TLS 1.3 is at the heart of secure modern internet communications. With the rise of quantum attacks, post-quantum TLS 1.3, built on post-quantum key encapsulation mechanisms (KEMs), has naturally become a major research focus. At Eurocrypt 2022, Huguenin-Dumittan and Vaudenay demonstrated that KEMs secure against chosen-plaintext attacks (CPA) are sufficient to construct a secure TLS 1.3 handshake in the random oracle model (ROM), but their security reduction incurs an 𝒪(q^6) loss, where q is the number of random oracle queries. Improving their security bounds was left as an open problem. To address this problem, Zhou et al. took the first step at Asiacrypt 2024, improving the loss factor to 𝒪(q^2) in the ROM and 𝒪(q^4) in the quantum ROM (QROM) for OW-CPA secure KEMs, and to 𝒪(q) (ROM) and 𝒪(q^2) (QROM) for IND-CPA secure KEMs. In this work, we advance the state-of-the-art by providing tighter security reductions for TLS 1.3 handshake based on CPA-secure KEMs. We introduce a new security notion, IND-1CCA-1MAC, and show that with a slight ciphertext expansion, the reduction losses can be significantly improved to 𝒪(q) (ROM) and 𝒪(q^2) (QROM) for OW-CPA secure KEMs, and to only 𝒪(1) in both models for IND-CPA secure KEMs. Moreover, we prove that without additional modifications such as ciphertext expansion, the loss of 𝒪(q) (ROM) and 𝒪(q^2) (QROM) is unavoidable. Finally, we analyze the security of TLS 1.3 from CPA-secure KEMs in the hybrid key exchange setting, and provide experimental evidence that ciphertext expansion is a practical trade-off for mitigating reduction losses.

Re-keying is one of the most effective techniques to protect symmetric ciphers against side-channel attacks. Since its introduction, numerous re-keying schemes have been developed. This paper proposes two new index-based re-keying schemes, HOP-1 and HOP-2, which are variants of Kocher's re-keying scheme. Specifically, through tree-based analysis of Kocher's re-keying scheme, we identify strategies to minimize key repetition in Kocher's approach. We compare HOP-1 and HOP-2 with existing re-keying schemes and evaluate their application to two leakage-resilient authenticated encryption schemes, highlighting their effectiveness relative to these other re-keying schemes.

Time-lock puzzles are unique cryptographic primitives that use computational complexity to keep information secret for some period of time, after which security expires. This topic, while over 25 years old, is still in a state where foundations are not well understood: For example, current analysis techniques of time-lock primitives provide no sound mechanism to build composed multi-party cryptographic protocols which use expiring security as a building block. Further, there are analyses that employ idealizations and simulators of unrealistic computational power to be an acceptable sound security argument. Our goal with this short paper is to advocate for understanding what approaches may lead to sound modeling beyond idealization, and what approaches may, in fact, be hopeless at this task of sound modeling. We explain in this paper how existing attempts at this subtle problem lack either composability, a fully consistent analysis, or functionality. The subtle flaws in the existing frameworks reduce to an impossibility result by Mahmoody et al., who showed that time-lock puzzles with super-polynomial gaps (between committer and solver) cannot be constructed from random oracles alone (or any repetitive computation where the next state is completely random given the prior state); yet still the analyses of algebraic puzzles today treat the solving process as if each step is a generic or random oracle. We point out that if the generation process relies on a trapdoor function that cannot be treated as a random oracle (to allow efficient generation while avoiding this impossibility result), then, to be consistent, the analysis of the solving process should also not treat such a trapdoor function (and its intermediate states) as a random oracle. We further prove that there are no time-lock puzzles schemes with efficient generation that are based only on random oracle or generic group operations. We also delineate additional issues with the proof techniques used for time-lock puzzles. Specifically, when a time-lock puzzle must retain privacy for some amount of time, the reduction should bound the running time of the simulator. It is not a valid security argument to give the simulator more running time than the adversary would require to solve a puzzle. We survey the adherence of various attempts to this principle, as well as the properties that different attempts achieve toward composition.

