International Association
for Cryptologic Research

Generalized Feistel schemes (GFSs) are extremely important and extensively researched cryptographic schemes. In this paper, we investigate the security of Type-1 GFS in quantum circumstances. On the one hand, in the qCCA setting, we give a new quantum polynomial time distinguisher on (d^2 -1)-round Type-1 GFS with branches d >3, which extends the previous results by d-2 rounds. This leads to a more efficient analysis of type-1 GFS, that is, the complexity of some previous key-recovery attacks is reduced by a factor of 2^(((d-2)k)/2), where k is the key length of the internal round function. On the other hand, for CAST-256, which is a certain block cipher based on Type-1 GFS, we give a 17-round quantum distinguisher in the qCPA setting. As a result, we construct an r(r > 17) round quantum key-recovery attack with complexity O(2^(37(r-17))/2 ).

The Ascon cipher suite, offering both authenticated encryption with associated data (AEAD) and hashing functionality, has recently emerged as the winner of the NIST Lightweight Cryptography (LwC) standardization process. The AEAD schemes within Ascon, namely Ascon-128 and Ascon-128a, have also been previously selected as the preferred lightweight authenticated encryption solutions in the CAESAR competition. In this paper, we present a tight and comprehensive security analysis of the Ascon AEAD schemes within the random permutation model. Existing integrity analyses of Ascon (and any Duplex AEAD scheme in general) commonly include the term $DT/2^c$, where $D$ and $T$ represent data and time complexities respectively, and $c$ denotes the capacity of the underlying sponge. In this paper, we demonstrate that Ascon achieves AE security when $T$ is bounded by $\min\{2^{\kappa}, 2^c\}$ (where $\kappa$ is the key size), and $DT$ is limited to $2^b$ (with $b$ being the size of the underlying permutation, which is 320 for Ascon). Our findings indicate that in accordance with NIST requirements, Ascon allows for a tag size as low as 64 bits while enabling a higher rate of 192 bits, surpassing the recommended rate.

Chameleon hash (CH) is a trapdoor hash function. Generally it is hard to find collisions, but with the help of trapdoor, finding collisions becomes easy. CH plays an important role in converting a conventional blockchain to a redactable one. However, most of the existing CH schemes are too weak to support redactable blockchain. The currently known CH schemes serving for redactable blockchain have the best security of so-called “full collision resistance (f-CR)”, but they are built either on random oracle model or rely on heavy tools like the simulation-sound extractable non-interactive zero-knowledge (SSE-NIZK) proof system. Moreover, up to now there is no CH scheme with post-quantum f-CR security in the standard model. Therefore, no CH can support redactable blockchain in a post-quantum way without relying on random oracles.

In this paper, we introduce a variant of CH, namely tagged chameleon hash (tCH). Tagged chameleon hash takes a tag into hash evaluations and collision finding algorithms. We define two security notions for tCH, collision resistance (CR) and full collision resistance (f-CR), and prove the equivalence between CR and f-CR when tCH works in the one-time tag mode. We propose a tCH scheme from lattice without using any NIZK proof, and prove that its collision resistance is (almost) tightly reduced to the Short Integer Solution (SIS) assumption in the standard model. We also show how to apply tCH to a blockchain in one-time tag mode so that the blockchain can be compiled to a redactable one. Our tCH scheme provides the first post-quantum solution for redactable blockchains, without resorting to random oracles or NIZK proofs. Besides, we also construct a more efficient tCH scheme with CR tightly reduced to SIS in the random oracle model, which may be of independent interest.

This note provides an update on Keccak performance on the ARMv7-M processors. Starting from the XKCP implementation, we have applied architecture-specific optimizations that have yielded a performance gain of up to 21% for the largest permutation instance.

At EUROCRYPT 2006, Kelsey and Kohno proposed the so-called chosen target forced-prefix (CTFP) preimage attack, where for any challenge prefix $P$, the attacker can generate a suffix $S$ such that $H(P\|S) = y$ for some hash value $y$ published in advance by the attacker. Consequently, the attacker can pretend to predict some event represented by $P$ she did not know before, and thus this type of attack is also known as the Nostradamus attack. At ASIACRYPT 2022, Benedikt et al. convert Kelsey et al.'s attack to a quantum one, reducing the time complexity from $\mathcal{O}(\sqrt{n}\cdot 2^{2n/3})$ to $\mathcal{O}(\sqrt[3]{n} \cdot 2^{3n/7})$. CTFP preimage attack is less investigated in the literature than (second-)preimage and collision attacks and lacks dedicated methods. In this paper, we propose the first dedicated Nostradamus attack based on the meet-in-the-middle (MITM) attack, and the MITM Nostradamus attack could be up to quadratically accelerated in the quantum setting. According to the recent works on MITM preimage attacks on AES-like hashing, we build an automatic tool to search for optimal MITM Nostradamus attacks and model the tradeoff between the offline and online phases. We apply our method to AES-MMO and Whirlpool, and obtain the first dedicated attack on round-reduced version of these hash functions. Our method and automatic tool are applicable to other AES-like hashings.

