International Association
for Cryptologic Research

In proof-of-stake (PoS) blockchains, stakeholders that extend the chain are selected according to the amount of stake they own. In S\&P 2019 the ``Ouroboros Crypsinous'' system of Kerber et al.\ (and concurrently Ganesh et al.\ in EUROCRYPT 2019) presented a mechanism that hides the identity of the stakeholder when adding blocks, hence preserving anonymity of stakeholders both during payment and mining in the Ouroboros blockchain. They focus on anonymizing the messages of the blockchain protocol, but suggest that potential identity leaks from the network-layer can be removed as well by employing anonymous broadcast channels.

In this work we show that this intuition is flawed. Even ideal anonymous broadcast channels do not suffice to protect the identity of the stakeholder who proposes a block.

We make the following contributions. First, we show a formal network-attack against Ouroboros Crypsinous, where the adversary can leverage network delays to distinguish who is the stakeholder that added a block on the blockchain. Second, we abstract the above attack and show that whenever the adversary has control over the network delay -- within the synchrony bound -- loss of anonymity is inherent for any protocol that provides liveness guarantees. We do so, by first proving that it is impossible to devise a (deterministic) state-machine replication protocol that achieves basic liveness guarantees and better than $(1-2\f)$ anonymity at the same time (where $\f$ is the fraction of corrupted parties). We then connect this result to the PoS setting by presenting the tagging and reverse tagging attack that allows an adversary, across several executions of the PoS protocol, to learn the stake of a target node, by simply delaying messages for the target. We demonstrate that our assumption on the delaying power of the adversary is realistic by describing how our attack could be mounted over the Zcash blockchain network (even when Tor is used). We conclude by suggesting approaches that can mitigate such attacks.

We study uncloneable quantum encryption schemes for classical messages as recently proposed by Broadbent and Lord. We focus on the information-theoretic setting and give several limitations on the structure and security of these schemes: Concretely, 1) We give an explicit cloning-indistinguishable attack that succeeds with probability $\frac12 + \mu/16$ where $\mu$ is related to the largest eigenvalue of the resulting quantum ciphertexts. 2) For a uniform message distribution, we partially characterize the scheme with the minimal success probability for cloning attacks. 3) Under natural symmetry conditions, we prove that the rank of the ciphertext density operators has to grow at least logarithmically in the number of messages to ensure uncloneable security. 4) The \emph{simultaneous} one-way-to-hiding (O2H) lemma is an important technique in recent works on uncloneable encryption and quantum copy protection. We give an explicit example which shatters the hope of reducing the multiplicative "security loss" constant in this lemma to below 9/8.

The k-XOR problem can be generically formulated as the following: given many n-bit strings generated uniformly at random, find k distinct of them which XOR to zero. This generalizes collision search (two equal elements) to a k-tuple of inputs.

This problem has become ubiquitous in cryptanalytic algorithms. Applications include variants in which the XOR operation is replaced by a modular addition (k-SUM) or other non-commutative operations (e.g., the composition of permutations). The case where a single solution exists on average is of special importance.

The generic study of quantum algorithms k-XOR (and variants) was started by Grassi et al. (ASIACRYPT 2018), in the case where many solutions exist. At EUROCRYPT 2020, Naya-Plasencia and Schrottenloher defined a class of "quantum merging algorithms" obtained by combining quantum search. They represented these algorithms by a set of "merging trees" and obtained the best ones through linear optimization of their parameters.

In this paper, we give a new, simplified representation of merging trees that makes their analysis easier. As a consequence, we improve the quantum time complexity of the Single-solution k-XOR problem by relaxing one of the previous constraints, and making use of quantum walks. Our algorithms subsume or improve over all previous quantum generic algorithms for Single-solution k-XOR. For example, we give an algorithm for 4-XOR (or 4-SUM) in quantum time $\widetilde{\mathcal{O}}(2^{7n/24})$.

In this work, we study disappearing cryptography in the bounded storage model. Here, a component of the transmission, say a ciphertext, a digital signature, or even a program, is streamed bit by bit. The stream is so large for anyone to store in its entirety, meaning the transmission effectively disappears once the stream stops.

We first propose the notion of online obfuscation, capturing the goal of disappearing programs in the bounded storage model. We give a negative result for VBB security in this model, but propose candidate constructions for a weaker security goal, namely VGB security. We then demonstrate the utility of VGB online obfuscation, showing that it can be used to generate disappearing ciphertexts and signatures. All of our applications are NOT possible in the standard model of cryptography, regardless of computational assumptions used.

