International Association
for Cryptologic Research

Masking is the main countermeasure against side-channel attacks on embedded devices. For cryptographic algorithms that combine Boolean and arithmetic masking, one must therefore convert between the two types of masking, without leaking additional information to the attacker. In this paper we describe a new high-order conversion algorithm between Boolean and arithmetic masking, based on table recomputation, and provably secure in the ISW probing model. We show that our technique is particularly efficient for masking structured LWE encryption schemes such as Kyber and Saber. In particular, for Kyber IND-CPA decryption, we obtain an order of magnitude improvement compared to existing techniques.

TCC'21 will take place will take place in Raleigh, United States on November 8-11 2021.

Registration for TCC 2021 is now open for both in person and remote attendees! Register early! The earliest we know the number of in-person attendees, the best the in-person experience will be. See: https://tcc.iacr.org/2021/registration.php

Stipends are available for students.

A special "in person" workshop will take place alongside TCC. Deadline to submit a talk is Oct. 13th. More details on the workshop can be found here: https://tcc.iacr.org/2021/inperson.php

We at Arcana(arcana.network) are looking for someone well versed with applied math and cryptography to help design our protocol and allied systems. Specifically, we are looking for knowledge in a significant subset of the following: (1) Verifiable Delay Function(VDF) implementation. (2)Distributed key generation(DKG) and management(DKMS). (3)Encryption schemes. (4)Game theoretical (dis)incentivisation on a decentralized network. (5)Distributed Random Number generation. (6)End to End Encryption schemes . (7)Zero Knowledge proofs. (8)Verifiable computation. Academics and part time consultants also welcome to apply.

Closing date for applications:

Contact: admin@arcana.network

More information: https://arcana.network

The Institute of Information Security and Dependability at KIT is looking for a Post-Doc with expertise in privacy-preserving cryptographic protocols with a focus on secure multi-party computation , ideally, having hands-on experiences with MPC-compilers. A track record in this field is expected, including publications at reputable conferences such as Crypto, Eurocrypt, ACM CCS, PETS, etc.

You will be a member of the KASTEL Security Research Labs (https://zentrum.kastel.kit.edu) and the Topic "Engineering Secure Systems" of the Helmholtz Association. Your research is dealing with cryptographic protocols for privacy-preserving computations, e.g., applied to mobility systems. It will result in both theoretical security concepts (protocol designs, security proofs, etc.) and their practical implementation (e.g., a demonstrator) for some application domain. The contract will initially be limited to 1 year, but can be extended.

If you are interested, please formally apply using the link given below. Besides your CV including a list of your publications, please also include the names of three references.

Closing date for applications:

Contact: Andy Rupp (andy.rupp@rub.de)

More information: https://www.pse.kit.edu/karriere/joboffer.php?id=96409&language=en

The high cost of IC design has made chip protection one of the first priorities of the semiconductor industry. In addition, with the growing number of untrusted foundries, the possibility of inside foundry attack is escalating. However, by taking advantage of polymorphic gates, the layouts of the circuits with different functionalities look exactly identical, making it impossible even for an inside foundry attacker to distinguish the defined functionality of an IC by looking at its layout. Moreover, since memristor is compatible with CMOS structure, it is possible to efficiently design hybrid memristor- CMOS circuits. In this paper, we propose a hardware obfuscation method based on polymorphic hybrid memristor-CMOS technology. Overhead of the polymorphic designs and the time complexity of possible attacks are discussed.

Group signatures allow group members to sign on behalf of the group anonymously. They are therefore well suited to storing data in a way that preserves the users’ privacy, while guaranteeing its authenticity. Garms and Lehmann (PKC’19) introduced a new type of group signatures that balance privacy with utility by allowing to selectively link subsets of the group signatures via an oblivious entity, the converter. The conversion takes a batch of group signatures and blindly transforms signatures originating from the same user into a consistent representation. Their scheme essentially targets a setting where the entity receiving fully unlinkable signatures and the converted ones is the same: only pseudonyms but not full signatures are converted, and the input to the converter is assumed to be well-formed. Thus, the converted outputs are merely linkable pseudonyms but no longer signatures. In this work we extend and strengthen such convertibly linkable group signatures. Conversion can now be triggered by malicious entities too, and the converted outputs can be publicly verified. This preserves the authentication of data during the conversion process. We define the security of this scheme and give a provably secure instantiation. Our scheme makes use of controlled-malleable NIZKs, which allow proofs to be mauled in a controlled manner. This allows signatures to be blinded, while still ensuring they can be verified during conversions.

