Traceable Receipt-free Encryption (TREnc) is a verifiable public-key encryption primitive introduced at Asiacrypt 2022. A TREnc allows randomizing ciphertexts in transit in order to remove any subliminal information up to a public trace that ensures the non-malleability of the underlying plaintext. A remarkable property of TREnc is the indistingui-shability of the randomization of chosen ciphertexts against traceable chosen-ciphertext attacks (TCCA). This property can support applications like voting, and it was shown that receipt-free non-interactive voting, where voters are unable to convince any third party of the content of their vote, can be generically built from a TREnc. While being a very promising primitive, the few existing TREnc mechanisms either require a secret coin CRS or are fairly demanding in time and space requirements. Their security proofs also come with a linear security degradation in the number of challenge ciphertexts. We address these limitations and offer two efficient public coin TREnc mechanisms tailored for the two common tallying approaches in elections: homomorphic and mixnet-based. The TCCA security of our mechanisms also enjoys an almost-tight reduction to SXDH, based on a new randomizable technique of independent interest in the random oracle model. A Rust implementation of our TREnc mechanisms demonstrates that we can verifiably encrypt 64 bits in less than a second, and full group elements in around 30 ms., which is sufficient for most real-world applications. While comparing with other solutions, we show that our approaches lead to the most efficient non-interactive receipt-free voting system to date.
The Raindrop block cipher is an award-winning algorithm of the recent Cryptographic Algorithm Design Competition in China. It employs a Feistel structure and has three versions Raindrop128-128, Raindrop128-256 and Raindrop256, which have a 128-bit block size with a 128- or 256-bit user key and a 256-bit block size with a 256-bit user key, respectively. In this paper, we observe that Raindrop can be transformed to an equivalent cipher with two rounds less than Raindrop (for each version), due to the position of the round key XOR operation. We also observe that when conducting impossible differential cryptanalysis of Raindrop, both inactive and active bit differences on plaintext and ciphertext as well as a few intermediate states may be exploited for some refined sorting conditions on plaintexts and ciphertexts to filter out preliminary satisfying plaintext/ciphertext pairs efficiently, and finally we exploit a few 12-round impossible differentials of Raindrop128 and Raindrop256 to make key-recovery attacks on 19-round Raindrop128-128, 21-round Raindrop128-256 and 20-round Raindrop256. Our attacks are better than any previously published cryptanalytic results on Raindrop in terms of the numbers of attacked rounds.
Oblivious Transfer ( ) is a fundamental cryptographic primitive, becoming a crucial component of a practical secure protocol. is typically implemented in software, and one way to accelerate its running time is by using hardware implementations. However, such implementations are vulnerable to side-channel attacks (SCAs). On the other hand, protecting interactive protocols against SCA is highly challenging because of their longer secrets (which include inputs and randomness), more complicated design, and running multiple instances. Consequently, there are no truly practical leakage-resistant protocols yet. In this paper, we introduce two tailored indistinguishability-based security definitions for leakage-resilient , focusing on protecting the sender’s state. Second, we propose a practical semi-honest secure protocol that achieves these security levels while minimizing the assumptions on the protocol’s building blocks and the use of a secret state. Finally, we extend our protocol to support sequential composition and explore efficiency-security tradeoffs.
FRI is a cryptographic protocol widely deployed today as a building block of many efficient SNARKs that help secure transactions of hundreds of millions of dollars per day. The Fiat-Shamir security of FRI-vital for understanding the security of FRI-based SNARKs-has only recently been formalized and established by Block et al. (ASI-ACRYPT '23). In this work, we complement the result of Block et al. by providing a thorough concrete security analysis of non-interactive FRI under various parameter settings from protocols deploying (or soon to be deploying) FRI today. We find that these parameters nearly achieve their desired security targets (being at most 1-bit less secure than their targets) for non-interactive FRI with respect to a certain security conjecture about the FRI Protocol. However, in all but one set of parameters, we find that the provable security of non-interactive FRI under these parameters is severely lacking, being anywhere between 21- and 63-bits less secure than the conjectured security. The conjectured security of FRI assumes that known attacks are optimal, the security of these systems would be severely compromised should a better attack be discovered. In light of this, we present parameter guidelines for achieving 100-bits of provable security for non-interactive FRI along with a methodology for tuning these parameters to suit the needs of protocol designers.