We push a little further the study of two characterizations of almost perfect nonlinear (APN) functions introduced in our recent monograph. We state open problems about them, and we revisit in their perspective a well-known result from Dobbertin on APN exponents. This leads us to new results about APN power functions and more general APN polynomials with coefficients in a subfield F_{2^k} , which ease the research of such functions and of differentially uniform functions, and simplifies the related proofs by avoiding tedious calculations. In a second part, we give slightly simpler proofs than in the same monograph, of two known results on Boolean functions, one of which deserves to be better known but needed clarification, and the other needed correction.

Current proofs of current multi-signature schemes yield bounds on adversary advantage that are loose, failing to match the indications of cryptanalysis, and failing to justify security of implementations of the schemes in the 256-bit groups that are the choice of practioners. We bridge this gap via proofs in the Algebraic Group Model (AGM). For classical 3-round schemes we give AGM proofs with tight bounds. We then give a new 2-round multi-signature scheme, as efficient as prior ones, for which we prove a tight AGM bound. These results are obtained via a framework in which a reduction is broken into a chain of sub-reductions involving intermediate problems. By giving as many as possible of the sub-reductions tightly in the standard model, we minimize use of the AGM, and also hedge the AGM proofs with standard-model ones from different starting points.

It has been common knowledge that for a stream cipher to be secure against generic TMD tradeoff attacks, the size of its internal state in bits needs to be at least twice the size of the length of its secret key. In FSE 2015, Armknecht and Mikhalev however proposed the stream cipher Sprout with a Grain-like architecture, whose internal state was equal in size with its secret key and yet resistant against TMD attacks. Although Sprout had other weaknesses, it germinated a sequence of stream cipher designs like Lizard and Plantlet with short internal states. Both these designs have had cryptanalytic results reported against them. In this paper, we propose the stream cipher Atom that has an internal state of 159 bits and offers a security of 128 bits. Atom uses two key filters simultaneously to thwart certain cryptanalytic attacks that have been recently reported against keystream generators. In addition, we found that our design is one of the smallest stream ciphers that offers this security level, and we prove in this paper that Atom resists all the attacks that have been proposed against stream ciphers so far in literature. On the face of it, Atom also builds on the basic structure of the Grain family of stream ciphers. However, we try to prove that by including the additional key filter in the architecture of Atom we can make it immune to all cryptanalytic advances proposed against stream ciphers in recent cryptographic literature.

Side-channel attacks are a threat to secrets stored on a device, especially if an adversary has physical access to the device. As an effect of this, countermeasures against such attacks for cryptographic algorithms are a well-researched topic. In this work, we deviate from the study of cryptographic algorithms and instead focus on the side-channel protection of a much more basic operation, the comparison of a known attacker-controlled value with a secret one. Comparisons sensitive to side-channel leakage occur in tag comparisons during the verification of message authentication codes (MACs) or authenticated encryption, but are typically omitted in security analyses. Besides, also comparisons performed as part of fault countermeasures might be sensitive to side-channel attacks. In this work, we present a formal analysis on comparing values in a leakage resilient manner by utilizing cryptographic building blocks that are typically part of an implementation anyway. Our results indicate that there is no need to invest additional resources into implementing a protected comparison operation itself if a sufficiently protected implementation of a public cryptographic permutation, or a (tweakable) block cipher, is already available. We complement our contribution by applying our findings to the SuKS message authentication code used by lightweight authenticated encryption scheme ISAP, and to the classical Hash-then-PRF construction.

