Intro.

I'm a lead cryptography researcher at PQShield GK, Japan, and also a collaborative researcher at the National Institute of Advanced Industrial Science and Technology (AIST), Japan. Before that, I was a senior cryptography researcher at AIST, Japan and a research consultant at PQShield Ltd, U.K.

My current area of interest is the intersection of theoretical and practical cryptography. Specifically, within Post-Quantum Cryptography (PQC), I focus on overcoming the numerous obstacles in turning efficient classically-secure schemes into PQ-secure ones. I enjoy constructing and providing security analysis of cryptographic primitives/protocols, using techniques tailored to PQC.
Some of my more recent research has focused on secure messaging protocols (e.g., Signal, MLS), threshold cryptography (e.g., Threshold Raccoon), and key exchanges (e.g., PQWireGuard).  See this and this blog posts where our Triple Ratchet Protocol was deployed by Signal. 

The PQShield R&D team is not always hiring permanent researchers, but we welcome applications for internships, PhD students, and post-docs. Feel free to contact us with your inquiry.

Contact: shuichi.katsumata [at] pqshield.com OR shuichi.katsumata000 [at] gmail.com

Preprint

• • Collisions in Supersingular Isogeny Graphs and the SIDH-based Identification Protocol. Wissam Ghantous, Shuichi Katsumata, Federico Pintore, Mattia Veroni. 

ePrint. 

International Conference

2026

• • Revisiting PQ WireGuard: A Comprehensive Security Analysis With a New Design Using Reinforced KEMs. Keitaro Hashimoto, Shuichi Katsumata, Guilhem Niot, Thom Wiggers.

IEEE S&P 2026. ePrint. 

2025

• • Unmasking TRaccoon: A Lattice-Based Threshold Signature with An Efficient Identifiable Abort Protocol. Rafael del Pino, Shuichi Katsumata, Guilhem Niot, Michael Reichle, Kaoru Takemure. 

CRYPTO 2025. ePrint. 

• • Triple Ratchet: A Bandwidth Efficient Hybrid-Secure Signal Protocol. Yevgeniy Dodis, Daniel Jost, Shuichi Katsumata, Thomas Prest, Rolfe Schmidt.

EUROCRYPT 2025. ePrint. 

• • How to Compare Two-Party Secure Messaging Protocols: A Quest for A More Efficient and Secure Post-Quantum Protocol. Benedikt Auerbach, Yevgeniy Dodis, Daniel Jost, Shuichi Katsumata, Rolfe Schmidt.

USENIX 2025. To Appear.

• • Bundled Authenticated Key Exchange: A Concrete Treatment of (Post-Quantum) Signal's Handshake Protocol.  Keitaro Hashimoto, Shuichi Katsumata, Thom Wiggers.

USENIX 2025. ePrint.

• Comprehensive Deniability Analysis of Signal Handshake Protocols: X3DH, PQXDH to Fully Post-Quantum with Deniable Ring Signatures. Shuichi Katsumata, Guilhem Niot, Ida Tucker, Thom Wiggers.

USENIX 2025. ePrint.

• Exploring How to Authenticate Application Messages in MLS: More  Efficient, Post-Quantum, and Anonymous Blocklistable. Keitaro Hashimoto, Shuichi Katsumata, Guillermo Pascual-Perez.

USENIX 2025. ePrint.

• • The Triple Ratchet Protocol: A Bandwidth Efficient  Hybrid-Secure Signal Protocol. Benedikt Auerbach, Yevgeniy Dodis, Daniel Jost, Shuichi Katsumata, Thomas Prest, Rolfe Schmidt.

Real World Crypto 2025. 

2024

• • Tighter Adaptive IBEs and VRFs: Revisiting Waters’ Artificial Abort. Goichiro Hanaoka, Shuichi Katsumata, Kei Kimura, Kaoru Takemure, Shota Yamada. 

TCC 2024. ePrint. 

• • Two-Round Threshold Signature from Algebraic One-More Learning with Errors. Thomas Espitau, Shuichi Katsumata, Kaoru Takemure. 

CRYPTO 2024. ePrint. 

• • Adaptively Secure 5 Round Threshold Signatures from MLWE/MSIS and DL with Rewinding. Shuichi Katsumata, Kaoru Takemure, Michael Reichle. 