Ring-LWE based homomorphic encryption computations in large depth use a combination of two techniques: 1) decomposition of big numbers into small limbs/digits, and 2) efficient cyclotomic multiplications modulo $X^N+1$. It was long believed that the two mechanisms had to be strongly related, like in the full-RNS setting that uses a CRT decomposition of big numbers over an NTT-friendly family of prime numbers, and NTT over the same primes for multiplications. However, in this setting NTT was the bottleneck of all large-depth FHE computations. A breakthrough result from Crypto'2023 by Kim et al. managed to overcome this limitation by introducing a second gadget decomposition and by showing that it indeed shifts the bottleneck and renders the cost of NTT computations negligible compared to the rest of the computation. In this paper, we extend this result (far) beyond the Full-RNS settings and show that we can completely decouple the big number decomposition from the cyclotomic arithmetic aspects. As a result, we get modulus switching/rescaling for free, and the memory footprint for storing relinearization keys across different levels is considerably lower compared to the CRT-based counterparts, by typically a factor $\ell/3$ where $\ell$ is the deepest level of multiplication depth supported. We verify both in theory and in practice that the performance of key-switching, external and internal products and automorphisms using our representation are similar or faster than the one achieved by Kim et al. Crypto'2023 paper, and we discuss the high impact of these results for people who work on low-level or hardware optimizations as well as the benefits of the new parametrizations for people currently working on compilers for FHE. We even manage to lower the running time of the gate bootstrapping of TFHE by eliminating 12.5% of its FFTs.

Compressed oracles (Zhandry, Crypto 2019) are a powerful technique to reason about quantum random oracles, enabling a sort of lazy sampling in the presence of superposition queries. A long-standing open question is whether a similar technique can also be used to reason about random (efficiently invertible) permutations. In this work, we make a step towards answering this question. We first define the compressed permutation oracle and illustrate its use. While the soundness of this technique (i.e., the indistinguishability from a random permutation) remains a conjecture, we show a curious 2-for-1 theorem: If we use the compressed permutation oracle methodology to show that some construction (e.g., Luby-Rackoff) implements a random permutation (or strong qPRP), then we get the fact that this methodology is actually sound for free.

This write-up summarizes the sampling analysis of the expander code from Brakedown [GLSTW21]. We elaborate their convexity argument for general linear expansion bounds, and we combine their approach with the one from Spielman [Sp96] to achieve asymptotic linear-time under constant field size. Choosing tighter expansion bounds we obtain more efficient parameters than [GLSTW21] for their 128 bit large field, reducing the encoding costs by 25% and beyond, and we provide a similar parameter set for the Mersenne prime field with modulus $p = 2^{31} - 1$, optimized by the combined Spielman-Brakedown approach.

We present Owl, an augmented password-authenticated key exchange (PAKE) protocol that is both efficient and supported by security proofs. Owl is motivated by recognized limitations in SRP-6a and OPAQUE. SRP-6a is the only augmented PAKE that has enjoyed wide use in practice to date, but it lacks the support of formal security proofs, and does not support elliptic curve settings. OPAQUE was proposed in 2018 as a provably secure and efficient alternative to SRP-6a, and was chosen by the IETF in 2020 for standardization, but open issues leave it unclear whether OPAQUE will replace SRP-6a in practice. Owl is obtained by efficiently adapting J-PAKE to an asymmetric setting, providing additional security against server compromise yet with lower computation than J-PAKE. Our scheme is provably secure, efficient and agile in supporting implementations in diverse multiplicative groups and elliptic curve settings. Owl is the first solution that provides systematic advantages over SRP-6a in terms of security, computation, message sizes, and agility. Owl’s agility across settings also contrasts ongoing issues related to how OPAQUE will instantiate a hash-to-curve operation in the elliptic curve setting (and what impact this will have on efficiency, security and forward compatibility with new elliptic curves in the future).

Fully Homomorphic Encryption (FHE) enables computations to be performed on encrypted data, so one can outsource computations of confidential information to an untrusted party. Ironically, FHE requires the client to generate massive evaluation keys and transfer them to the server side where all computations are supposed to be performed. In this paper, we propose LFHE, the Light-key FHE variant of the FHEW scheme introduced by Ducas and Micciancio in Eurocrypt 2015, and its improvement TFHE scheme proposed by Chillotti et al. in Asiacrypt 2016. In the proposed scheme the client generates small packed evaluation keys, which can be transferred to the server side with much smaller communication overhead compared to the original non-packed variant. The server employs a key reconstruction technique to obtain the evaluation keys needed for computations.

