties disappear progressively and up to 100 % of all Boolean functions in 6 variables are potentially vulnerable. A major limitation for all our attacks, is that they work only for a fraction of the key space. However in some cases, this fraction of the key space is very large for the full 16-round DES.

Format-Preserving Encryption (FPE) schemes accept plaintexts from any finite set of values (such as social security numbers or birth dates) and produce ciphertexts that belong to the same set. They are extremely useful in practice since they make it possible to encrypt existing databases or communication packets without changing their format. Due to industry demand, NIST had standardized in 2016 two such encryption schemes called FF1 and FF3. They immediately attracted considerable cryptanalytic attention with decreasing attack complexities. The best currently known attack on the Feistel construction FF3 has data and memory complexity of ${O}(N^{11/6})$ and time complexity of ${O}(N^{17/6})$, where the input belongs to a domain of size $N \times N$.

In this paper, we present and experimentally verify three improved attacks on FF3. Our best attack achieves the tradeoff curve $D=M=\tilde{O}(N^{2-t})$, $T=\tilde{O}(N^{2+t})$ for all $t \leq 0.5$. In particular, we can reduce the data and memory complexities to the more practical $\tilde{O}(N^{1.5})$, and at the same time, reduce the time complexity to $\tilde{O}(N^{2.5})$.

We also identify another attack vector against FPE schemes, the related-domain attack. We show how one can mount powerful attacks when the adversary is given access to the encryption under the same key in different domains, and show how to apply it to efficiently distinguish FF3 and FF3-1 instances.

We prove that Kilian's four-message succinct argument system is post-quantum secure in the standard model when instantiated with any probabilistically checkable proof and any collapsing hash function (which in turn exist based on the post-quantum hardness of Learning with Errors).

At the heart of our proof is a new "measure-and-repair" quantum rewinding procedure that achieves asymptotically optimal knowledge error.

Event date: 26 September to 28 September 2021
Submission deadline: 27 May 2021
Notification: 3 August 2021

Event date: 6 September 2021
Submission deadline: 21 May 2021
Notification: 2 July 2021

Event date: 10 January to 12 January 2022
Submission deadline: 1 September 2021
Notification: 1 November 2021

Cryptography, Security & Privacy Research Group at Koç University has one opening at the post-doctoral researcher level. Accepted applicants may receive competitive salary, housing (accommodation) support, health insurance, computer, travel support, and lunch meal card.

Your duties include performing research on cryptography, security, and privacy in line with our research group's focus, as well as directing graduate and undergraduate students in their research and teaching. The project funding is related to cryptography, game theory and mechanism design, and blockchain technologies.

Applicants are expected to have already obtained their Ph.D. degrees in Computer Science or related discipline with a thesis topic related to the duties above.

For more information about joining our group and projects, visit

https://crypto.ku.edu.tr/work-with-us/

Submit your application via email including

• full CV,
• 1-3 sample publications where you are the main author,
• a detailed research proposal,
• and 2-3 reference letters sent directly by the referees.

Application and start dates are flexible.

Closing date for applications:

Contact: Assoc. Prof. Alptekin Küpçü
https://member.acm.org/~kupcu

More information: https://crypto.ku.edu.tr/work-with-us/

Cryptography, Security & Privacy Research Group at Koç University has multiple openings at every level. Accepted Computer Science and Engineering applicants may receive competitive scholarships including monthly stipend, tuition waiver, housing (accommodation) support, health insurance, computer, travel support, and lunch meal card.

Your duties include performing research on cryptography, security, and privacy in line with our research group's focus, assist teaching, as well as collaborating with other graduate and undergraduate students. Computer Science, Mathematics, Cryptography, or related background is necessary.

For applying online, and questions about the application-process for M.Sc. and Ph.D. positions, visit

https://gsse.ku.edu.tr/en/admissions/application-requirements

All applications must be completed online. Applications with missing documents will not be considered. Applications via e-mail will not be considered. Application Requirements:

1. CV
2. Recommendation Letters (2 for MSc, 3 for PhD)
3. TOEFL (for everyone whose native language is not English, Internet Based: Minimum Score 80)
4. GRE scores (required from non-Turkish nationals)
5. Official transcripts from all the universities attended
6. Statement of Purpose
7. Area of Interest Form filled online

https://gsse.ku.edu.tr/en/admissions/how-to-apply/

We also have non-thesis Cyber Security M.Sc. program:

https://cybersecurity.ku.edu.tr/tuition/

For more information about joining our group and projects, visit

https://crypto.ku.edu.tr/work-with-us/

Closing date for applications:

Contact: https://gsse.ku.edu.tr/en/admissions/how-to-apply/

More information: https://gsse.ku.edu.tr/en/admissions/application-requirements

Cryptography, Security & Privacy Research Group at Koç University has multiple openings for summer research interns (at both undergraduate and graduate level). Apply via:

http://kusrp.ku.edu.tr

For more information about joining our group and projects, visit

https://crypto.ku.edu.tr/work-with-us/

All applications must be completed online. Applications with missing documents will not be considered. Applications via e-mail will not be considered. Application Requirements:

1. CV
2. 2 Recommendation Letters
3. Official transcripts from all the universities attended
4. Statement of Purpose
5. Application Form filled online

Deadline is 11 April 2021.

Closing date for applications:

Contact: http://kusrp.ku.edu.tr

More information: http://kusrp.ku.edu.tr

We introduce a class of interactive protocols, which we call sumcheck arguments, that establishes a novel connection between the sumcheck protocol (Lund et al. JACM 1992) and folding techniques for Pedersen commitments (Bootle et al. EUROCRYPT 2016).