Cryptanalysis of symmetric-key ciphers, e.g., linear/differential cryptanalysis, requires an adversary to know the internal structures of the targeted ciphers. On the other hand, deep learning-based cryptanalysis has attracted significant attention because the adversary is not assumed to have knowledge of the targeted ciphers except the interfaces of algorithms. Such a blackbox attack is extremely strong; thus we must design symmetric-key ciphers that are secure against deep learning-based cryptanalysis. However, previous attacks do not clarify what features or internal structures affect success probabilities; therefore it is difficult to employ the results of such attacks to design deep learning-resistant symmetric-key ciphers. In this paper, we focus on toy SPN block ciphers (small PRESENT and small AES) and propose deep learning-based output prediction attacks. Due to its small internal structures, we can build learning models by employing the maximum number of plaintext/ciphertext pairs, and we can precisely calculate the rounds in which full diffusion occurs. We demonstrate the following: (1) our attacks work against a similar number of rounds attacked by linear/differential cryptanalysis, (2) our attacks realize output predictions (precisely plaintext recovery and ciphertext prediction) that are much stronger than distinguishing attacks, and (3) swapping the order of components or replacement components affects the success probabilities of the attacks. It is particularly worth noting that swapping/replacement does not affect the success probabilities of linear/differential cryptanalysis. We expect that our results will be an important stepping stone in the design of deep learning-resistant symmetric key ciphers.

Branching program is an important component of indistinguishability obfuscation (IO) schemes, its size greatly influences the efficiencies of corresponding IO schemes. There are two major candidates of branching programs, Bar86 branching program and IK00 branching program. Bar86 branching program was shown to efficiently recognize NC$^1$ circuits. In specific, a Boolean circuit of depth $d$ can be recognized by a Bar86 branching program of length not larger than $4^d$ (Therefore of size not larger than $5^2 \times 4^d$). In this short paper we present similar result about IK00 branching program. We show that IK00 branching program efficiently recognizes NC$^1$ circuits, that is, a Boolean circuit of depth $d$ can be recognized by an IK00 branching program of size not larger than $(n+1) \times 4^d$, where $n$ is input length. Our result may be a negative evidence for IK00 branching program to efficiently recognize polynomial-time computable functions.

In our previous paper we introduced a novel SNARK-based construction, called Zendoo, that allows Bitcoin-like blockchains to create and communicate with sidechains of different types without knowing their internal structure. We also introduced a specific construction, called Latus, allowing creation of fully verifiable sidechains. But in there we omitted a detailed description of an incentive scheme for Latus that is an essential element of a real decentralized system. This paper fills the gap by introducing details of the incentive scheme for the Latus sidechain. Represented ideas can also be adopted by other SNARK-based blockchains to incentivize decentralized proofs creation.

In 1989, Shamir presented an efficient identification scheme (IDS) based on the permuted kernel problem (PKP). After 21 years, PKP was generalized by Lampe and Patarin, who were able to build an IDS similar to Shamir's one, but using the binary field. This binary variant presented some interesting advantages over Shamir's original IDS, such as reduced number of operations and inherently resistance against side-channel attacks. In the security analysis, considering the best attacks against the original PKP, the authors concluded that none of these existing attacks appeared to have a significant advantage when attacking the binary variant. In this paper, we propose the first attack that targets the binary PKP. The attack is analyzed in detail, and its practical performance is compared with our theoretical models. For the proposed parameters originally targeting 79 and 98 bits of security, our attack can recover about 100% of all keys using less than $2^{63}$ and $2^{77}$ operations, respectively.

State-separating proofs (SSP) is a recent methodology for structuring game-based cryptographic proofs in a modular way. While very promising, this methodology was previously not fully formalized and came with little tool support. We address this by introducing SSProve, the first general verification framework for machine-checked state-separating proofs. SSProve combines high-level modular proofs about composed protocols, as proposed in SSP, with a probabilistic relational program logic for formalizing the lower-level details, which together enable constructing fully machine-checked crypto proofs in the Coq proof assistant. Moreover, SSProve is itself formalized in Coq, including the algebraic laws of SSP, the soundness of the program logic, and the connection between these two verification styles.

Code-based cryptographic schemes are highly regarded among the quantum-safe alternatives to current standards. Yet, designing code-based signatures using traditional methods has always been a challenging task, and current proposals are still far from the target set by other post-quantum primitives (e.g. lattice-based). In this paper, we revisit a recent work using an innovative approach for signing, based on the hardness of the code equivalence problem. We introduce some optimizations and provide a security analysis for all variants considered. We then show that the new parameters produce instances of practical interest.

Recently, a light-weight authenticated key-exchange (AKE) scheme has been proposed. The scheme provides mutual authentication. It is asymmetric in nature by delegating complex cryptographic operations to resource-equipped servers, and carefully managing the workload on resource-constrained Smart meter nodes by using Physically Unclonable Functions. The prototype Smart meter built using commercial-off-the-shelf products is enabled with a low-cost countermeasure against load-modification attacks, which goes side-by-side with the proposed protocol. An attack against this AKE scheme has been recently proposed claiming that the server can be breached to mount spoofing attacks. It relies on the assumption that the result of an attack against authenticated key-exchange protocol is determined before the attacker learns the session key. In this short paper, we discuss the attack’s validity and describe the misinterpretation of the AKE protocol’s security definition.