This paper describes a practical side-channel power analysis on CRYSTALS-Kyber key-encapsulation mechanism. In particular, we analyse the polynomial multiplication in the decapsulation phase to recover the secret key in a semi-static setting. The power analysis attack was performed against the KYBER512 implementation from pqm4 running on STM32F3 M4-cortex CPU.

We consider the related-tweak impossible differential cryptanalysis of \texttt{TweAES}. It is one of the underlying primitives of Authenticated Encryption with Associated Data (AEAD) scheme \texttt{ESTATE} which was accepted as one of second-round candidates in the NIST Lightweight Cryptography Standardization project. Firstly, we reveal several properties of \texttt{TweAES}, which show what kinds of distinguishers are more effective in recovering keys. With the help of automatic solver Simple Theorem Prover (STP), we achieve many 5.5-round related-tweak impossible differentials with fixed input differences and output differences that just have one active byte. Then, we implement 8-round key recovery attacks against \texttt{TweAES} based on one of these 5.5-round distinguishes. Moreover, another 5.5-round distinguisher that has four active bytes at the end is utilized to mount a 7-round key recovery attack against \texttt{TweAES}, which needs much lower attack complexities than the 6-round related-tweak impossible differential attack of \texttt{TweAES} in the design document. Our 8-round key recovery attack is the best one against \texttt{TweAES} in terms of the number of rounds and complexities so far.

The final exponentiation affects the efficiency of pairing computations especially on pairing-friendly curves with high embedding degree. We propose an efficient method for computing the hard part of the final exponentiation on the KSS18 curve at 192-bit security level. Implementations indicate that the computation of the final exponentiation can be 8.74% faster than the previously fastest result.

This paper presents the design and evaluation of Wendy, the first Byzantine consensus protocol that achieves optimal latency (two phases), linear authenticator complexity, and optimistic responsiveness. Wendy's core technical contribution is a novel aggregate signature scheme that allows leaders to prove, with constant pairing cost, that an operation did not commit. This No-commit proof addresses prior liveness concerns in protocols with linear authenticator complexity (including view change), allowing Wendy to commit operations in two-phases only.

A variety of post-quantum cryptographic schemes are currently undergoing standardization in the National Institute of Standards and Technology's post-quantum cryptography standardization process. It is well known from classical cryptography that actual implementations of cryptographic schemes can be attacked by exploiting side-channels, e.g. timing behavior, power consumption or emanation in the electromagnetic field. Although several of the reference implementations currently in the third and final standardization round are - to some extent - implemented in a timing-constant fashion, resistance against other side-channels is not taken into account yet. Implementing sufficient countermeasures, however, is challenging.

We therefore exemplarily examine CRYSTALS-Kyber, which is a lattice-based key encapsulation mechanism currently considered as a candidate for standardization. By analyzing the power consumption side-channel during message encoding we develop four more and compare six different implementations with an increasing degree of countermeasures.

We show that introducing randomization countermeasures is crucial as all examined implementations aiming at reducing the leakage by minimizing the Hamming distance of the processed intermediate values only are vulnerable against single-trace attacks when implemented on an ARM Cortex-M4.

Micropayments are one of the challenges in cryptocurrencies.

The problems in realizing micropayments in the blockchain are the low throughput and the high blockchain transaction fee.

As a solution, decentralized probabilistic micropayment has been proposed. The winning amount is registered in the blockchain, and the tickets are issued to be won with probability $p$, which allows us to aggregate approximately $\frac{1}{p}$ transactions into one.

Unfortunately, existing solutions do not allow for ticket transferability, and the smaller $p$, the more difficult it is to use them in the real world.

We propose a novel decentralized probabilistic micropayment Transferable Scheme. It allows tickets to be transferable among users. By allowing tickets to be transferable, we can make $p$ smaller.

We also propose a novel Proportional Fee Scheme. This is a scheme where each time a ticket is transferred, a portion of the blockchain transaction fee will be charged.

With the proportional fee scheme, users will have the advantage of sending money with a smaller fee than they would generally send through the blockchain.

For example, sending one dollar requires only ten cents.

This paper presents the first adaptively simulation secure functional encryption (FE) schemes for attribute-weighted sums. In such an FE scheme, encryption takes as input N pairs of attribute {(x_i, z_i )}_{i \in [N]} for some N \in \mathbb{N} where the attributes {x_i}_{i \in [N]} are public while the attributes {z_i}_{i \in [N]} are private. The indices i \in [N] are referred to as the slots. A secret key corresponds to some weight function f, and decryption recovers the weighted sum \sum_{i \in [N]} f(x_i)z_i. This is an important functionality with a wide range of potential real life applications. In the proposed FE schemes attributes are viewed as vectors and weight functions are arithmetic branching programs (ABP). We present two schemes with varying parameters and levels of adaptive security.

