International Association
for Cryptologic Research

Let $(N,e)$ be a public key of the RSA cryptosystem, and $d$ be the corresponding private key. In practice, we usually choose a small $e$ for quick encryption. In this paper, we improve partial private key exposure attacks against RSA with MSBs of $d$ and small $e$. The key idea is that under such a setting we can usually obtain more information about the prime factors of $N$ and then, by solving a univariate modular polynomial equation using Coppersmith's method, $N$ can be factored in polynomial time. Compared to previous results, we reduce the number of the leaked bits in $d$ that are needed to mount the attack by $\log_2 (e)$ bits. For $e=65537$, previous work required an additional enumeration of 17 bits to achieve our new bound, resulting in a $2^{10}$ (or 1,024x) increase in time consumption. Furthermore, our experiments show that for a $1024$-bit modulus $N$, our attack can achieve the theoretical bound on a simple personal computer, which verifies the new method.

Recently, the NSA has proposed a block cipher called ARADI and a mode of operation called LLAMA for memory encryption applications. In this note, we comment on this proposal, on its suitability for the intended application, and describe an attack on LLAMA that breaks confidentiality of ciphertext and allows a straightforward forgery attack breaking integrity of ciphertext (INT-CTXT) using a related-IV attack. Both attacks have negligible complexity.

The notion of (Receiver-) Anamorphic Encryption was put forth recently to show that a dictator (i.e., an overreaching government), which demands to get the receiver’s private key and even dictates messages to the sender, cannot prevent the receiver from getting an additional covert anamorphic message from a sender. The model required an initial private collaboration to share some secret. There may be settings though where an initial collaboration may be impossible or performance-wise prohibitive, or cases when we need an immediate message to be sent without private key generation (e.g., by any casual sender in need). This situation, to date, somewhat limits the applicability of anamorphic encryption.

To overcome this, in this work, we put forth the new notion of “public-key anamorphic encryption,” where, without any initialization, any sender that has not coordinated in any shape or form with the receiver, can nevertheless, under the dictator control of the receiver’s private key, send the receiver an additional anamorphic secret message hidden from the dictator. We define the new notion with its unique new properties, and then prove that, quite interestingly, the known CCA-secure Koppula-Waters (KW) system is, in fact, public-key anamorphic.

We then describe how a public-key anamorphic scheme can support a new hybrid anamorphic encapsulation mode (KDEM) where the public-key anamorphic part serves a bootstrapping mechanism to activate regular anamorphic messages in the same ciphertext, thus together increasing the anamorphic channel capacity.

Looking at the state of research thus far, we observe that the initial system (Eurocrypt’22) that was shown to have regular anamorphic properties is the CCA-secure Naor-Yung (and other related schemes). Here we identify that the KW CCA-secure scheme also provides a new type of anamorphism. Thus, this situation is hinting that there may be a connection between some types of CCA-secure schemes and some type of anamorphic schemes (in spite of the fact that the goals of the two primitives are fundamentally different); this question is foundational in nature. Given this, we identify a sufficient condition for a “CCA-secure scheme which is black-box reduced from a CPA secure scheme” to directly give rise to an “anamorphic encryption scheme!” Furthermore, we identify one extra property of the reduction, that yields a public-key anamorphic scheme as defined here.

Smart farming uses different vehicles to manage all the operations on the farm. These vehicles should be put to good use for secure data transmission. The Vangala et al.'s key agreement scheme [IEEE TIFS, 18 (2023), 904-9193] is designed for agricultural IoT networks. In this note, we show that the scheme fails to keep anonymity, instead pseudonymity. The scheme simply thinks that anonymity is equivalent to preventing the real identity from being recovered. But the true anonymity means that the adversary cannot attribute different sessions to target users. To the best of our knowledge, it is the first time to clarify the differences between anonymity and pseudonymity.

