Blind signatures are a central tool for privacy-preserving protocols. They allow users to obtain signatures from a signer without the signer seeing the signed message. For instance, it enables electronic cash: signatures correspond to coins which can be issued by the bank in a privacy-preserving manner via blind signing. To mitigate the risk of key compromise, threshold blind signatures allow the distribution of the signing key amongst N parties. While recent works have focused on improving the security of this primitive in the classical setting, no construction is known to date in the post-quantum setting. We present the first construction of a threshold blind signature secure in the post-quantum setting, based on lattices. We prove its security under an interactive variant of the SIS assumption introduced in [Agrawal et al., CCS’22]. Our construction has a reasonable overhead of a factor of roughly 1.4 X to 2.5 X in signature size over comparable non-threshold blind signatures over lattices under heuristic but natural assumptions.

The advent of quantum computing poses a fundamental threat to classical cryptographic assumptions. While algorithms such as RSA and Elliptic-Curve Cryptography are secure against classical adversaries, they would be efficiently broken by a sufficiently powerful quantum adversary. Yet, despite rapid industrial and academic progress, the timeline for achieving a Cryptographically Relevant Quantum Computer (CRQC) remains uncertain and opaque. In this work, we propose a mechanism to monitor quantum capabilities through economic incentives. We introduce QCAP, a trustless distributed protocol for deploying a quantum canary address alert. QCAP enables the creation of publicly auditable cryptographic challenges whose solutions would reveal the existence of quantum computers capable of breaking the Elliptic Curve Discrete Logarithm problem. The protocol is decentralized, secure, efficient, and verifiable, featuring adjustable difficulty and native Bitcoin compatibility. A proof-of-concept implementation demonstrates the feasibility of QCAP as a Bitcoin-based early-warning system for the emergence of quantum computational power.

This paper systematically analyzes the security of the two-branch Unified Feistel Lai Massey (UFLM) structure with independent random round functions under chosen plaintext and chosen ciphertext attacks, focusing on its indistinguishability from a random permutation. UFLM uses an invertible linear layer represented as a $2 \times 2$ block matrix $\varphi$ with blocks $A_{11}, A_{12}, A_{21}, A_{22}$. Previously, Dai et al. proved that when $A_{12}$ is invertible, $4$-round UFLM achieves CCA security and resists up to $\mathcal{O}(2^{n/2})$ queries, where the UFLM input is $2n$ bits. Our work imposes no restriction on $A_{12}$. We determine the minimal number of rounds for UFLM to achieve CPA and CCA security, fully determined by the parameters $T(A_{12}^{\top}, A_{11}^{\top})$ and $T(A_{12}, A_{22})$. For UFLM with enough rounds to be secure, the query bound is primarily determined by the rank of $A_{12}$. For all UFLM with too few rounds to be secure, we present successful distinguishing attacks that require at most four queries. Our results rigorously show, for the first time, that when $A_{12}$ has full rank, UFLM requires the fewest rounds to achieve CPA and CCA security and attains the highest query bound. Nevertheless, when $A_{12}$ is not full rank, CPA and CCA security can still be achieved by increasing the number of rounds unless $T(A_{12}^{\top}, A_{11}^{\top}) = \infty$ or $T(A_{12}, A_{22}) = \infty$. At last, for involutory $\varphi$, we find UFLM achieves CPA and CCA security if and only if $A_{12}$ has full rank.

The security of lattice-based cryptography relies critically on the concrete hardness of the approximate shortest vector problem (Approx-SVP). For cryptographic-sized instances, existing Approx-SVP rank reduction conditions may be overly aggressive, as they implicitly assume access to a large number of extremely short lattice vectors. In this work, we systematize and refine Approx-SVP rank reduction conditions from a feasibility perspective. We identify that, in the context of the dimension-for-free (D4f) technique, the existence of a single sufficiently short vector is the essential requirement, and we derive two refined and compact rank reduction conditions accordingly. The first condition is based on geometric properties of lattice sieving, while the second incorporates a basis-quality-dependent probabilistic bound. These results are validated through extensive experiments on high-dimensional lattices, where the compact condition outperforms prior methods by up to a factor of $60$ in dimensions $850$ and $925$. To reliably realize these conditions in high dimensions, we present APBKZ, an adaptive Pump-based lattice reduction strategy that dynamically selects the blocksize and D4f parameters according to the evolving Gram-Schmidt profile. We further introduce HeadAPBKZ, a head-focused execution mode that restricts reduction to a critical prefix once the rank reduction condition is satisfied. Combining these advances, we develop an improved concrete security estimation framework for the MSIS problem. Applied to Dilithium, our analysis indicates that, when compact rank reduction behavior is integrated with the D4f technique, the estimated concrete security of Dilithium is reduced by 3.65-6.09 bits relative to the conservative Core-SVP baseline, providing a more realistic concrete assessment. We also derive an analytic approximation whose closed-form estimates remain close to the numerical estimation, with discrepancies of only about 1.1 bits.

