Group signatures are an important cryptographic primitive providing both anonymity and accountability to signatures. Accountable ring signatures (ARS) combine features from ring signatures (RS) and group signatures (GS), and can be directly transformed into either of both. While there exist extensive works on constructing GS from various post-quantum assumptions, there has not been any research using isogeny-based assumptions. In this work, we propose the first isogeny-based GS, which is a direct result of our isogeny-based ARS. Our schemes are based on the decisional CSIDH assumption (D-CSIDH) and are proven secure under the quantum random oracle model (QROM). This work is also the first post-quantum ARS and GS that are proven QROM-secure.
Group signatures are an important cryptographic primitive providing both anonymity and accountability to signatures. Accountable ring signatures (ARS) combine features from ring signatures (RS) and group signatures (GS), and can be directly transformed into either of both. While there exist extensive works on constructing GS from various post-quantum assumptions, there has not been any research using isogeny-based assumptions. In this work, we propose the first isogeny-based GS, which is a direct result of our isogeny-based ARS. Our schemes are based on the decisional CSIDH assumption (D-CSIDH) and are proven secure under the quantum random oracle model (QROM). This work is also the first post-quantum ARS and GS that are proven QROM-secure.
Abstract With the rapid growth of online users, protecting user privacy in access control scenarios has become a critical challenge in the field of information security. Group signatures serve as a fundamental cryptographic primitive that enables users to sign on behalf of a group, providing anonymity while supporting traceability of signers. However, traditional group signature schemes, which rely on number-theoretic assumptions vulnerable to quantum algorithms, face significant security threats in the advent of quantum computing. In this paper, we propose a module lattice-based group signature scheme (ML-GS). ML-GS leverages FIPS 204 standard, a recently standardized signature scheme by the National Institute of Standards Technology (NIST), and integrates a dualrejection-sampling signature mechanism with the K-PKE encryption scheme from FIPS 203 standard, forming a “sign-hybrid-encrypt” hybrid structure to ensure both efficiency and traceability. In the key generation phase, we introduce a module Gaussian preimage sampling algorithm that reduces public key size and supports dynamic user enrollment. The security of ML-GS is formally proven in the random oracle model under the Module Learning with Error (MLWE) and Module Short Integer Solution (MSIS) assumptions. Experimental results demonstrate that, compared to existing scheme with equivalent security level, the ML-GS scheme achieves significant improvements in both time and storage overhead.
Advances in quantum computing have heightened concerns about information security in the post-quantum era. While traditional cryptographic schemes rooted in number- theoretic assumptions (e.g., integer factorization, elliptic curve discrete logarithm, and discrete logarithm problems) remain foundational to modern security infrastructures, their vulnerabilities in quantum computing environments have become apparent. To address this, researchers have shifted focus toward quantum-resistant mathematical problems. Among these, isogeny-based cryptography has emerged as a strong candi- date within the five primary post-quantum cryptographic approaches, garnering sig- nificant academic interest. Recent progress in isogeny-based cryptographic schemes includes SIDH (though recent cryptanalysis by Castryck and Decru has compromised its security), CSIDH, and SQISign. Despite progress, the inherent structural characteristics of isogenies impose limitations on design flexibility compared to other post-quantum alternatives. The goal of the thesis is to develop isogeny-based cryptographic protocols with ad- vanced functionalities. We develop isogeny-based blind signatures based on techniques from CSIDH, CSI-FiSh, and other works. We also develop ring signature schemes us- ing SQISign. Finally, we develop a verifiable encryption scheme (based on CSIDH) and bit commitment scheme (based on the Charles–Goren–Lauter isogeny-based hash function). For both schemes, we give ways to prove to a verifier that the encrypted/ committed bits satisfy a given Boolean equation.
Ring signatures, a novel cryptographic primitive, were introduced by Rivest, Shamir, and Tauman at ASIACRYPT 2001. It allows a member of a group to sign a message on behalf of an ad-hoc group of users, known as a ring, without revealing its identity. Ring signatures, initially introduced for applications such as whistle-blowing and confidential disclosures, have since gained significant relevance and widespread adoption across various domains, including blockchain technologies, ad-hoc networks, anonymous transactions, location-based services, cloud computing platforms, and outsourced computation. However, traditional ring signatures are vulnerable if the signers’ keys are compromised. To resolve this vulnerability, Liu and Wong introduced the notion of forward-secure ring signatures at IJNS 2008. Forward-security ensures that even if a signer’s secret key is compromised, signatures generated in prior time periods remain secure. It significantly reduces the damage caused by secret key exposure, making it crucial for long-term security in applications such as whistle-blowing or anonymous authentication. However, most existing forward-secure ring signature schemes are based on number-theoretic assumptions, making them vulnerable to quantum adversaries. Yu and Wang proposed a quantum-safe forward-secure ring signature scheme at ICICS 2023, and Yu et al. further extended this work in Computer Networks (2025), retaining the same ring signature scheme with forward security. But the scheme suffers from large signatures that grow linearly with the ring size. In this paper, we propose a lattice-based forward-secure ring signature scheme in the quantum random oracle model ( QROM ) that simultaneously achieves security against quantum adversaries and compact signature size. Notably, the signature size is logarithmic in the number of members in the ring, enabling significantly better scalability compared to prior constructions.