This approach allowed us to achieve the FHE scheme with the packed evaluation key transferring size of less than a Megabyte, which is an order of magnitude improvement compared to the best-known methods.

Anonymous Credentials are an important tool to protect user's privacy for proving possession of certain credentials. Although various efficient constructions have been proposed based on pre-quantum assumptions, there have been limited accomplishments in the post-quantum and especially practical settings. This research aims to derive new methods that enhance the current state of the art.

To achieve this, we make the following contributions. By distilling prior design insights, we propose a new primitive to instantiate \emph{signature with protocols}, called commit-transferrable signature (\CTS). When combined with a multi-theorem straight-line extractable non-interactive zero-knowledge proof of knowledge (\NIZKPoK), $\CTS$ gives a modular approach to construct anonymous credentials. We then show efficient instantiations of $\CTS$ and the required \NIZKPoK from lattices, which are believed to be post-quantum hard. Finally, we propose concrete parameters for the $\CTS$, \NIZKPoK, and the overall Anonymous Credentials, based on Module-\SIS~and Ring-\LWE. This would serve as an important guidance for future deployment in practice.

We present a three-round protocol for threshold ECDSA signing with malicious security against a dishonest majority, which information-theoretically UC-realizes a standard threshold signing functionality, assuming ideal commitment and two-party multiplication primitives. Our work improves upon and fully subsumes the DKLs $t$-of-$n$ and 2-of-$n$ protocols. This document focuses on providing a succinct but complete description of the protocol and its security proof, and contains little expository text.

In 2013, the Snowden revelations have shown subversion of cryptographic implementations to be a relevant threat. Since then, the academic community has been pushing the development of models and constructions to defend against adversaries able to arbitrarily subvert cryptographic implementations. To capture these strong capabilities of adversaries, Russell, Tang, Yung, and Zhou (CCS'17) proposed CPA-secure encryption in a model that utilizes a trusted party called a watchdog testing an implementation before use to detect potential subversion. This model was used to construct subversion-resilient implementations of primitives such as random oracles by Russell, Tang, Yung, and Zhou (CRYPTO'18) or signature schemes by Chow et al. (PKC'19) but primitives aiming for a CCA-like security remained elusive in any watchdog model. In this work, we present the first subversion-resilient authenticated encryption scheme with associated data (AEAD) without making use of random oracles. At the core of our construction are subversion-resilient PRFs, which we obtain from weak PRFs in combination with the classical Naor-Reingold transformation. We revisit classical constructions based on PRFs to obtain subversion-resilient MACs, where both tagging and verification are subject to subversion, as well as subversion-resilient symmetric encryption in the form of stream ciphers. Finally, we observe that leveraging the classical Encrypt-then-MAC approach yields subversion-resilient AEAD. Our results are based on the trusted amalgamation model by Russell, Tang, Yung, and Zhou (ASIACRYPT'16) and the assumption of honest key generation.

Recent advances in the capabilities of large language models such as GPT-4 have spurred increasing concern about our ability to detect AI-generated text. Prior works have suggested methods of embedding watermarks in model outputs, by $\textit{noticeably}$ altering the output distribution. We ask: Is it possible to introduce a watermark without incurring $\textit{any detectable}$ change to the output distribution?

To this end we introduce a cryptographically-inspired notion of undetectable watermarks for language models. That is, watermarks can be detected only with the knowledge of a secret key; without the secret key, it is computationally intractable to distinguish watermarked outputs from those of the original model. In particular, it is impossible for a user to observe any degradation in the quality of the text. Crucially, watermarks should remain undetectable even when the user is allowed to adaptively query the model with arbitrarily chosen prompts. We construct undetectable watermarks based on the existence of one-way functions, a standard assumption in cryptography.

Recently, there have been several proposals for secure computation with fair output delivery that require the use of a bulletin board abstraction (in addition to a trusted execution environment (TEE)). These proposals require all protocol participants to have read/write access to the bulletin board. These works envision the use of (public or permissioned) blockchains to implement the bulletin board abstractions. With the advent of consortium blockchains which place restrictions on who can read/write contents on the blockchain, it is not clear how to extend prior proposals to a setting where (1) not all parties have read/write access on a single consortium blockchain, and (2) not all parties prefer to post on a public blockchain.

In this paper, we address the above by showing the first protocols for fair secure computation in the multi-blockchain setting. More concretely, in a $n$-party setting where at most $t < n$ parties are corrupt, our protocol for fair secure computation works as long as (1) $t$ parties have access to a TEE (e.g., Intel SGX), and (2) each of the above $t$ parties are on some blockchain with each of the other parties. Furthermore, only these $t$ parties need write access on the blockchains.