Robust message authentication codes (MACs) and authenticated encryption (AE) schemes that provide authenticity in the presence of side-channel leakage are essential primitives. These constructions often rely on primitives designed for strong leakage protection, among others including the use of strong-unpredictable (tweakable) block-ciphers. This paper extends the strong-unpredictability security definition to the versatile and new forkcipher primitive. We show how to construct secure and efficient MAC and AEs that guarantee authenticity in the presence of leakage. We present a leakage-resistant MAC, ForkMAC, and two leakage-resistant AE schemes, ForkDTE1 and ForkDTE2, which use forkciphers instead of traditional secure (tweakable) block-ciphers as compared to the prior art. We prove and analyze their security in the presence of leakage based on a strong unpredictable forkcipher. A comparison with the state-of-the-art in terms of both security and efficiency is followed in the paper. Key advantages and highlights promoted by the proposed constructions are that for the minimal assumptions they require, unpredictability with leakage-based security, the tag-generation of ForkMAC is the most efficient among leakage-resilient MAC proposals, equivalent to HBC. ForkDTE 1 and 2 have a more efficient encryption than any other scheme, achieving integrity with leakage (and also providing misuse-resistance).

In early August 2024, three NSA researchers -- Patricia Greene, Mark Motley, and Bryan Weeks -- published the technical specifications for a new low-latency block cipher, ARADI, along with its corresponding authenticated encryption mode, LLAMA, which is specifically designed for memory encryption applications. Their manuscript offered minimal security analysis of the design, only briefly discussing the differential, linear and algebraic properties of cipher's underlying components. In this work, we present a set of distinguishers for the round reduced ARADI block cipher, discovered using the automated cryptanalysis tool CLAASP. More precisely, using CLAASP, we evaluate the resistance of ARADI against avalanche, statistical and continuous diffusion tests, differential and linear distinguishers, impossible differentials, algebraic attacks, and neural distinguishers. Accordingly, we give distinguishers that reach up to 9 out of 16 rounds of ARADI. We hope these preliminary findings will encourage further in-depth cryptanalysis of the cipher to enhance confidence in its security.

Framed within the general context of cyber-security, standard cryptographic constructions often represent an enabling technology for associated solutions. Alongside or in combination with their design, therefore, the implementation of such constructions is an important challenge: beyond delivering artefacts that are usable in practice, implementation can impact many quality metrics (such as efficiency and security) which determine fitness-for-purpose. A rich design space of implementation techniques can be drawn on in order to address this challenge, but threat- and opportunity-driven innovation based on clear understanding and empirical evidence remains vital.

In at least some use-cases, software-based implementation of cryptography is important, e.g., because it delivers an attractive trade off or is mandated for some reason. Such an implementation is heavily influenced both by 1) the Instruction Set Architecture (ISA) it is expressed using, and 2) the micro-architecture it is executed using. For example, the extent to which a general-purpose ISA can support more domain-specific requirements of a cryptographic construction will influence how the latter is mapped to the former (i.e., which implementation techniques are viable) and behavioural properties of doing so (e.g., the execution latency stemming from use of a given implementation technique).

This paper attempts to systematise the topic of cryptographic Instruction Set Extensions (ISEs), which represent an approach to provision of a platform where such support is more explicit and extensive. At a high level, the goal is to improve understanding of what is an extensive and somewhat inter-disciplinary body of literature (e.g., spanning academia and industry, hardware and software, as well as cryptographic and non-cryptographic publication venues). We argue that doing so will help to maximise the quality of subsequent work on this and associated topics.

Side-channel analysis on complex SoC devices with high-frequency microprocessors and multitasking operating systems presents significant challenges in practice due to the high costs of trace acquisition and analysis, generally involving tens of thousands to millions of traces. This work uses a cryptographic execution process on a Broadcom 2837 SoC as a case study to explore ways to reduce costs in electromagnetic side-channel analysis. In the data acquisition phase, we propose an efficient electromagnetic probe positioning strategy that does not require additional tool assistance, significantly accelerating the collection of effective electromagnetic traces. In the side-channel analysis phase, we investigate the combined use of preprocessing techniques, where the optimal preprocessing approach successfully reduces the number of required electromagnetic traces by 12 times, significantly improving the success rate of attacks. Additionally, we implement profiling attacks on such devices, including traditional template attacks, MLP-based, and CNN-based side-channel analysis, demonstrating that even minimal modeling costs can yield excellent analysis performance. Our study confirms the feasibility of low-cost side-channel analysis on complex SoCs and indicates that the sensitive applications running on these devices still require protection.