Broadbent and Islam (TCC '20) proposed a quantum cryptographic primitive called quantum encryption with certified deletion. In this primitive, a receiver in possession of a quantum ciphertext can generate a classical certificate that the encrypted message is deleted. Though they proved that their construction is information theoretically secure, a drawback is that the construction is limited to the setting of one-time symmetric key encryption (SKE) where a sender and receiver have to share a common key in advance and the key can be used only once. In this paper, we construct a (reusable-key) public key encryption (PKE) and attribute-based encryption (ABE) with certified deletion. Our PKE with certified deletion is constructed assuming the existence of IND-CPA secure PKE, and our ABE with certified deletion is constructed assuming the existence of indistinguishability obfuscation and one-way function.

We extend a basic key agreement model with a hidden identifier source to a multi-user model with joint secrecy and privacy constraints over all entities that do not trust each other. Different entities that use different measurements of the same remote source through broadcast channels (BCs) to agree on mutually-independent local secret keys are considered. This model is the proper multi-user extension of the basic model since the encoder and decoder pairs are not assumed to trust other pairs, unlike assumed in the literature. Strong secrecy constraints imposed jointly on all secret keys, which is more stringent than separate secrecy leakage constraints for each secret key considered in the literature, are satisfied. Inner bounds for maximum key rate, and minimum privacy-leakage and storage rates are proposed for any finite number of entities. Inner and outer bounds for degraded and less-noisy BCs are given to illustrate cases with strong privacy. A multi-enrollment model that is used for common physical unclonable functions (PUFs) is also considered to establish inner and outer bounds for key-leakage-storage regions that differ only in the Markov chains imposed. For this special case, the encoder and decoder measurement channels have the same channel transition matrix and secrecy leakage is measured for each secret key separately. We illustrate cases for which it is useful to have multiple enrollments as compared to a single enrollment and vice versa.

Differential privacy has been widely applied to provide privacy guarantees by adding random noise to the function output. However, it inevitably fails in many high-stakes voting scenarios, where voting rules are required to be deterministic. In this work, we present the first framework for answering the question: ``How private are commonly-used voting rules?" Our answers are two-fold. First, we show that deterministic voting rules provide sufficient privacy in the sense of distributional differential privacy (DDP). We show that assuming the adversarial observer has uncertainty about individual votes, even publishing the histogram of votes achieves good DDP. Second, we introduce the notion of exact privacy to compare the privacy preserved in various commonly-studied voting rules, and obtain dichotomy theorems of exact DDP within a large subset of voting rules called generalized scoring rules.

There is a difference between a system having no known attacks and the system being secure---as cryptographers know all too well. Once we begin talking about the implementations of systems this issue becomes even more prominent since the amount of material which needs to be scrutinised skyrockets. Historically, lack of transparency and low standards for e-voting system implementations have resulted in a culture where open source code is seen as a gold standard; however, this ignores the issue of the comprehensibility of that code.

In this work we make concrete empirical recommendations based on our, and others, experiences and findings from examining the source code of e-voting systems. We highlight that any solution used for significant elections should be well designed, carefully analysed, deftly built, accurately documented and expertly maintained. Until e-voting system implementations are clear, comprehensible, and open to public scrutiny security standards are unlikely to improve.

We present Orthros, a 128-bit block pseudorandom function. It is designed with primary focus on latency of fully unrolled circuits. For this purpose, we adopt a parallel structure comprising two keyed permutations. The round function of each permutation is similar to Midori, a low-energy block cipher, however we thoroughly revise it to reduce latency, and introduce different rounds to significantly improve cryptographic strength in a small number of rounds. We provide a comprehensive, dedicated security analysis. For hardware implementation, Orthros achieves the lowest latency among the state-of-the-art low-latency primitives. For example, using the STM 90nm library, Orthros achieves a minimum latency of around 2.4 ns, while other constructions like PRINCE, Midori-128 and QARMA_{9}-128-\sigma_{0} achieve 2.56 ns, 4.10 ns, 4.38 ns respectively.