In an optimistic setting where parties behave honestly, our protocol runs completely off-chain.

This paper introduces Nimble, a cloud service that helps applications running in trusted execution environments (TEEs) to detect rollback attacks (i.e., detect whether a data item retrieved from persistent storage is the latest version). To achieve this, Nimble realizes an append-only ledger service by employing a simple state machine running in a TEE in conjunction with a crash fault-tolerant storage service. Nimble then replicates this trusted state machine to ensure the system is available even if a minority of state machines crash. A salient aspect of Nimble is a new reconfiguration protocol that allows a cloud provider to replace the set of nodes running the trusted state machine whenever it wishes—without affecting safety. We have formally verified Nimble’s core protocol in Dafny, and have implemented Nimble such that its trusted state machine runs in multiple TEE platforms (Intel SGX and AMD SNP-SEV). Our results show that a deployment of Nimble on machines running in different availability zones can achieve from tens of thousands of requests/sec with an end-to-end latency of under 3.2 ms (based on an in-memory key-value store) to several thousands of requests/sec with a latency of 30ms (based on Azure Table).

With the emergence of Miner Extractable Value (MEV), block construction markets on blockchains have evolved into a competitive arena. Following Ethereum's transition from Proof of Work (PoW) to Proof of Stake (PoS), the Proposer Builder Separation (PBS) mechanism has emerged as the dominant force in the Ethereum block construction market.

This paper presents an in-depth longitudinal study of the Ethereum block construction market, spanning from the introduction of PoS and PBS in September 2022 to May 2023. We analyze the market shares of builders and relays, their temporal changes, and the financial dynamics within the PBS system, including payments among builders and block proposers---commonly referred to as bribes. We introduce an MEV-time law quantifying the expected MEV revenue wrt. the time elapsed since the last proposed block. We provide empirical evidence that moments of crisis (e.g. the FTX collapse, USDC stablecoin de-peg) coincide with significant spikes in MEV payments compared to the baseline.

Despite the intention of the PBS architecture to enhance decentralization by separating actor roles, it remains unclear whether its design is optimal. Implicit trust assumptions and conflicts of interest may benefit particular parties and foster the need for vertical integration. MEV-Boost was explicitly designed to foster decentralization, causing the side effect of enabling risk-free sandwich extraction from unsuspecting users, potentially raising concerns for regulators.

In this paper, we introduce a new approach to efficiently compute TFHE bootstrapping keys for (predefined) multiple users. Hence, a fixed number of users can enjoy the same level of efficiency as in the single key setting, keeping their individual input privacy. Our construction relies on a novel algorithm called homomorphic indicator, which can be of independent interest. We provide a detailed analysis of the noise growth and a set of secure parameters suitable to be used in practice. Moreover, we compare the complexity of our technique with other state-of-the-art constructions and show which method performs better in what parameter sets, based on our noise analysis. We also provide a prototype implementation of our technique. To the best of our knowledge, this is the first implementation of TFHE in the multiparty setting.

Mobile contact discovery is a convenience feature of messengers such as WhatsApp or Telegram that helps users to identify which of their existing contacts are registered with the service. Unfortunately, the contact discovery implementation of many popular messengers massively violates the users' privacy as demonstrated by Hagen et al. (NDSS '21, ACM TOPS '23). Unbalanced private set intersection (PSI) protocols are a promising cryptographic solution to realize mobile private contact discovery, however, state-of-the-art protocols do not scale to real-world database sizes with billions of registered users in terms of communication and/or computation overhead.

In our work, we make significant steps towards truly practical large-scale mobile private contact discovery. For this, we combine and substantially optimize the unbalanced PSI protocol of Kales et al. (USENIX Security '19) and the private information retrieval (PIR) protocol of Kogan and Corrigan-Gibbs (USENIX Security '21). Our resulting protocol has a total communication overhead that is sublinear in the size of the server's user database and also has sublinear online runtimes. We optimize our protocol by introducing database partitioning and efficient scheduling of user queries. To handle realistic change rates of databases and contact lists, we propose and evaluate different possibilities for efficient updates. We implement our protocol on smartphones and measure online runtimes of less than 2s to query up to 1024 contacts from a database with more than two billion entries. Furthermore, we achieve a reduction in setup communication up to factor 32x compared to state-of-the-art mobile private contact discovery protocols.

We remark that the key agreement scheme [IEEE Trans. Veh. Technol. 2021, 70(2): 1736--1751] fails to keep anonymity and untraceability, because the user $U_k$ needs to invoke the public key $PK_{U_j}$ to verify the signature generated by the user $U_j$. Since the public key is compulsively linked to the true identity $ID_{U_j}$ for authentication, any adversary can reveal the true identity by checking the signature.