Private Stream Aggregation (PSA) allows clients to send encryptions of their private values to an aggregator that is then able to learn the sum of these values but nothing else. It has since found many applications in practice, e.g. for smart metering or federated learning. In 2018, Becker et al. proposed the first lattice-based PSA scheme LaPS (NDSS 2018), with putative post-quantum security, which has subsequently been patented. In this paper, we describe two attacks on LaPS that break the claimed aggregator obliviousness security notion, where the second attack even allows to recover the secret keys of the clients, given enough encryptions. Moreover, we review the PSA literature for other occurrences of the responsible flawed proof steps. By explicitly tracking down and discussing these flaws, we clarify and hope to contribute to the literature on PSA schemes, in order to prevent further insecure schemes in practice. Finally, we point out that a Real-or-Random variant of the security notion that is often used as a substitute to make proofs easier, is not well-defined and potentially weaker than the standard PSA security notion. We propose a well defined variant and show that it is equivalent to the standard security notion of PSA under mild assumptions.
Privacy-preserving machine learning (PPML) enables multiple distrusting parties to jointly train ML models on their private data without revealing any information beyond the final trained models. In this work, we study the client-aided two-server setting where two non-colluding servers jointly train an ML model on the data held by a large number of clients. By involving the clients in the training process, we develop efficient protocols for training algorithms including linear regression, logistic regression, and neural networks. In particular, we introduce novel approaches to securely computing inner product, sign check, activation functions (e.g., ReLU, logistic function), and division on secret shared values, leveraging lightweight computation on the client side. We present constructions that are secure against semi-honest clients and further enhance them to achieve security against malicious clients. We believe these new client-aided techniques may be of independent interest. We implement our protocols and compare them with the two-server PPML protocols presented in SecureML (Mohassel and Zhang, S&P'17) across various settings and ABY2.0 (Patra et al., Usenix Security'21) theoretically. We demonstrate that with the assistance of untrusted clients in the training process, we can significantly improve both the communication and computational efficiency by orders of magnitude. Our protocols compare favorably in all the training algorithms on both LAN and WAN networks.
Authenticated key exchange (AKE) is a fundamental cryptographic protocol that establishes a secure channel over the Internet. The security for AKE is defined as a security game between an adversary and the challenger. Especially, partners and freshness of the session are used to identify trivial attacks by the adversary. Roughly, partners are instances that derive the same session key, and freshness determines whether an adversary's behavior constitutes trivial attacks. In this work, we reconsider security definitions for AKE and point out the shortcomings of the conventional partners and freshness definition. Then, we propose a new robust and strong security definition. First, we propose a new definition of partners based on round identifiers. Second, we propose a new freshness definition, which captures more non-trivial attacks than the conventional ones. We introduce a new notion of miscommunicators to identify the adversary's behavior more accurately than conventional definitions. This allows, for example, the behavior of sending the first message as-is and tampering with the second message to be viewed as a non-trivial attack, which was considered a trivial attack in conventional definitions. Our new security definition is strictly stronger than the conventional one. As evidence of this, we provide a new construction of AKE that is secure in the conventional definition but insecure in the new definition.
Password-Authenticated Key Exchange (PAKE) establishes a secure channel between two parties who share a password. Asymmetric PAKE is a variant of PAKE, where one party stores a hash of the password to preserve security under the situation that the party is compromised. The security of PAKE and asymmetric PAKE is often analyzed in the framework of universal composability (UC). Abdalla et al. (CRYPTO '20) relaxed the UC security of PAKE and showed that the relaxed security still guarantees reasonable properties. This relaxation makes it possible to prove the security in the UC framework for several PAKE protocols. In this paper, we propose a relaxed functionality of asymmetric PAKE by following the approach of Abdalla et al. We prove that the SPAKE2+ protocol UC-realizes this functionality. We also define a more relaxed functionality and prove that a variant of the AuCPace protocol UC-realizes it.