CRYPTO 2024. ePrint. 

• Raccoon: A Masking-Friendly Signature Proven in the Probing Model. Rafael del Pino, Shuichi Katsumata, Thomas Prest, Mélissa Rossi.

CRYPTO 2024. ePrint.

• • Threshold Raccoon:  Practical Threshold Signatures from Standard Lattice Assumptions. Rafael del Pino, Shuichi Katsumata, Mary Maller, Fabrice Mouhartem, Thomas Prest, Markku-Juhani O. Saarinen.

EUROCRYPT 2024. ePrint. slides. 

• • More Efficient Protocols for Post-Quantum Secure Messaging.  Keitaro Hashimoto, Shuichi Katsumata, Eamonn W. Postlethwaite, Thomas Prest, Bas Westerbaan.

Real World Crypto 2024. (Contents from [KKPP20@ASIACRYPT] ePrint. + [HKKPW21@CCS] ePrint.)

• • Breaking Parallel ROS: Implication for Isogeny and Lattice-based Blind Signatures. Shuichi Katsumata, Yi-Fu Lai, Michael Reichle. 

PKC 2024. ePrint. 

• • Anonymous Reputation Systems with Revocation, Revisited. Ryuya Hayashi, Shuichi Katsumata, Yusuke Sakai.

FC 2024. Preproceeding.

2023

• • Practical Round-Optimal Blind Signatures in the ROM from Standard Assumptions. Shuichi Katsumata, Michael Reichle, Yusuke Sakai.

ASIACRYPT 2023. ePrint.  

• • CSI-Otter: Isogeny-based (Partially) Blind Signatures from the Class Group Action with a Twist. Shuichi Katsumata, Yi-Fu Lai, Jason T. LeGrow, Ling Qin.

CRYPTO 2023. ePrint. video.

• • Signature for Objects: Formalizing How to Authenticate Physical Data and More.  Ryuya Hayashi, Taiki Asano, Junichiro Hayata, Takahiro Matsuda, Shota Yamada, Shuichi Katsumata, Yusuke Sakai, Tadanori Teruya, Jacob Schuldt, Nuttapong Attrapadung, Goichiro Hanaoka, Kanta Matsuura, Tsutomu Matsumoto.

FC 2023. Proceeding. 

• • Metadata Protection for MLS and Its Variants.  Keitaro Hashimoto, Shuichi Katsumata, Thomas Prest.

Real World Crypto 2023. video. (Contents from [HKP22@CCS]. ePrint.)

2022

• • How to Hide MetaData in MLS-Like Secure Group Messaging: Simple, Modular, and Post-Quantum.  Keitaro Hashimoto, Shuichi Katsumata, Thomas Prest.

ACM CCS 2022. ePrint. 

• • A New Framework For More Efficient Round-Optimal Lattice-Based (Partially) Blind Signature via Trapdoor Sampling. Rafael del Pino, Shuichi Katsumata. 

CRYPTO 2022. ePrint.

• • Group Signatures and More from Isogenies and Lattices: Generic, Simple, and Efficient. Ward Beullens, Samuel Dobson, Shuichi Katsumata, Yi-Fu Lai, Federico Pintore. 

EUROCRYPT 2022. ePrint. 

2021

• • Statistical ZAPs from Group-Based Assumptions. Geoffroy Couteau, Shuichi Katsumata, Elahe Sadeghi, Bogdan Ursu.
    
    TCC 2021. ePrint. video (by Elahe Sadeghi)

  • A New Simple Technique to Bootstrap Various Lattice Zero-Knowledge Proofs to QROM Secure NIZKs. Shuichi Katsumata.
    
    CRYPTO 2021. ePrint. video. (Longer 50mins version is available below.) 

  • A Concrete Treatment of Efficient Continuous Group Key Agreement via Multi-Recipient PKEs. Keitaro Hashimoto, Shuichi Katsumata, Eamonn W. Postlethwaite, Thomas Prest, Bas Westerbaan.
    
    ACM CCS 2021. ePrint. video (by Keitaro Hashimoto).

  • Revisiting Fuzzy Signatures: Towards a More Risk-Free Cryptographic Authentication System based on Biometrics. Shuichi Katsumata,  Takahiro Matsuda, Wataru Nakamura, Kazuma Ohara, Kenta Takahashi.