(a) We first present a one-slot scheme that achieves adaptive security in the simulation-based security model against a bounded number of ciphertext queries and an arbitrary polynomial number of secret key queries both before and after the ciphertext queries. This is the best possible level of security one can achieve in the adaptive simulation-based framework. From the relations between the simulation-based and indistinguishability-based security frameworks for FE, it follows that the proposed FE scheme also achieves indistinguishability- based adaptive security against an a-priori unbounded number of ciphertext queries and an arbitrary polynomial number of secret key queries both before and after the ciphertext queries. Moreover, the scheme enjoys compact ciphertexts that do not grow with the number of appearances of the attributes within the weight functions.

(b) Next, bootstrapping from the one-slot scheme, we present an unbounded-slot scheme that achieves simulation-based adaptive security against a bounded number of ciphertext and pre-ciphertext secret key queries while supporting an a-priori unbounded number of post-ciphertext secret key queries. The scheme achieves public parameters and secret key sizes independent of the number of slots N and a secret key can decrypt a ciphertext for any a-priori unbounded N. Further, just like the one-slot scheme, this scheme also has the ciphertext size independent of the number of appearances of the attributes within the weight functions. However, all the parameters of the scheme, namely, the master public key, ciphertexts, and secret keys scale linearly with the bound on the number of pre-ciphertext secret key queries.

Our schemes are built upon asymmetric bilinear groups of prime order and the security is derived under the standard (bilateral) k-Linear (k-Lin) assumption. Our work resolves an open problem posed by Abdalla, Gong, and Wee in CRYPTO 2020, where they presented an unbounded-slot FE scheme for attribute-weighted sum achieving only semi-adaptive simulation security. At a technical level, our work extends the recent adaptive security framework of Lin and Luo [EUROCRYPT 2020], devised to achieve compact ciphertexts in the context of indistinguishability-based payload-hiding security, into the setting of simulation-based adaptive attribute-hiding security.

Let $n=2m$. In the present paper, we study the binomial Boolean functions of the form $$f_{a,b}(x) = \mathrm{Tr}_1^{n}(a x^{2^m-1 }) +\mathrm{Tr}_1^{2}(bx^{\frac{2^n-1}{3} }), $$ where $m$ is an even positive integer, $a\in \mathbb{F}_{2^n}^*$ and $b\in \mathbb{F}_4^*$. We show that $ f_{a,b}$ is a bent function if the Kloosterman sum $$K_{m}\left(a^{2^m+1}\right)=1+ \sum_{x\in \mathbb{F}_{2^m}^*} (-1)^{\mathrm{Tr}_1^{m}(a^{2^m+1} x+ \frac{1}{x})}$$ equals $4$, thus settling an open problem of Mesnager. The proof employs tools including computing Walsh coefficients of Boolean functions via multiplicative characters, divisibility properties of Gauss sums, and graph theory.

Black-box accumulation (BBA) is a cryptographic protocol that allows users to accumulate and redeem points, e.g. in payment systems, and offers provable security and privacy guarantees. Loosely speaking, the transactions of users remain unlinkable, while adversaries cannot claim a false amount of points or use points from other users. Attempts to spend the same points multiple times (double spending) reveal the identity of the misbehaving user and an undeniable proof of guilt. Known instantiations of BBA rely on classical number-theoretic assumptions, which are not post-quantum secure. In this work, we propose the first lattice-based instantiation of BBA, which is plausibly post- quantum secure. It relies on the hardness of the Learning with Errors (LWE) and Short Integer Solution (SIS) assumptions and is secure in the Random Oracle Model (ROM). Our work shows that a lattice-based instantiation of BBA can be realized with a communication cost per transaction of about 199 MB if built on the zero-knowledge protocol by Yang et al. (CRYPTO 2019) and the CL-type signature of Libert et al. (ASIACRYPT 2017). Without any zero-knowledge overhead, our protocol requires 1.8 MB communication.