The strength of Elliptic curve cryptography (ECC) relies on curve choice. This work analyzes weak keys in standardized curves, i.e., private keys within small subgroups of the auxiliary group $\mathbb{Z}^*_p$. We quantify weak key prevalence across standardized curves, revealing a potential vulnerability due to numerous small divisors in auxiliary group orders. To address this, we leverage the implicit "baby-steps giant-steps algorithm", which transforms the complex elliptic curve discrete logarithm problem into a simpler problem within $\mathbb{Z}^*_p$. This enables efficient detection of weak keys in small-order subgroups. Our findings highlight the importance of rigorous key testing in applications using standardized ECC. While random weak keys are unlikely, malicious actors could exploit this by manipulating key generation libraries. To this end, we show how users can assess their private key vulnerabilities and mitigate risks by eliminating weak keys. Hence, this work contributes to improved ECC security through proactive key management practices.

In a typical network, a DNS(SEC) message over 1232 bytes would either be fragmented into several UDP/IP packets or require a re-transmit over TCP. Unfortunately, IP fragmentation is considered unreliable and a non-trivial number of servers do not support TCP.

We present $\texttt{QNAME}$-Based Fragmentation ($\mathsf{QBF}$): a DNS layer fragmentation scheme that fragments/re-assembles large post-quantum DNS(SEC) messages over UDP in just 1 round-trip while using only standard DNS records. Our experiments show that DNSSEC over $\mathsf{QBF}$, with either Falcon-512, Dilithium-2 or SPHINCS$^{+}$ as the zone signing algorithm, is practically as fast as the currently deployed ECDSA-P256 and RSA-2048 setups in resolving $\texttt{QTYPE}$ $\texttt{A}$ queries.

We present $\mathsf{SL\text{-}DNSSEC}$: a backward-compatible protocol that leverages a quantum-safe KEM and a MAC to perform signature-less $\mathsf{(SL)}$ DNSSEC validations in a single UDP query/response style. Our experiments targeting NIST level I security for QTYPE A query resolution show that $\mathsf{SL\text{-}DNSSEC}$ is practically equivalent to the presently deployed RSA-2048 in terms of bandwidth usage and resolution speeds. Compared to post-quantum signatures, $\mathsf{SL\text{-}DNSSEC}$ reduces bandwidth consumption and resolution times by up to $95\%$ and $60\%$, respectively. Moreover, with response size $<$ query size $\leq 1232$ bytes, $\mathsf{SL\text{-}DNSSEC}$ obviates the long-standing issues of IP fragmentation, TCP re-transmits and DDoS amplification attacks.

TFHE is a homomorphic encryption scheme supporting fast bootstrapping. There are two kinds of bootstrapping in TFHE: programmable bootstrapping (also known as gate bootstrapping) and circuit bootstrapping. Circuit bootstrapping offers more functionality than programmable bootstrapping, but requires heavier computational cost and larger evaluation key size. A recent work by Wang et al. improving circuit bootstrapping using homomorphic trace evaluation seems to mitigate its heavy cost, while we observe some flaws in their error analysis.

In this paper, we patch the circuit bootstrapping method proposed by Wang et al. with correct error analysis and extend the ciphertext modulus from a prime modulus to a power-of-two modulus, enabling FFT-based implementation of our patched method. In addition, we propose a high precision WWL+ method by adopting GLWE keyswitching, improving the circuit bootstrapping time (resp. key size) of WoP-PBS proposed by Bergerat et al. by factors from $3.26$ to $7.22$ (resp. $2.39$ to $2.63$). We also patch the parameter selection used in the AES evaluation by the WWL+ method, obtaining $26.301$s for a single AES evaluation in a single thread.

Secure multi-party computation (MPC) enables multiple distrusting parties to jointly compute a function while keeping their inputs private. Computing the AES block cipher in MPC, where the key and/or the input are secret-shared among the parties is important for various applications, particularly threshold cryptography.