A commitment protocol enables a sender to fix a message without revealing or altering it until later, which is crucial for various cryptographic protocols and applications. However, traditional commitments risk coercion, where an entity, named dictator, can force premature opening or involuntary commitments. This coercive entity was initially explored in encryption by Persiano et al. at EUROCRYPT’22. This work investigates how to circumvent the dictator in commitment, considering the dictator’s enhanced ability to influence the sender by tampering with public parameters. We note that from inception to today, commitments were defined and employed under a passive adversary, and this work, therefore, conceptually demonstrates how the “anamorphic perspective” can enrich the relevant adversarial settings of basic primitives. Specifically, we formalize dictator’s capabilities for undermining commitments-ranging from weak to strong models-and establish reductions among them. To counter coercion, we propose “anamorphic commitment”, a novel primitive that allows the sender to embed a covert message within an innocent-looking commitment to alert to coercion or transmit secret information. We define the syntax, security models, and robustness of anamorphic commitment, and present two generic constructions with distinct performance advantages that can integrate seamlessly with the standard commitments.

Adversarial Missingness (AM) attacks aim to manipulate model fitting by carefully engineering a missing data problem to achieve a specific malicious objective. AM attacks are significantly different from prior data poisoning attacks in that no malicious data inserted and no data is maliciously perturbed. Current AM attacks are feasible only under the assumption that the modeler (victim) uses full-information maximum likelihood methods to handle missingness. This work aims to remedy this limitation of AM attacks; in the approach taken here, the adversary achieves their goal by solving a bi-level optimization problem to engineer the adversarial missingness mechanism, where the lower level problem incorporates a differentiable approximation of the targeted missingness remediation technique. As instantiations of this framework, AM attacks are provided for three popular techniques: (i) complete case analysis, (ii) mean imputation, and (iii) regression-based imputation for general empirical risk minimization (ERM) problems. Experiments on real-world data show that AM attacks are successful with modest levels of missingness (less than 20%). Furthermore, we show on the real-world Twins dataset that AM attacks can manipulate the estimated average treatment effect (ATE) as an instance of the general ERM problems: the adversary succeeds in not only reversing the sign, but also in substantially inflating the ATE values from a true value of -1.61% to a manipulated one as high as 10%. These experimental results hold when the ATE is calculated using multiple regression-based estimators with different architectures, even when the adversary is restricted to modifying only a subset of the training data. The goals of this work are to: (i) establish the vulnerability to AM attacks of a significantly wider class of missingness remediation strategies than established in prior work, and (ii) brings the AM threat model to the attention of the community, as there are currently no defense strategies for these attacks.

Recent advancements in post-Quantum secure signing have revitalized interest in one-time signatures, such as Lamport's, and their many signature extensions. Predominantly based on standard hash functions, these signatures avoid reliance on number theoretic assumptions. Existing methods utilize a commitment array, with de-commitment contingent on the hashed message's representation bits. State-of-the-art variants incorporate pseudorandom functions. This study introduces a novel method utilizing a probabilistic ''set membership data structure" derived from hash functions. It involves accessing a long array with $k$ independent hash functions for each message, analogous to Bloom filters. This stateless signature scheme is adjustable to accommodate any pre-set maximum number of signatures by modulating the array's length. The key concept is the partial loading of the de-committed array, ensuring validation of signed messages, non-validation of unsigned messages, and signature unforgeability (forgery equates to decommitment without the private key). This approach extends to improving one-time or bounded-message Constructions, like the Naor-Yung extension, for regular signature applications in the new Hash-Based Stateless Signature (HBSS) scheme.

Git services such as GitHub, have been widely used to manage projects and enable collaborations among multiple entities. Just as in messaging and cloud storage, where end-to-end security has been gaining increased attention, such a level of security is also demanded for Git services. Content in the repositories (and the data/code supply-chain facilitated by Git services) could be highly valuable, whereas the threat of system breaches has become routine nowadays. However, existing studies of Git security to date (mostly open source projects) suffer in two ways: they provide only very weak security, and they have a large overhead. In this paper, we initiate the needed study of efficient end-to-end encrypted Git services. Specifically, we formally define the syntax and critical security properties, and then propose two constructions that provably meet those properties. Moreover, our constructions have the important property of platform-compatibility: They are compatible with current Git servers and reserve all basic Git operations, thus can be directly tested and deployed on top of existing platforms. Furthermore, the overhead we achieve is only proportional to the actual difference caused by each edit, instead of the whole file (or even the whole repository) as is the case with existing works. We implemented both constructions and tested them directly on several public GitHub repositories. Our evaluations show (1) the effectiveness of platform-compatibility, and (2) the significant efficiency improvement we got (while provably providing much stronger security than prior ad-hoc treatments).

