Zero-Knowledge Proof of Progress: Secure Multi-Phase Capture-the-Flag Competitions

Threshold Authorization Without Threshold Signatures: Signature-Agnostic MPC Custody

Digital-asset custody has been built on threshold multi-party approval: no operation proceeds unless $t$ of $n$ parties approve, and fewer than t compromised parties can neither authorize nor learn the authorization secret. Threshold signature schemes (TSS) have been the standard mechanism, but the post-quantum transition disrupts this model: standardized hash-based signatures resist efficient threshold signing, and lattice-based threshold protocols remain an emerging research track. We present a dual-gate architecture that separates member authentication from threshold authorization. Each member signs its approval with an ordinary signature under any EUF-CMA scheme; the quorum jointly produces a threshold seal from Shamir-shared secrets bound to the operation. The seal is the base instance of a programmable authorization computation: simple quorum is the minimal policy, while richer policies can evaluate secret-shared state without making the member-signature scheme part of that computation. The signature scheme is a deployment parameter: migrating from ECDSA to SLH-DSA or ML-DSA is a key rotation, not a protocol redesign, and members holding keys in commodity HSMs participate through the standard sign API. The architecture can be deployed wherever the asset-control path supports programmable verification, such as smart contracts, vault modules, or HSMs guarding a master key, and produces an enforcement-layer authorization rather than a native chain signature. Below-threshold secrecy is information-theoretic; an adversary holding $\geq t$ signing keys but no coefficient shares still cannot produce the seal.

Optimizing and Implementing Threshold MAYO

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 (PQCrypto 2025), 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.

Subtitle: A FROST-based Threshold Schnorr Signature Scheme with Identifiable Aborts and Fully Adaptive Security from AOMDL

AcadVault: A Privacy-First Blockchain Credential Framework Achieving O(1) Batch Issuance via Merkle Tree and SSS-based Distributed Key Recovery

A national audit that revealed 10,000 fake credentials among 400,035 public employees demonstrates the severity of the global threat posed by academic credential fraud. Although blockchain-based credential systems provide a promising countermeasure, existing architectures suffer from four persistent shortcomings: (1) credentials stored in plaintext on IPFS expose personally identifiable information (PII) to anyone who obtains the content identifier (CID); (2) batch issuance entails N independent transactions with O(N) cumulative gas scaling; (3) single-point-of-failure master-key management leaves encrypted credentials vulnerable to loss of keys; and (4) verification events are not logged on-chain, precluding lifecycle analytics. In this paper, we present AcadVault, a privacy-preserving decentralized application that addresses these limitations with four contributions: (1) Hierarchical Deterministic (HD) subkeys for ChaCha20-Poly1305 AEAD encryption provide per-credential confidentiality and integrity for all credentials stored on IPFS with zero centralized key storage; (2) a sorted-pair Merkle tree protocol performs O(N) client-side leaf hashing and commits all N credential hashes in a single O(1) on-chain transaction, achieving 99.88% per-credential gas reduction at N=1,000; (3) Shamir’s Secret Sharing (t,n)-threshold scheme over GF(2^8) distributes decryption key custody among institutional guardians using information-theoretically secure shares; and (4) seven indexed on-chain event types provide immutable lifecycle tracking for issuance, revocation, verification, and key recovery. Validated on the Ethereum Sepolia testnet, AcadVault shows 62–77% gas saving per single operation and 99.88% batch saving at N=1,000 via O(1) on-chain Merkle anchoring, offering scalable and cost-effective grounds for institutional adoption.

Verifiable and Fair Registered Attribute-Based Multi-Hop Proxy Re-Encryption Scheme for LLM Agents

As Large Language Model (LLM) agents emerge as intelligent coordinators, they intensify the demand for secure sharing and forwarding of sensitive data in data-driven systems. Existing Attribute-Based Proxy Re-Encryption (ABPRE) offers fine-grained access control and secure data forwarding, but suffers from the key escrow problem and lacks efficient verifiability and fairness guarantees. In this paper, we propose the first Verifiable and Fair Registered ABPRE (VF-RABPRE) scheme to eliminate reliance on trusted authorities and support multi-hop re-encryption for flexible multi-agent data sharing. To ensure efficient verifiability and fairness, we combine a lightweight verifiable tag and a non-interactive zero-knowledge proof to detect misbehavior of the proxy and prevent false accusations. As a trade-off between security and overhead, we design a more secure fairness mechanism that does not reveal plaintext using zero-knowledge Succinct Non-interactive Arguments of Knowledge (zkSNARK). Additionally, we extend VF-RABPRE with dynamic user registration and outsourced decryption, supporting flexible registration and efficient decryption for data users. Finally, we formally prove the security of our scheme and implement a prototype to evaluate its performance. Experimental results demonstrate that VF-RABPRE outperforms state-of-the-art ABPRE schemes and achieves practical efficiency, making it well-suited for secure data sharing in scenarios empowered by LLM agents.

KORD: Breaking the Key-Generation Bottleneck in Dealerless FSS via Protocol--Hardware Co-Design

Function secret sharing (FSS) has become a core primitive in privacy-preserving computation. However, each FSS invocation requires a fresh pair of function keys generated by a trusted dealer , expands the system's trust boundary and hinders practical deployment. Existing dealerless protocols eliminate this dependency, but incur substantial communication and a number of interaction rounds that grows linearly with the input bit-width, making key generation a major bottleneck. This paper present KORD, a protocol--hardware co-design that dramatically reduces the cost of dealerless FSS key generation. At its core is a pair of special-purpose chips that establish a common root of trust through mutual attestation and, within it, reconstruct FSS keys---eliminating the need for a dealer. This root of trust further forms a security boundary within which KORD restructures the generation protocol, collapsing the interaction of prior dealerless protocols into a single round, independent of GGM depth. A cross-key scheduling scheme then interleaves independent GGM-tree traversals, sustaining high computational throughput. KORD reduces per-key-generation communication by 7,633--70,274$\times$ over the state-of-the-art distributed FSS protocol across a comprehensive suite of FSS building blocks. Post-route analysis projects 12.75 million 32-bit DPF keys per second at 204 MHz using 21.5K LUTs, with 99.8% AES lane utilization. On private ResNet-18 inference, KORD cuts the share of end-to-end time spent on key generation from over 96% to 11.9%.