In this work, we propose a family of dedicated, high-performance MPC protocols to compute the non-linear S-box part of AES in the honest majority setting. Our protocols come in both semi-honest and maliciously secure variants. The core technique is a combination of lookup table protocols based on random one-hot vectors and the decomposition of finite field inversion in $GF(2^8)$ into multiplications and inversion in the smaller field $GF(2^4)$, taking inspiration from ideas used for hardware implementations of AES. We also apply and improve the analysis of a batch verification technique for checking inner products with logarithmic communication. This allows us to obtain malicious security with almost no communication overhead, and we use it to obtain new, secure table lookup protocols with only $O(\sqrt{N})$ communication for a table of size $N$, which may be useful in other applications.

Our protocols have different trade-offs, such as having a similar round complexity as previous state-of-the-art but $37\%$ lower bandwidth costs, or having $27\%$ fewer rounds and $16\%$ lower bandwidth costs. An experimental evaluation in various network conditions using three party replicated secret sharing shows improvements in throughput between $23\%$ and $27\%$ in the semi-honest setting. For malicious security, we improve throughput by $46\%$ and $270\%$ in LAN and by up to $453\%$ in WAN due to a new multiplication verification protocol.

In recent years a new class of symmetric-key primitives over $\mathbb{F}_p$ that are essential to Multi-Party Computation and Zero-Knowledge Proofs based protocols has emerged. Towards improving the efficiency of such primitives, a number of new block ciphers and hash functions over $\mathbb{F}_p$ were proposed. These new primitives also showed that following alternative design strategies to the classical Substitution-Permutation Network (SPN) and Feistel Networks leads to more efficient cipher and hash function designs over $\mathbb{F}_p$ specifically for large odd primes $p$. In view of these efforts, in this work we build an \emph{algebraic framework} that allows the systematic exploration of viable and efficient design strategies for constructing symmetric-key (iterative) permutations over $\mathbb{F}_p$. We first identify iterative polynomial dynamical systems over finite fields as the central building block of almost all block cipher design strategies. We propose a generalized triangular polynomial dynamical system (GTDS), and based on the GTDS we provide a generic definition of an iterative (keyed) permutation over $\mathbb{F}_p^n$. Our GTDS-based generic definition is able to describe the three most well-known design strategies, namely SPNs, Feistel networks (FN) and Lai--Massey (LM). Consequently, the block ciphers that are constructed following these design strategies can also be instantiated from our generic definition. Moreover, we find that the recently proposed \texttt{Griffin} design, which neither follows the Feistel nor the SPN design, can be described using the generic GTDS-based definition. We also show that a new generalized Lai--Massey construction can be instantiated from the GTDS-based definition. The latter results confirm that our GTDS-based definition is able to instantiate cryptographic permutations that are beyond SPN, FN and LM based. We further provide generic (security) analysis of the GTDS including an upper bound on the differential uniformity and the correlation.

The proliferation of attacks to cloud computing, coupled with the vast amounts of data outsourced to online services, continues to raise major concerns about the privacy for end users. Traditional cryptography can help secure data transmission and storage on cloud servers, but falls short when the already encrypted data needs to be processed by the cloud provider. An emerging solution to this challenge is fully homomorphic encryption (FHE), which enables computations directly on encrypted data, and recent works have focused on developing new processor designs tailored for native processing of FHE data. In this work, we introduce PulpFHE, an optimized instruction set extension tailored for the next generation of FHE processors. Our proposed FHE instructions offer native support for non-linear operations on encrypted data, and enable significantly faster homomorphic computations for a broad range of realistic applications.

Time-Lock Puzzles (TLPs) have been developed to securely transmit sensitive information into the future without relying on a trusted third party. Multi-instance TLP is a scalable variant of TLP that enables a server to efficiently find solutions to different puzzles provided by a client at once. Nevertheless, existing multi-instance TLPs lack support for (verifiable) homomorphic computation. To address this limitation, we introduce the "Multi-Instance partially Homomorphic TLP" (MH-TLP), a multi-instance TLP supporting efficient verifiable homomorphic linear combinations of puzzles belonging to a client. It ensures anyone can verify the correctness of computations and solutions. Building on MH-TLP, we further propose the "Multi-instance Multi-client verifiable partially Homomorphic TLP" (MMH-TLP). It not only supports all the features of MH-TLP but also allows for verifiable homomorphic linear combinations of puzzles from different clients. Our schemes refrain from using asymmetric-key cryptography for verification and, unlike most homomorphic TLPs, do not require a trusted third party. A comprehensive cost analysis demonstrates that our schemes scale linearly with the number of clients and puzzles.