ACM CCS 2021. arXiv. video.

• • Round-Optimal Blind Signatures in the Plain Model from Classical and Quantum Standard Assumptions.  Shuichi Katsumata, Ryo Nishimaki, Shota Yamada, Takashi Yamakawa.

EUROCRYPT 2021. ePrint. video (by Ryo Nishimaki).

• • An Efficient and Generic Construction for Signal’s Handshake (X3DH): Post-Quantum, State Leakage Secure, and Deniable. Keitaro Hashimoto, Shuichi Katsumata, Kris Kwiatkowski,  Thomas Prest.

PKC 2021. ePrint. video (by Keitaro Hashimoto).

2020

• • Adaptively Secure Inner Product Encryption from LWE.  Shuichi Katsumata, Ryo Nishimaki, Shota Yamada, Takashi Yamakawa.

ASIACRYPT 2020. ePrint. video (by Shota Yamada).

• • Scalable Ciphertext Compression Techniques for Post-Quantum KEMs and their Applications. Shuichi Katsumata, Kris Kwiatkowski,  Federico Pintore, Thomas Prest.

ASIACRYPT 2020. ePrint. video (by Thomas Prest).

• •  Calamari and Falafl: Logarithmic (Linkable) Ring Signatures from Isogenies and Lattices.  Ward Beullens, Shuichi Katsumata, Federico Pintore.

ASIACRYPT 2020. ePrint. video (by Federico  Pintore).

• • Adaptively Secure Constrained Pseudorandom Functions in the Standard Model.  Alex Davidson, Shuichi Katsumata, Ryo Nishimaki, Shota Yamada, Takashi Yamakawa.

CRYPTO 2020. ePrint. video (by Takashi Yamakawa).

*(Historical Note: This is a major updated version of eprint:DKNYY18 with many new results, which itself is a merged version of eprint:DavNis18 and eprint:KatYam18.)

• • Exposing Private User Behaviors of Collaborative Filtering via Model Inversion Techniques. Seira Hidano, Takao Murakami, Shuichi Katsumata, Shinsaku Kiyomoto, Goichiro Hanaoka.

PETS/PoPETS 2020. Proceedings. 

• • Non-Interactive Zero-Knowledge in Pairing-Free Groups from Weaker Assumptions. Geoffroy Couteau, Shuichi Katsumata, Bogdan Ursu. 

EUROCRYPT 2020. ePrint. video (by Bogdan Ursu).

• • Compact NIZKs from Standard Assumptions on Bilinear Maps.  Shuichi Katsumata, Ryo Nishimaki, Shota Yamada, Takashi Yamakawa.

EUROCRYPT 2020. ePrint. video.

• • Lossy CSI-FiSh: Efficient Signature Scheme with Tight Reduction to Decisional CSIDH-512. Ali El Kaafarani, Shuichi Katsumata, Federico Pintore

PKC 2020. ePrint. video (by Federico Pintore).

2019

• • Exploring Constructions of Compact NIZKs from Various Assumptions. Shuichi Katsumata, Ryo Nishimaki, Shota Yamada, Takashi Yamakawa.

CRYPTO 2019. ePrint. video. 

• • Identity-Based Encryption with Security against the KGC: A Formal Model and Its Instantiation from Lattices. Keita Emura, Shuichi Katsumata, Yohei Watanabe. 

ESORICS 2019. ePrint.

• • Group Signatures without NIZK: From Lattices in the Standard Model. Shuichi Katsumata, Shota Yamada.

EUROCRYPT 2019. ePrint.

• • Designated Verifier/Prover and Preprocessing NIZKs from Diffie-Hellman Assumptions. Shuichi Katsumata, Ryo Nishimaki, Shota Yamada, Takashi Yamakawa.

EUROCRYPT 2019. ePrint. video. 

• • Non-Zero Inner Product Encryption Schemes from Various Assumptions: LWE, DDH and DCR. Shuichi Katsumata, Shota Yamada.

PKC 2019. ePrint.

• • Lattice-Based Revocable (Hierarchical) IBE with Decryption Key Exposure Resistance. Shuichi Katsumata, Takahiro Matsuda, Atsushi Takayasu.

PKC 2019. ePrint.