Continuous group key agreement (CGKA) allows a group of users to maintain a continuously updated shared key in an asynchronous setting where parties only come online sporadically and their messages are relayed by an untrusted server. CGKA captures the basic primitive underlying group messaging schemes. Current solutions including TreeKEM (“Messaging Layer Security” (MLS) IETF RFC 9420) cannot handle concurrent requests while retaining low communication complexity. The exception being CoCoA, which is concurrent while having extremely low communication complexity (in groups of size n and for m concurrent updates the communication per user is log (n) , i.e., independent of m). The main downside of CoCoA is that in groups of size n, users might have to do up to log (n) update requests to the server to ensure their (potentially corrupted) key material has been refreshed. In this work we present a “fast healing” concurrent CGKA protocol, named DeCAF, where users will heal after at most log (t) requests, with t being the number of corrupted users. While also suitable for the standard central-server setting, our protocol is particularly interesting for realizing decentralized group messaging, where protocol messages (add, remove, update) are being posted on some append-only data structure rather than sent to a server. In this setting, concurrency is crucial once the rate of requests exceeds, say, the rate at which new blocks are added to a blockchain. In the central-server setting, CoCoA (the only alternative with concurrency, sub-linear communication and basic post-compromise security) enjoys much lower download communication. However, in the decentralized setting – where there is no server which can craft specific messages for different users to reduce their download communication – our protocol significantly outperforms CoCoA. DeCAF heals in fewer epochs ( log (t) vs. log (n) ) while incurring a similar per epoch per user communication cost.
We present a tweakable wide block cipher called Mystrium and show it as the fastest such primitive on low-end processors that lack dedicated AES or other cryptographic instructions, such as ARM Cortex-A7. Mystrium is based on the provably secure double-decker mode, that requires a doubly extendable cryptographic keyed (deck) function and a universal hash function. We build a new deck function called Xymmer that for its compression part uses Multimixer-128, the fastest universal hash for such processors, and for its expansion part uses a newly designed permutation, 𝒢_512 . Deck functions can also be used in modes to build encryption, authenticated encryption, and authentication schemes, and hence, Xymmer is of independent interest. The current state-of-the-art wide tweakable block cipher Adiantum-XChaCha12-AES encrypts 4096-byte messages at 11.5 cycles per byte on ARM Cortex-A7, while for Mystrium it is 6.8 cycles per byte while having a higher claimed security.
We instantiate the hash-then-evaluate paradigm for pseudorandom functions (PRFs), (k,x):=(k,(x)) , which builds a PRF from a weak PRF via a public pre-processing random oracle . In applications to secure multiparty computation (MPC), only the low-complexity performs secret-depending operations. Our construction replaces by f(k_,(x)) , where f is a non-adaptive PRF and the key k_ is public and thus known to the distinguishing adversary. We show that, perhaps surprisingly, several existing weak PRF candidates are plausibly also secure when their inputs are generated by f(k_,(.)) . Firstly, analogous cryptanalysis applies (because pseudorandomness of f implies good statistical properties) and/or secondly an attack against the weak PRF with such pseudorandom inputs generated by f would imply surprising results such as key agreement from the hardness of the high-noise version of the Learning Parity with Noise (LPN) when implementing both and f from this assumption. Our simple transformation of replacing (· ) public pre-processing by f(k_,(x)) public pre-processing applies to the entire family of PRF-style functions. Specifically, we obtain results for oblivious PRFs, which are a core building block for password-based authenticated key exchange (PAKE) and private set intersection (PSI) protocols, and we also obtain results for pseudorandom correlation functions (PCF), which are a key tool for silent oblivious transfer (OT) extension.