With the advent of the Internet of Things (IoT), e-health has become one of the main topics of research. Due to the sensitivity of patient information, patient privacy seems challenging. Nowadays, patient data is usually stored in the cloud in healthcare programs, making it difficult for users to have enough control over their data. The recent increment in announced cases of security and surveillance breaches compromising patients' privacy call into question the conventional model, in which third-parties gather and control immense amounts of patients' Healthcare data. In this work, we try to resolve the issues mentioned above by using blockchain technology. We propose a blockchain-based protocol suitable for e-health applications that does not require trust in a third party and provides an efficient privacy-preserving access control mechanism. Transactions in our proposed system, unlike Bitcoin, are not entirely financial, and we do not use conventional methods for consensus operations in blockchain like Proof of Work (PoW). It is not suitable for IoT applications because IoT devices have resources-constraints. Usage of appropriate consensus method helps us to increase network security and efficiency, as well as reducing network cost, i.e., bandwidth and processor usage. Finally, we provide security and privacy analysis of our proposed protocol.

Isogeny-based cryptography is known as one of the promising approaches to the emerging post-quantum public key cryptography. In cryptography, an IDentification (ID) protocol is a primitive that allows someone's identity to be confirmed. We present an efficient variation of the isogeny-based interactive ID scheme used in the base form of the CSI-FiSh signature [BKV19], which was initially proposed by Couveignes-Rostovtsev-Stolbunov [Cou06, RS06}, to support a larger challenge space, and consequently achieve a better soundness error rate in each execution. To this end, we prolong the public key of the basic ID protocol with some $\it{well-formed}$ elements that are generated by particular factors of the secret key. Due to the need for a well-formed (or structured) public key, the (secret and public) keys are generated by a trusted authority. Our analysis shows that, for a particular security parameter, by extending a public key of size 64 B to 2.1 MB, the prover and verifier of our ID protocol can be more than 14$\times$ faster than the basic ID protocol which has a binary challenge space, and moreover, the proof in our case will be about 13.5$\times$ shorter. Using standard techniques, we also turn the presented ID protocol into a signature scheme that is as efficient as the state-of-the-art CSI-FiSh signature, and is existentially unforgeable under chosen message attacks in the (quantum) random oracle model. However, in our signature scheme, a verifier should get the public key of a signer from a trusted authority, which is standard in a wide range of current uses of signatures. Finally, we show how to eliminate the need for a trusted authority in our proposed ID protocol.

We introduce report and trace ring signature schemes, balancing the desire for signer anonymity with the ability to report malicious behaviour and subsequently revoke anonymity. We contribute a formal security model for report and trace ring signatures that incorporates established properties of anonymity, unforgeability and traceability, and captures a new notion of reporter anonymity. We present a construction of a report and trace ring signature scheme, proving its security and analysing its efficiency, comparing with the state of the art in the accountable ring signatures literature. Our analysis demonstrates that our report and trace scheme is efficient, particularly for the choice of cryptographic primitives that we use to instantiate our construction. We contextualise our new primitive with respect to related work, and highlight, in particular, that report and trace ring signature schemes protect the identity of the reporter even after tracing is complete.

Password-based authentication is a central tool for end-user security. As part of this, password hashing is used to ensure the security of passwords at rest. If quantum computers become available at sufficient size, they are able to significantly speed up the computation of preimages of hash functions. Using Grover's algorithm, at most, a square-root speedup can be achieved, and thus it is expected that quantum password guessing also admits a square-root speedup. However, password inputs are not uniformly distributed but highly biased. Moreover, typical password attacks do not only compromise a random user's password but address a large fraction of all users' passwords within a database of millions of users.

In this work, we study those quantum large-scale password guessing attacks for the first time. In comparison to classical attacks, we still gain a square-root speedup in the quantum setting when attacking a constant fraction of all passwords, even considering strongly biased password distributions as they appear in real-world password breaches. We verify the accuracy of our theoretical predictions using the LinkedIn leak and derive specific recommendations for password hashing and password security for a quantum computer era.

We propose to adapt ”low-algebra” digital signature schemes SPHINCS+ and PICNIC, present in the NIST-PQC contest, to the limitations of resource-bounded low-end devices. For this, we replaced the cryptographic primitives (hash functions and symmetric ciphers) of these schemes with lightweight alternatives presented in the NIST-LWC contest. With these specifically conceived primitives, we improve the performance of the signature schemes and still preserve the NIST’s security levels. Regarding SPHINCS+, besides replacing the hash function, we also take into consideration relaxing some parameters and introduce a new notion: security as life expectancy. Furthermore, we also introduce an attack to the SPHINCS+ scheme that takes advantage of the usage of FORS on this scheme and the way its leaves are calculated. Also, we give some solutions on how to avoid this attack. Additionally, a modification of PICNIC is introduced as PICNIC+WOTS where PICNIC is used to generate the secret keys for several WOTS+ signatures significantly reducing the size and signature time of each signature.