2018

• • Tighter Security Proofs for GPV-IBE in the Quantum Random Oracle Model. Shuichi Katsumata, Shota Yamada, Takashi Yamakawa.

ASIACRYPT 2018. (Invited to Journal of Cryptography.) ePrint.

• • Attribute-Based Signatures for Unbounded Languages from Standard Assumptions. Yusuke Sakai, Shuichi Katsumata, Nuttapong Attrapadung, Goichiro Hanaoka.

ASIACRYPT 2018. ePrint.

• • Attribute-Based Signatures for Unbounded Circuits in the ROM and Efficient Instantiations from Lattices. Ali El Kaafarani, Shuichi Katsumata.

PKC 2018. ePrint.

• • Anonymous Reputation Systems Achieving Full Dynamicity from Lattices. Ali El Kaafarani, Shuichi Katsumata, Ravital Solomon.

FC 2018. Preproceeding.

2017

• • On the Untapped Potential of Encoding Predicates by Arithmetic Circuits and Their Applications. Shuichi Katsumata.

ASIACRYPT 2017. ePrint. video.

• • Model Inversion Attacks for Prediction Systems: Without Knowledge of Non-Sensitive Attributes. Seira Hidano, Takao Murakami, Shuichi Katsumata, Shinsaku Kiyomoto, Goichiro Hanaoka.

PST 2017. Proceedings.

2016

• • Partitioning via Non-linear Polynomial Functions: More Compact IBEs from Ideal Lattices and Bilinear Maps. Shuichi Katsumata, Shota Yamada.

ASIACRYPT 2016. ePrint. video. 

2015

• • Robust Cost Sensitive Support Vector Machine. Shuichi Katsumata, Akiko Takeda.

AISTATS 2015. Proceedings.

2014

• • Constructing Subspace Membership Encryption through Inner Product Encryption. Shuichi Katsumata, Noboru Kunihiro.

ProvSec 2014. Proceedings.

Journal Articles

2025

• • Two-Round Threshold Signature from Algebraic One-More Learning with Errors. Thomas Espitau, Shuichi Katsumata, Kaoru Takemure. 

Journal of Cryptology, Volume 38, 2025.  Springer. (Full version of [EKT24@C]).

2024

• • CSI-Otter: Isogeny-based (Partially) Blind Signatures from the Class Group Action with a Twist. Shuichi Katsumata, Yi-Fu Lai, Jason T. LeGrow, Ling Qin.

Designs, Codes and Cryptography, 2023. Springer. (Full version of [KLLQ23@C]).

• • Compact NIZKs from Standard Assumptions on Bilinear Maps.  Shuichi Katsumata, Ryo Nishimaki, Shota Yamada, Takashi Yamakawa.

Journal of Cryptology, Volume 37, 2024. Springer. (Full version of [KNYY20@EC]).

2023

• • Group Signatures and More from Isogenies and Lattices: Generic, Simple, and Efficient. Ward Beullens, Samuel Dobson, Shuichi Katsumata, Yi-Fu Lai, Federico Pintore. 

Designs, Codes and Cryptography, 2023. Springer. (Full version of [BDKLP22@EC]

• • Direct Computation of Branching Programs and its Applications to More Efficient Lattice-based Cryptography. Shuichi Katsumata, Toi Tomita, Shota Yamada.

Designs, Codes and Cryptography, 2023. Springer. 

2022

• • An Efficient and Generic Construction for Signal’s Handshake (X3DH): Post-Quantum, State Leakage Secure, and Deniable. Keitaro Hashimoto, Shuichi Katsumata, Kris Kwiatkowski,  Thomas Prest.

Journal of Cryptology, Volume 35, 2022. Springer. (Full version of [HKKP22@PKC]).

• • Identity-Based Encryption with Security against the KGC: A Formal Model and Its Instantiation from Lattices. Keita Emura, Shuichi Katsumata, Yohei Watanabe. 

Theoretical Computer Science, Volume 900, 2022. Elsevier. (Full version of [EKW19@ESORICS]).

2021

• • Compact Designated Verifier NIZKs from the CDH Assumption Without Pairings.  Shuichi Katsumata, Ryo Nishimaki, Shota Yamada, Takashi Yamakawa.
    