Rekeying is an effective technique for protecting symmetric ciphers against side-channel and key-search attacks. Since its introduction, numerous rekeying schemes have been developed [40]. We introduce Post-Quantum Stateless Auditable Rekeying (PQ-STAR), a novel post-quantum secure [12] stateless rekeying scheme with audit support. PQ-STAR is presented in three variants of increasing security guarantees: (i) Plain PQ-STAR lets an authorized auditor decrypt and verify selected ciphertexts; (ii) Commitment-based PQ-STAR with the additional binding guarantee from the commitments, preventing a malicious sender from potentially claiming a random or wrong session key. (iii) Zero-knowledge PQ-STAR equips each session key with a signature-based zero-knowledge proof (ZKP), which proves that the session key was derived honestly, without ever revealing the secret preimage. We provide informal arguments that all variants achieve key-uniqueness, index-hiding, and forward-secrecy, even if a probabilistic polynomial-time (PPT) adversary arbitrarily learns many past session keys. PQ-STAR provides a verified, stateless, and audit-capable rekeying primitive that can be seamlessly integrated as a post-quantum upgrade for existing symmetric-key infrastructures.

The central security issue of outsourced 3D printing (aka AM: Additive Manufacturing), an industry that is expected to dominate manufacturing, is the protection of the digital design (containing the designers' model, which is their intellectual property) shared with the manufacturer. Here, we show, for the first time, that side-channel attacks are, in fact, a concrete serious threat to existing industrial grade 3D printers, enabling the reconstruction of the model printed (regardless of employing ways to directly conceal the design, e.g. by encrypting it in transit and before loading it into the printer). Previously, such attacks were demonstrated only on fairly simple FDM desktop 3D printers, which play a negligible role in manufacturing of valuable designs. We focus on the Powder Bed Fusion (PBF) AM process, which is popular for manufacturing net-shaped parts with both polymers and metals. We demonstrate how its individual actuators can be instrumented for the collection of power side-channel information during the printing process. We then present our approach to reconstruct the 3D printed model solely from the collected power side-channel data. Further, inspired by Differential Power Analysis, we developed a method to improve the quality of the reconstruction based on multiple traces. We tested our approach on two design models with different degrees of complexity. For different models, we achieved as high as 90.29 % of True Positives and as low as 7.02 % and 9.71 % of False Positives and False Negatives by voxel-based volumetric comparison between reconstructed and original designs. The lesson learned from our attack is that the security of design files cannot solely rely on protecting the files themselves in an industrial environment, but must instead also rely on assuring no leakage of power, noise and similar signals to potential eavesdroppers in the printer's vicinity.

Rekeying is an effective technique for protecting symmetric ciphers against side-channel and key-search attacks. Since its introduction, numerous rekeying schemes have been developed [16]. We introduce Post-Quantum Stateless Auditable Rekeying (PQ-STAR), a novel post-quantum secure [4] stateless rekeying scheme with audit support. PQ-STAR is presented in three variants of increasing security guarantees: (i) Plain PQ-STAR lets an authorized auditor decrypt and verify selected ciphertexts; (ii) Commitment-based PQ-STAR with the additional binding guarantee from the commitments, preventing a malicious sender from potentially claiming a random or wrong session key. (iii) Zero-knowledge PQ-STAR equips each session key with a signature-based zero-knowledge proof (ZKP), which proves that the session key was derived honestly, without ever revealing the secret preimage. We provide informal arguments that all variants achieve key-uniqueness, index-hiding, and forward-secrecy, even if a probabilistic polynomial-time (PPT) adversary arbitrarily learns many past session keys. PQ-STAR provides a verified, stateless, and audit-capable rekeying primitive that can be seamlessly integrated as a post-quantum upgrade for existing symmetric-key infrastructures.

Off-chain blockchain contracts are useful for parties who wish to keep the details of their contract's business logic private. The ad hoc installation of blockchain instances on mutually agreed-upon servers forms a reliable infrastructure for such contracts. We detail the core post-quantum algorithmic ingredients for structuring ad hoc blockchain (AHB) infrastructure and contracts. The initialization of AHB involves agreement on servers and blockchain software, such as Ethereum or the post-quantum SodsBC versions [13]. These choices, in turn, are approved by the digital signatures of the contract binding parties. A contract in AHB is executed by an MPC-based zero-knowledge proof of a hash (SHA) preimage commitment. Our efficient implementation extends zero-knowledge proofs based on MPC-in-theHead (or MitH, in short) to the case of distributed verified signatures with a global zero-knowledge verification of knowledge of secret shared preimage. AHB guarantees that the preimage is released when a group with a predefined number of participants running the AHB agrees that the off-chain contract conditions are met. A new key aspect of AHB is the opportunity to eliminate all but the current state of the MPC history while executing the MPC, as we can verify that the result is identical to the declared SHA value of the preimage. Although AHB is a multi-round MPC, it transfers and stores less data than MitH. Furthermore, it maintains the same level of privacy, as participants are exposed solely to provably random information.