Reed–Solomon (RS) codes underpin a wide range of cryptographic protocols, from verifiable secret sharing (VSS) to blockchain data availability (DA). An RS code commitment enables a prover to distribute codeword fragments among many parties while allowing each recipient to verify that its fragment is consistent with a RS code. Existing constructions either rely on homomorphic polynomial commitments, which incur redundant commitments and expensive group operations, or use FRI-based interactive proofs, which achieve fast proving but suffer from prohibitively large communication. We present UltraProofs, a new RS code commitment framework that achieves linear-time proof generation while remaining compatible with any multilinear polynomial commitment. Our key technical contribution is an evaluation-consolidation protocol that reduces $n$ evaluation proofs at distinct RS points to a single randomized evaluation, eliminating redundant commitments and removing the need for homomorphic structure. We further design a tailored multilinear PCS, LightLigero, which achieves $O(\lambda \log n)$ proof size and maintains $O(n \log n)$ prover time. UltraProofs attains asymptotically optimal prover complexity with concrete speedups in practice: in VSS, it reduces prover time by $2.4\times$ and proof size by $4\times$ compared to HydraProofs (S&P'25); in DA, it cuts per-node communication by up to $2{\sim}5\times$ relative to FRIDA (Crypto'24) while retaining similar prover cost. By decoupling verifiable RS encoding from any specific PCS instantiation, UltraProofs provides a flexible, efficient, and modular foundation for large-scale verifiable storage and distributed cryptographic systems.

HiAE is a well-designed AEAD scheme using AES round functions, delivering outstanding performance on both ARM and x86 processors. Some existing AEAD schemes such as Rocca (ToSC 2021) have been shown to be vulnerable when attackers can repeatedly query the decryption oracle with forged ciphertexts and random tags until a valid tag is accepted. Motivated by this, we use algebraic techniques to analyze the security of HiAE under the same setting. Firstly, we employ the meet-in-the-middle technique and guess-and-determine technique to recover the state and derive a key-related equation resulting from two layers of AES round functions. Secondly, by adopting an algebraic approach to study the properties of the round function, we decompose the equation into byte-level equations for divide-and-conquer. Finally, we utilize the guess-and-determine technique to recover the key. Collectively, these techniques enable us to present the full key-recovery attack on HiAE. Our attack achieves a data complexity of $2^{130}$ and a time complexity of approximately $2^{209}$, leveraging both encryption and decryption oracles with a success probability of 1. We emphasize that our attack considers a stronger scenario than HiAE's original security model, and thus does not invalidate its original security claims.

Threshold signatures distribute trust across multiple parties, eliminating single points of failure and reducing insider and key-exfiltration risks—properties that are increasingly important for high-assurance deployments and recently emphasized by NIST’s Multi-Party Threshold Cryptography (MPTC) initiative. We present a practical t-out-of-n threshold variant and emulation of MAYO, a post-quantum signature candidate to NIST’s call for additional signatures. Our proposal builds upon the threshold MAYO design of Celi, Escudero and Niot (PQCrypto2025), which we significantly refine to achieve practical performance. To this end, we introduce two algorithmic modifications to MAYO tailored for the distributed setting: (1) Explicit-Salt MAYO, which allows for pre-determined salts to enable a single-round online phase; and (2) Depth-Reduced MAYO, which restructures the signing algorithm to minimize the depth of secret-dependent operations. We then propose a unified protocol framework that integrate these techniques, plus other MPC specific optimizations, with the goal of minimizing online latency. Finally, we provide a concrete instantiation and local emulation in the dishonest majority setting, secure against active adversaries. Our emulation shows that threshold signing is practical at typical threshold sizes and amenable to deployment. By releasing an open-source implementation and reporting end-to-end performance, this work offers a concrete reference for the thresholdization of post-quantum signatures. Clearly the aforementioned framework is not limited to MAYO, and can be applied to the UOV family of signatures more generally.