Release of unverified plaintexts (RUP) security is an important target for robustness in AE schemes. It is also highly crucial for lightweight (LW) implementations of online AE schemes on memory-constrained devices. Surprisingly, very few online AEAD schemes come with provable guarantees against RUP integrity and not one with any well-defined RUP confidentiality. In this work, we first propose a new strong security notion for online AE schemes called OAE-RUP that captures security under blockwise processing of both encryption (which includes nonce-misuse) and decryption (which includes RUP). Formally, OAE-RUP combines the standard RUP integrity notion INT-RUP with a new RUP confidentiality notion sOPRPF (strong Online PseudoRandom Permutation followed by a pseudorandom Function). sOPRPF is based on the concept of "strong online permutations" and can be seen as an extension of the well-known CCA3 notion (Abed et al., FSE 2014) that captures arbitrary-length inputs. An OAE-RUP-secure scheme is resistant against nonce-misuse as well as leakage of unverified plaintexts where the integrity remains unaffected, and the confidentiality of any encrypted plaintext is preserved up to the leakage of the longest prefix with the leaked plaintexts and the leakage of the length of the longest prefix with the nonce-repeating ciphertexts. We then prove the OAE-RUP security of the SAEF mode. SAEF is a ForkAE mode (Asiacrypt 2019) that is optimized for authenticated encryption of short messages and processes the message blocks sequentially and in an online manner. At SAC 2020, it was shown that SAEF is also an online nonce misuse-resistant AE (OAE), offering enhanced security against adversaries that make blockwise adaptive encryption queries. It has remained an open question if SAEF also resists attacks against blockwise adaptive decryption adversaries or, more generally, when the decrypted plaintext is released before verification (RUP). Our proofs are conducted using the coefficients H technique, and they show that, without any modifications, SAEF is OAE-RUP secure up to the birthday bound, i.e., up to 2(n/2) processed data blocks, where n is the block size of the forkcipher.
We present an encrypted multi-map, a fundamental data structure underlying searchable encryption/structured encryption. Our protocol supports updates and is designed for applications demanding very strong data security. Not only it hides the information about queries and data, but also the query, access, and volume patterns. Our protocol utilizes a position-based ORAM and an encrypted dictionary. We provide two instantiations of the protocol, along with their operation-type-revealing variants, all using PathORAM but with different encrypted dictionary instantiations (AVL tree or BSkiplist). Their efficiency has been evaluated through both asymptotic and concrete complexity analysis, outperforming prior work while achieving the same level of strong security. We have implemented our instantiations and evaluated their performance on two real-world email databases (Enron and Lucene). We also discuss the strengths and limitations of our construction, including its resizability, and highlight that optimized solutions, even with heavy network utilization, may become practical as network speed improves.
We present techniques for constructing zero-knowledge argument systems from garbled circuits, extending a theoretical framework by Hazay and Venkitasubramaniam (J. Crypto, 2020) to the following directions: The above result is then applied to develop circuit-represented compositions of Σ -protocols. For predicates P_1,… ,P_n individually associated with a Σ -protocol, and a predicate C represented by a Boolean circuit, we construct a Σ -protocol for proving C(P_1,… ,P_n) = 1. This result answers positively an open question posed by Abe, et al., at TCC 2021.
Zero-knowledge shuffle arguments are a useful tool for constructing mix-nets which enable anonymous communication. We propose a new shuffle argument using a novel technique that probabilistically checks that each weighted set of input elements corresponds to some weighted set of output elements, with weights from the same set as the input element weights. We achieve this using standard discrete log assumptions and the shortest integer solution (SIS) assumption. Our shuffle argument has prover and verifier complexity linear in the size of the shuffled set, and communication complexity logarithmic both in the shuffled set size and security parameter.
Substitution-Permutation Networks (SPNs) are a popular and powerful technique for designing block ciphers. Confusion-Diffusion Networks (CDNs), as formalised by Dodis et al. at Eurocrypt'16, treat unkeyed SPNs as a means of extending the domain of public permutations. Dodis et al. showed that 5-round CDNs are indifferentiable from an ideal permutation, and subsequent works by Da et al. at Indocrypt'21 and Nandi et al. at C2SI'23 have established that 2-round CDNs cannot achieve the weaker notion of sequential indifferentiability even with non-linear diffusion layers. In this paper we show for the first time that 3-round CDNs with linear diffusion layers can achieve indifferentiability from an ideal permutation.