Subversion-resilient cryptography has garnered increasing attention in recent years due to growing concerns about cryptographic subversions in real-world applications. Among the existing countermeasures, the notion of cryptographic reverse firewalls (RFs), initially proposed by Mironov and Stephens-Davidowitz (EUROCRYPT 2015) and later extended by Chakraborty et al. (EUROCRYPT 2022) to the universally composable (UC) model, has proven to be a powerful tool for building subversion-resilient cryptographic protocols. In this work, we focus on designing subversion-resilient authenticated key exchange (AKE) protocols, which are critical components of secure Internet communication. We present the first generic framework for subversion-resilient UC-secure AKE protocols leveraging RFs. Inspired by the state-of-the-art advancements by Chakraborty et al. (ASIACRYPT 2024), we address subversions: where a party’s implementation is covertly altered to exfiltrate secrets or behave unpredictably when triggered by adversarial inputs. A key contribution of our work is the introduction of a new AKE functionality which, for the first time, incorporates security against key control, an essential aspect of achieving subversion resilience. We also provide a concrete instantiation of our framework, demonstrating its feasibility in practice. Notably, the RFs in our proposed AKE protocol are transparent, an important property of RF as defined originally, which allows deployment of RF without all parties explicitly knowing about it and allows robust security. Achieving transparency for RFs has been widely regarded as challenging, particularly when addressing broader subversion attacks (e.g., input-trigger attacks) in the UC model. Our approach, thus, not only advances the state of AKE protocol design, but also offers insights into building other subversion-resilient protocols in the UC model using transparent RFs.

To date, Anamorphic Cryptography [EC22] has been developed to support adding an anamorphic message within a ciphertext carrying a primary message. The anamorphic message remains hidden even in the presence of a strong adversary that possesses the receiver’s key and/or determined the sent primary message. In this paper, we expand one-to-one encrypted anamorphic communication to one-to-many anamorphism, naturally assuming communication over a broadcast channel. What we show is that using a previously designed public-key encryption scheme, two things can happen: First, the receiver of an added hidden message may be a party different from the actual receiver (i.e., a shadow party) who has initially collaborated with the sender. Secondly, and perhaps more surprisingly, the receiving party need not be a singleton, and can be a number of different shadow (i.e., anonymous) groups, each receiving a different anamorphic message, where all these messages are extracted from a single one-receiver ciphertext. The idea of having multiple hidden channels to different shadow groups is highly handy if, for example, the anamorphic messages are warnings with operational instructions, sent to the groups and will be received by a group even if the adversary is able to temporarily cut off all but one members of a channel. More specifically,

The cloud has become pervasive, and we ask: how can we protect cloud data against the cloud itself? For secure user-to-user communication via a cloud server, End-to-End encryption has been formally studied, building on existing TLS channels without requiring new primitives. However, enabling user-to-same-user secure outsourced data storage - solving the analogous problem of “privacy from the server” while (1) relying on existing infrastructure and (2) supporting user mobility, remains open. Existing proposals, like password-protected secret sharing, target the same goal but are incompatible with existing cloud storage services. Specifically, they lack the simplicity needed to directly utilize existing cloud storage without requiring changes on the cloud side. Here, we propose a novel system for securely storing private data in existing cloud storage with the help of a key server (necessary given the requirements). In our system, user data is secure against threats from the cloud server, the key server, and illegitimate users. Only the legitimate user can access the data on any device using a correct passphrase. Most importantly, our system does not require the storage server to support any newly programmable operations. Moreover, leveraging the existing App login, our system requires only one passphrase, which never leaves the user’s device and remains hidden from both servers. The security is proved under formal models, and its efficiency is demonstrated by experiments conducted on Amazon S3. Notably, a preliminary variant, based on our principles, was deployed by Snapchat in their My Eyes Only module, serving hundreds of millions of users!