Blind signatures are fundamental cryptographic primitives enabling privacy-preserving authentication and have seen renewed interest in the post-quantum literature. Existing efficient constructions predominantly rely on Fischlin’s generic paradigm instantiated over lattice assumptions, while blinding techniques for Σ-protocol-based blind signatures remain sparse beyond lattices. Moreover, achieving provable concurrent security under polynomially many sessions has been a longstanding open challenge for this approach in the post-quantum literature as evidenced by the recent attacks in EC’24 and PKC’24. This work broadens the landscape of post-quantum blind signatures by introducing novel techniques and proposing four frameworks based on general cryptographic group actions, without requiring commutativity. Our constructions admit instantiations under diverse post-quantum assumptions, including CSIDH (isogeny-based), LESS (code-based, NIST round-two), and more. These frameworks offer flexible trade-offs in assumptions (from interactive one-more to the standard inversion problem) and key/signature sizes, and culminate in a construction that achieves security under polynomially many concurrent sessions. This enables the first efficient blind signatures from isogenies and codes with provable concurrent security with 4.5 and 64.7 KB respectively. We also outline several directions for optimization and further instantiations for future work.
Traceable signatures (Kiayas et al., 2004) is an anonymous digital signature system that extends the tracing power of opening authority in group signatures. There are many known constructions of traceable signatures but all are based on number-theoretical/pairing assumptions. For such reason, they may not be secure in the presence of quantum computers. This work revisits the notion of traceable signatures and presents a lattice-based construction provably secure in the quantum random oracle model (QROM).
The potential development of large-scale quantum computers has raised serious concerns about the security of classical cryptographic systems.In response, the field of post-quantum cryptography has emerged to design cryptographic schemes that remain secure even against quantum adversaries.Isogeny-based cryptography is the most recent addition to post-quantum cryptography and started garnering attention in the 2010s.Digital signature schemes are a fundamental part of public key cryptography, providing authenticity, integrity, and non-repudiation for digital messages.They are essential for secure communication, software updates, electronic transactions, and virtually all systems requiring trust and verification.Although the traditional digital signature scheme consists only of a single prover and verifier, numerous variants have been developed over the years to address specific needs, such as multi-signatures, ring signatures, group signatures, and proxy signatures.These variants extend the functionality of traditional signatures to support more complex security models in real-world applications, such as contact tracing.As isogeny-based cryptography is a relatively young research field, many commonly used cryptographic primitives, such as variants of signature schemes, have not yet been fully developed or widely implemented.This leaves room for exploration and optimization in designing secure and efficient isogeny-based alternatives, while taking advantage of its strength with its small keys and signature compared to other post-quantum based constructions such as lattice.Multi-signature schemes have been extensively studied for decades and form a well-established area of research in cryptography.These schemes enable a group of users to collaboratively generate a single, compact signature on a shared message, ensuring that all designated parties have approved the content.Their efficiency and security properties make them particularly valuable in applications such as blockchain transactions and distributed systems.Active security in multi-signature schemes requires the signature to be unforgeable even in the worst-case scenario, where malicious signers would collude to generate a multi-signature in a group with a single honest signer.Our first contribution is to propose an actively secure isogeny-based multi-signature scheme.Proxy signature schemes have been the subject of extensive research over the years, offering cryptographic solutions for secure delegation of signing authority.These schemes allows an original signer to designate a proxy signer to generate signatures on their behalf, within the limitations imposed by the original signer.Such mechanisms are particularly useful in settings like secure communications, delegated access control, and corporate approval systems where it is essential for operations to continue even in the absence of the designated signer.Proxy I would like to express my deepest gratitude to everyone who has supported me throughout my Ph.D. journey.First and foremost, I am deeply grateful to my supervisor, Professor Atsuko Miyaji, for giving me a chance to experience the field of research as an exchange student before accepting me once again as a Ph.D student.Her invaluable support and guidance throughout the course of those 3 years as a Ph.D student enabled me to make it this far.Additionally, I am deeply grateful to the NEC Corporation for its C&C Fellowship, as well as the Jisedai Program.Their generous support eased my financial burdens, allowing to focus completely on my research.I am also deeply grateful to the professors, staff, and colleagues at Miyaji Lab, who were always there whenever I needed it.I want to especially thank Hide Miyaji, for welcoming me with open arms as soon as I arrived for the first time in Japan and supporting me throughout my Ph.D journey.I am also profoundly grateful to Mohamed Bourefis, my French comrade, who shares a very similar