Lookup arguments have recently attracted a lot of developments due to their applications in the constructions of succinct non-interactive arguments of knowledge (SNARKs). A closely related topic is subsequence arguments in which one can prove that string s is a subsequence of another string t , i.e., deleting some characters in t can achieve s . A dual notion, namely, non-subsequence arguments, is to prove that s is not a subsequence of t . These problems have a lot of important applications in DNA sequence analysis, internet of things, blockchains, natural language processing, speech recognition, etc. However, despite their applications, they are not well-studied in cryptography, especially succinct arguments for non-subsequences with efficient proving time and sublinear verification time. In this work, we propose the first succinct non-subsequence argument. Our solution applies the sumcheck protocol and is instantiable by any multivariate polynomial commitment schemes (PCSs). We achieve an efficient prover whose running time is linear in the size of sequences s , t and their respective alphabet . Our proof is succinct and the verifier time is sublinear assuming the employed PCS has succinct commitments and sublinear verification time. When instantiating with Sona PCS (EUROCRYPT’24), we achieve proof size 𝒪(log _2|s| + log _2|t|+log _2| |) , prover time 𝒪(|s|+|t|+| |) and verifier time 𝒪(√(|s|)+√(|t|)+√(| |)) . Extending our technique, we can achieve a batch subsequence argument for proving in batch k interleaving subsequence and non-subsequence arguments without proof size suffering a linear blow-up in k.
EagleSign is one of the 40 "Round 1 Additional Signatures" that is accepted for consideration in the supplementary round of the Post-Quantum Cryptography standardization process, organized by NIST. Its design is based on structured lattices, and it boasts greater simplicity and performance compared to the two lattice signatures already selected for standardization: Falcon and Dilithium. In this paper, we show that those claimed advantages come at the cost of security. More precisely, we show that the distribution of EagleSign signatures leaks information about the private key, to the point that only a few hundred signatures on arbitrary known messages suffice for a full key recovery, for all proposed parameters. A related vulnerability also affects EagleSign-V2, a subsequent version of the scheme specifically designed to thwart the initial attack. Although a larger number of signatures is required for key recovery, the idea of the attack remains largely similar. Both schemes come with proofs of security that we show are flawed.
Cryptographic wallets are an essential tool in Blockchain networks to ensure the secure storage and maintenance of an user's cryptographic keys. Broadly, wallets can be divided into three categories, namely custodial, non-custodial, and shared-custodial wallets. The first two are centralized solutions, i.e., the wallet is operated by a single entity, which inherently introduces a single point of failure. Shared-custodial wallets, on the other hand, are maintained by two independent parties, e.g., the wallet user and a service provider, and hence avoid the single point of failure centralized solutions. Unfortunately, current shared-custodial wallets suffer from significant privacy issues. In our work, we introduce password-authenticated deterministic wallets (PADW), a novel and efficient shared-custodial wallet solution, which exhibits strong security and privacy guarantees. In a nutshell, in a PADW scheme, the secret key of the user is shared between the user and the server. In order to generate a signature, the user first authenticates itself to the server by providing a password and afterwards engages in an interactive signing protocol with the server. Security is guaranteed as long as at most one of the two parties is corrupted. Privacy, on the other hand, guarantees that a corrupted server cannot link a transaction to a particular user. We formally model the notion of PADW schemes and we give an instantiation from blind Schnorr. Our construction supports deterministic key derivation, a feature that is widely used in practice by existing wallet schemes, and it does not rely on any heavy cryptographic primitives. In the full version of this work, we prove our scheme secure against adaptive adversaries in the random oracle model and under standard assumptions. That is, our proof only relies on the assumption that the Schnorr signature scheme is unforgeable and that a public key encryption scheme is CCA-secure.