Informally, we consider a general notion of bilinear commitment over modules, and show that the sumcheck protocol applied to a certain polynomial associated with the commitment scheme yields a succinct argument of knowledge for openings of the commitment. Building on this, we additionally obtain succinct arguments for the NP-complete language R1CS over certain rings.

Sumcheck arguments enable us to recover as a special case numerous prior works in disparate cryptographic settings (such as discrete logarithms, pairings, groups of unknown order, lattices), providing one abstract framework to understand them all. Further, we answer open questions raised in prior works, such as obtaining a lattice-based succinct argument from the SIS assumption for satisfiability problems over rings.

We elaborate an approach for determining the order of an elliptic curve from the family $\mathcal{E}_p = \{E_a: y^2 = x^3 + a \pmod p, a \not = 0\}$ where $p$ is a prime number $> 3$. The essence of this approach consists in combining the well-known Hasse bound with an explicit formula for that order reduced to modulo $p$. It allows to advance an efficient technique of complexity $O(\log^2 p)$ for computing simultaneously the six orders associated with the family $\mathcal{E}_p$ when $p \equiv 1 \pmod 3$, thus improving the best known algorithmic solution for that problem with an order of magnitude.

The high demand for customer-centric applications such as secure cloud storage laid the foundation for the development of user-centric security protocols with multiple security features in recent years. But, the current state-of-art techniques primarily emphasized only one type of security feature i.e., either homomorphism or non-malleability. In order to fill this gap and provide a common platform for both homomorphic and non-malleable cloud applications, we have introduced a new public key based probabilistic encryption switching (i.e., homomorphism to/from non-malleability property switching during the encryption phase without changing the underlying security structure) scheme by introducing a novel Contiguous Chain Bit Pair Encryption (CC-BPE) and Discrete Chain Bit Pair Encryption (DC-BPE) techniques for plaintext bits encryption and using quadratic residuosity based trapdoor function of Freeman et al. [13] for intermediate ciphertext connections. The proposed scheme generates O ( m +2 log N ) bits of ciphertext where m &#8712; N and m < n , n &#8712; N is the plaintext size, N is the RSA composite. This security extension would be helpful to cover both homomorphism and non-malleability cloud applications. The superior performance of the proposed scheme has been tested in comparison to existing methods and is reported in this paper.

Key-Alternating Feistel (KAF) ciphers are a popular variant of Feistel ciphers whereby the round functions are defined as $x \mapsto F(k_i \oplus x)$, where k_i are the round keys and F is a public random function. Most Feistel ciphers, such as DES, indeed have such a structure. However, the security of this construction has only been studied in the classical CPA/CCA models. We provide the first security analysis of KAF ciphers in the key-dependent message (KDM) attack model, where plaintexts can be related to the private key. This model is motivated by cryptographic schemes used within application scenarios such as full-disk encryption or anonymous credential systems.

We show that the four-round KAF cipher, with a single function $F$ reused across the rounds, provides KDM security for a non-trivial set of KDM functions. To do so, we develop a generic proof methodology, based on the H-coefficient technique, that can ease the analysis of other block ciphers in such strong models of security.

Blockchain has been widely used in finance, logistics, copyright and other fields with its outstanding characteristics such as non-centralization, collective maintenance, openness, transparency and non-tamperability. However, as transactions are stored in plaintext in the blockchain for public verification, the anonymity and privacy of users can not be guaranteed and this hampers many financial applications. How to protect the privacy of transactions is worthy further research.

In this paper, we have proposed two regulatory and efficient confidential transaction schemes using homomorphic encrytion and zero-knowledge proof. The first one improves the efficiency of the existing ElGamal based scheme while preserves its privacy. The second one employs the Paillier encryption with homomorphic property and it empowers regulators with greater power to obtain transaction-related specific content. The core of ElGamal based scheme is the Modified ElGamal algorithm, which changes the form of the standard ElGamal algorithm and expands it into four ciphertexts such that $(m,r)$ in the transaction can be decrypted. The Paillier based scheme is mainly to combine Paillier encryption with ElGamal encryption. Contrast to other ElGamal based scheme, the combination makes any token amount can be directly decrypted without calculating a discrete logarithm problem. As any $(m,r)$ in transactions can be decrypted directly, game theory is applied to further reduce transaction size. In our construction, transactions are about 1.1KB.

The Department of Mathematics in the School of Sciences and Humanities at Nazarbayev University invites applications for several positions in applied mathematics at the rank of Instructor or Assistant Professor. The department currently has 23 members. Faculty specializations include statistics, numerical analysis, applied mathematics, analysis, and algebra/number theory, including cryptography. Appointments are for three years with the possibility of renewal.

Responsibilities of these positions:

• Teach undergraduate courses in mathematics;
• Advise students in academic matters;
• Administrative and service work at the departmental, school, and university level;
• Faculty appointed at the Assistant Professor level will also be expected to teach graduate courses in mathematics, supervise undergraduate and graduate student research and capstone projects, apply for grants, and develop new courses.

Applicants should submit a cover letter, a curriculum vitae, research and teaching statements, and contact information for at least three references, who will be asked to submit letters of recommendation. At least one of the letters of recommendation should address the candidate's teaching.

Closing date for applications:

Contact: Daniel Oliveira da Silva at daniel.dasilva@nu.edu.kz

More information: https://jobs.smartrecruiters.com/NazarbayevUniversity1/743999738292544-instructor-and-assistant-professor-position-department-of-mathematics-school-of-sciences-and-humanities