    Journal of Cryptology, Volume 34, 2021. Springer. (Merged and revised version of  [KNYY19@EC] and [KNYY19@C]).
    
    

  • Tighter Security Proofs for GPV-IBE in the Quantum Random Oracle Model. Shuichi Katsumata, Shota Yamada, Takashi Yamakawa.

Journal of Cryptology, Volume 34, 2021. Springer. (Full version of [KYY18@AC]).

2020

• • Lattice-Based Revocable (Hierarchical) IBE with Decryption Key Exposure Resistance. Shuichi Katsumata, Takahiro Matsuda, Atsushi Takayasu.

Theoretical Computer Science, Volume 809, 2020. Elsevier. (Full version of [KMT19@PKC]).

2018

• • Model Inversion Attacks for Prediction Systems: Without Knowledge of Non-Sensitive Attributes. Seira Hidano, Takao Murakami, Shuichi Katsumata, Shinsaku Kiyomoto, Goichiro Hanaoka.

IEICE Transactions E101-D(11), 2018. (Full version of [HMKKH18@PST]).

2017

• • Constructing Subspace Membership Encryption through Inner Product Encryption. Shuichi Katsumata, Noboru Kunihiro.

IEICE Transactions E100-A(9), 2017. (Full version of [KK14@ProvSec]).

• • Taking Post-Quantum Cryptography from Theory to Practice: A Case Study with Signal.
    INDOCRYPT 2025, Keynote Speaker, December 2025. (slides).

  • The Triple Ratchet Protocol: A Bandwidth Efficient  Hybrid-Secure Signal Protocol.
    Real World Crypto, March 2025. (slides).

  • Post-Quantum Cryptography: The Path From Theory to Practice.
    New Concepts and Paradigms of Security for Next Generation 〜Post Quantum, AI and Drones〜, January, 2025. (slides)

  • Overseas Trends in the Transition to Post-Quantum Cryptography (耐量子計算機暗号への移行に関する海外の動向と課題).
    75th ISS Square Suihei Workshop, July, 2024. (slides)

  • Challenges of Schnorr-like Post-Quantum Blind Signatures.
    AB+ Attributes and Blindness, May, 2024. (slides)

  • CSI-Otter: Isogeny-based (Partially) Blind Signatures from the Class Group Action with a Twist.
    22th Forum on Information Technology, September, 2023. (slides)

  • Challenges and Solutions to Post-Quantum Secure Messaging.
    ACNS 2023, Keynote Speaker, June, 2023. (slides).
    The 3rd Yanqi Lake International PQC Standardization and Application Workshop, July, 2023.

  • Metadata Protection for MLS and Its Variants.
    Real World Crypto, March, 2023. (slides, video).

  • Post-Quantum Cryptography and Secure Messaging. 
    MIT Technology Review: Innovators Under 35 Japan Summit 2022 in Nihonbashi, December, 2022. 

  • A Survey on the NIST PQC Standardization (耐量子計算暗号の標準化動向). 
    Hardware Security Forum, December, 2022. 

  • Secure Messaging in a Post-Quantum World. 
    PQShield Webinar, November, 2022. 

  • Revisiting Fuzzy Signatures: Towards a More Risk-Free Cryptographic Authentication System based on Biometrics. 
    21th Forum on Information Technology, September, 2022. 

  • A New Simple Technique to Bootstrap Various Lattice Zero-Knowledge Proofs to QROM Secure NIZKs.
    Monash Univsersity, Cybersecurity Seminars, July, 2021. (slides, video).
    Workshop on Cryptography and Information Security (WCIS), September, 2021.
    Design and Evaluation for New-generation Cryptography,  November. 2021. 

  • Calamari and Falafl: Logarithmic (Linkable) Ring Signatures from Isogenies and Lattices.
    ENS de Lyon/Royal Holloway/CWI Cryptography Seminar, March, 2021. (slides).

  • Compact NIZKs from Various Assumptions.
    MPI/RUB Seminar, December, 2020. (slides).

  • Exploring Constructions of Compact NIZKs from Various Assumptions.
    Workshop on Cryptography and Information Security (WCIS), September, 2019. (slides).

• • Exploring Links Between Lattice-based NIZKs and Various Signature Schemes (in the Standard Model).
    The Workshop on Modern Trends in Cryptography, June, 2019. (slides).