Let $N=pq$ be the product of two balanced prime numbers $p$ and $q$. In 2002, Elkamchouchi, Elshenawy, and Shaban introduced an interesting RSA-like cryptosystem that, unlike the classical RSA key equation $ed - k (p-1)(q-1) = 1$, uses the key equation $ed - k (p^2-1)(q^2-1) = 1$. The scheme was further extended by Cotan and Te\c seleanu to a variant that uses the key equation $ed - k (p^n-1)(q^n-1) = 1$, where $n \geq 1$. Furthermore, they provide a continued fractions attack that recovers the secret key $d$ if $d < N^{0.25n}$. In this paper we improve this bound using a lattice based method. Moreover, our method also leads to the factorisation of the modulus $N$, while the continued fractions one does not (except for $n=1,2,3,4$).

Redis (Remote Dictionary Server) is a general purpose, in-memory database that supports a rich array of functionality, including various Probabilistic Data Structures (PDS), such as Bloom filters, Cuckoo filters, as well as cardinality and frequency estimators. These PDS typically perform well in the average case. However, given that Redis is intended to be used across a diverse array of applications, it is crucial to evaluate how these PDS perform under worst-case scenarios, i.e., when faced with adversarial inputs. We offer a comprehensive analysis to address this question. We begin by carefully documenting the different PDS implementations in Redis, explaining how they deviate from those PDS as described in the literature. Then we show that these deviations enable a total of 10 novel attacks that are more severe than the corresponding attacks for generic versions of the PDS. We highlight the critical role of Redis' decision to use non-cryptographic hash functions in the severity of these attacks. We conclude by discussing countermeasures to the attacks, or explaining why, in some cases, countermeasures are not possible.

A threshold key encapsulation mechanism (TKEM) facilitates the secure distribution of session keys among multiple participants, allowing key recovery through a threshold number of shares. TKEM has gained significant attention, especially for decentralized systems, including blockchains. However, existing constructions often rely on trusted setups, which pose security risks such as a single point of failure, and are limited by fixed participant numbers and thresholds. To overcome this, we propose a dynamic TKEM with a transparent setup, allowing for a flexible selection of recipients and thresholds without relying on trusted third parties in the setup phase. In addition, our construction does not rely on pairing operations. We prove the security of our TKEM under the decisional Diffie-Hellman assumption, ensuring selective chosen-ciphertext security and decapsulation consistency. Our proof-of-concept implementation highlights the practicality and efficiency of this approach, advancing the field of threshold cryptography.

Proving resistance to conventional attacks, e.g., differential, linear, and integral attacks, is essential for designing a secure symmetric-key cipher. Recent advances in automatic search and deep learning-based methods have made this time-consuming task relatively easy, yet concerns persist over expertise requirements and potential oversights. To overcome these concerns, Kimura et al. proposed neural network-based output prediction (NN) attacks, offering simplicity, generality, and reduced coding mistakes. NN attacks could be helpful for designing secure symmetric-key ciphers, especially the S-box-based block ciphers. Inspired by their work, we first apply NN attacks to Simon, one of the AND-Rotation-XOR-based block ciphers, and identify structures susceptible to NN attacks and the vulnerabilities detected thereby. Next, we take a closer look at the vulnerable structures. The most vulnerable structure has the lowest diffusion property compared to others. This fact implies that NN attacks may detect such a property. We then focus on a biased event of the core function in vulnerable Simon-like ciphers and build effective linear approximations caused by such an event. Finally, we use these linear approximations to reveal that the vulnerable structures are more susceptible to a linear key recovery attack than the original one. We conclude that our analysis can be a solid step toward making NN attacks a helpful tool for designing a secure symmetric-key cipher.