• • On the Untapped Potential of Encoding Predicates by Arithmetic Circuits and Their Applications.
    NTT-JFLI-U.Tokyo Workshop on Cryptography, January, 2018.
    Tokyo Crypto Day, March, 2018. (slides).
    ISEC, May, 2018. 

• • Partitioning via Non-linear Polynomial Functions: More Compact IBEs from Ideal Lattices and Bilinear Maps.
    10th Workshop on Public Key Cryptography: Secure Constructions and Its Applications, February, 2017.
    ISEC, May, 2017. (slides). 

• • Lattice IBE without Reduction Loss in the Random Oracle Model.
    SCIS/CSS Invited Session in IWSEC, September, 2016.

Program Committee

• • RWC2026, PRIVCRYPT2025, EUROCRYPT2025, ASIACRYPT2024, PQCrypto2024, EUROCRYPT2024, CRYPTO2023, PKC2023, CT-RSA2023, PQCifris2022, CT-RSA2022, PKC2022, ProvSec2021, ASIACRYPT2021, PKC2021, APKC2021, ProvSec2020.

External Reviewer

• • CRYPTO, EUROCRYPT, ASIACRYPT, TCC, PKC, AsiaCCS, ACNS, PQCrypto, ACISP, APKC, IWSEC, DCC, etc...

Local Organizing Committee

• • New Trends in Post-Quantum Cryptography (NewtPQC) 2024. 

  • Symposium on Cryptography and Information Security (SCIS23)

  • Computer Security Symposium 2021 (CSS21).

  • Oxford Post-Quantum Cryptography Workshop 2019. 

Standardization/Guidelines

• “CRYPTREC Cryptographic Technology Guideline - Advanced Cryptography -” 2022. 

Media

• MIT Technology Review "Trajectory of U35 Innovators", May, 2025. (in Japanese)

• Nikkei xTECH, July 2023. (in Japanese)

Affiliation

• • Lead cryptography researcher at PQShield.

  • Collaborative researcher at the National Institute of Advanced Industrial Science and Technology (AIST).

• Tsujii Shigeo Security Award, 2023, for the paper: How to Hide MetaData in MLS-Like Secure Group Messaging: Simple, Modular, and Post-Quantum (with Keitaro Hashimoto and Thomas Prest.)

• MIT Technology Review, Innovators Under 35 Japan, 2022. Japanese Website. English Website.

•  Tsujii Shigeo Security Award, 2022, for the paper: A Concrete Treatment of Efficient Continuous Group Key Agreement via Multi-Recipient PKEs (with Keitaro Hashimoto, Eamonn W. Postlethwaite, Thomas Prest, Bas Westerbaan.)

• Symposium on Cryptography and Information Security 2021 (SCIS 2021), Best Paper Award (received by Keitaro Hashimoto)

& Tsujii Shigeo Security Award, 2021.
- for the paper: An Efficient and Generic Construction for Signal’s Handshake (X3DH): Post-Quantum, State Leakage Secure, and Deniable. (with Keitaro Hashimoto, Kris Kwiatkowski,  Thomas Prest.)

• Tsujii Shigeo Security Award, 2021, for the paper: Scalable Ciphertext Compression Techniques for Post-Quantum KEMs and their Applications. (with Kris Kwiatkowski,  Federico Pintore, Thomas Prest.)

• Dean's Award for Outstanding Achievement from Graduate School of Frontier Science (Ph.D) (top honor in the department), The University of Tokyo. May, 2019. 

• Invited to Journal of Cryptography from ASIACRYPT 2018.

• Computer Security Symposium 2017 (CSS2017), Best Student Paper Award.

& Tsujii Shigeo Security Award, 2018.
- for the paper: On the Untapped Potential of Encoding Predicates by Arithmetic Circuits and Their Applications. 

• Research Fellowships for Doctoral Scientists, Japan Society for the Promotion of Science (JSPS). 2017.

• Symposium on Cryptography and Information Security 2016 (SCIS 2016), Best Paper Award. 

& Tsujii Shigeo Security Award, 2017.
- for the paper: Partitioning via Non-linear Polynomial Functions: More Compact IBEs from Ideal Lattices and Bilinear Maps. (with Shota Yamada.)