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IEEE International Conference on Trust, Security and Privacy in Computing and Communications (TrustCom)

Post-Quantum Anonymity of Key Encapsulation Mechanism and Its Applications in Password Authenticated Key Exchange Protocols

JiaYv An, Yankun Gao, Lei Han

Published Nov 14, 2025• 0 citations
Publisher

Abstract

Driven by advances in the standardization of NIST’s Post-Quantum Cryptography (PQC) algorithms, research on constructing Password Authenticated Key Exchange (PAKE) protocols using Key Encapsulation Mechanisms (KEMs) has gained considerable attention. Several PAKE protocols have established precise security requirements for the underlying KEM. Arriaga et al. introduced a PAKE protocol called CHIC, which requires the KEM to achieve anonymity under plaintext-checkable attacks (ANO-1PCA). An anonymous KEM does not leak any information about which public key was used for encryption. It protects the identities and privacy of participants and is critical in preventing identity-guessing attacks, therefore, anonymity is also called key-privacy. The anonymity of NIST PQC Round-3 candidates has recently been analyzed, some of which satisfied ANO-CCA.Huguenin-Dumittan and Vaudenay proposed two efficient constructions, TCHand TH, to create KEM from public-key encryption (PKE) schemes. Jiang et al. introduced TRHbased on TH. These constructions can transform any OW-CPA PKE into an IND-qCCA KEM efficiently and avoid re-encryption. However, prior to this work, no research has been conducted to determine whether these constructions preserve or lift anonymity.In this paper, we proved that for any OW-CPA, DS-IND PKE, the construction KEMRH= TRH[PKE,H] achieves ANO1-CCA in the quantum random oracle model (QROM). We then construct a KEM called KyberRH, and provide an algorithm for the CHIC protocol using KyberRH.

DOI
10.1109/trustcom66490.2025.00189

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Driven by advances in the standardization of NIST’s Post-Quantum Cryptography (PQC) algorithms, research on constructing Password Authenticated Key Exchange (PAKE) protocols using Key Encapsulation Mechanisms (KEMs) has gained considerable attention. Several PAKE protocols have established precise security requirements for the underlying KEM. Arriaga et al. introduced a PAKE protocol called CHIC, which requires the KEM to achieve anonymity under plaintext-checkable attacks (ANO-1PCA). An anonymous KEM does not leak any information about which public key was used for encryption. It protects the identities and privacy of participants and is critical in preventing identity-guessing attacks, therefore, anonymity is also called key-privacy. The anonymity of NIST PQC Round-3 candidates has recently been analyzed, some of which satisfied ANO-CCA.Huguenin-Dumittan and Vaudenay proposed two efficient constructions, TCHand TH, to create KEM from public-key encryption (PKE) schemes. Jiang et al. introduced TRHbased on TH. These constructions can transform any OW-CPA PKE into an IND-qCCA KEM efficiently and avoid re-encryption. However, prior to this work, no research has been conducted to determine whether these constructions preserve or lift anonymity.In this paper, we proved that for any OW-CPA, DS-IND PKE, the construction KEMRH= TRH[PKE,H] achieves ANO1-CCA in the quantum random oracle model (QROM). We then construct a KEM called KyberRH, and provide an algorithm for the CHIC protocol using KyberRH.
Transport Layer Security (TLS) protocols are essential for ensuring the confidentiality, integrity, and authentication of data communications. Mutual authentication between client and server is crucial in high-security scenarios such as finance and government. However, existing TLS protocols typically rely on certificate-based public key infrastructures (PKI) for client authentication, which increases deployment complexity and introduces risks associated with third-party trust. In contrast, password-authenticated TLS protocols use memorable passwords for client authentication, offering advantages in simplicity and usability. Nonetheless, such protocols remain vulnerable to server compromise and fail to provide sufficient security against quantum adversaries.To address these challenges, we propose a novel TLS protocol based on asymmetric password-authenticated key exchange (PAKE). The protocol employs an asymmetric PAKE mechanism wherein only password-derived information is stored on the server, while the client derives its private key from the password. This design effectively mitigates the risk of server leakage attacks. The protocol further integrates two types of post-quantum secure key encapsulation mechanisms (KEMs): an OW-ChCCA (One-Way Checkable against Chosen-Ciphertext Attacks) secure KEM for server key exchange and authentication, and an OW-PCA (One-Way Plaintext-Checkable Attacks) secure KEM for binding the client’s password-derived private key to enable client authentication. Both KEMs support multi-user, multi-challenge settings and admit tight, post-quantum secure instantiations.Furthermore, this paper provides a formal security analysis of the proposed protocol within an optimized multi-stage key exchange model. By combining the random oracle and ideal cipher models with a game-based proof methodology, we demonstrate that the protocol achieves mutual authentication and forward secrecy, while also providing tight security bounds and post-quantum resilience. This work thus presents a promising solution for constructing post-quantum password-authenticated TLS protocols.
This paper proposes a new generic Encrypt-thenMAC (EtM) transformation for constructing IND-CCA secure Key Encapsulation Mechanisms (KEMs) in the post-quantum setting. The transformation builds on a public-key encryption scheme satisfying one-wayness under plaintext-checking attacks (OW-PCA) and an existentially unforgeable MAC, and formally achieves tight IND-CCA security in the random oracle model via a rigorous security reduction. Unlike the widely used FujisakiOkamoto (FO) transformation, our approach eliminates the need for ciphertext re-encryption during decapsulation-a major source of computational overhead and side-channel vulnerability in lattice-based implementations. We instantiate the transformation using the ML-KEM (FIPS 203) subroutine, yielding the ML-KEM-EtM variant. Experimental results show that ML-KEMEtM reduces decapsulation cycle counts by72−79%72-79 {\%}72−79%across security levels, with only a modest increase in encapsulation cost(1.8−7.3%)(1.8-7.3 {\%})(1.8−7.3%)and ciphertext size(1.0−2.1%)(1.0-2.1 {\%})(1.0−2.1%). The scheme thus offers a practical and efficient a lternative to F O-based KEMs, especially in performance-critical scenarios such as post-quantum TLS handshakes, where decapsulation efficiency is paramount.
Rapid advances in quantum computing, together with the announcement by the National Institute of Standards and Technology (NIST) to define new standards for digitalsignature, encryption, and key-establishment protocols, have created significant interest in post-quantum cryptographic schemes. This paper introduces Kyber (part of CRYSTALS - Cryptographic Suite for Algebraic Lattices - a package submitted to NIST post-quantum standardization effort in November 2017), a portfolio of post-quantum cryptographic primitives built around a key-encapsulation mechanism (KEM), based on hardness assumptions over module lattices. Our KEM is most naturally seen as a successor to the NEWHOPE KEM (Usenix 2016). In particular, the key and ciphertext sizes of our new construction are about half the size, the KEM offers CCA instead of only passive security, the security is based on a more general (and flexible) lattice problem, and our optimized implementation results in essentially the same running time as the aforementioned scheme. We first introduce a CPA-secure public-key encryption scheme, apply a variant of the Fujisaki-Okamoto transform to create a CCA-secure KEM, and eventually construct, in a black-box manner, CCA-secure encryption, key exchange, and authenticated-key-exchange schemes. The security of our primitives is based on the hardness of Module-LWE in the classical and quantum random oracle models, and our concrete parameters conservatively target more than 128 bits of postquantum security.
The Secure Shell (SSH) protocol is one of the first security protocols on the Internet to upgrade itself to resist attacks against future quantum computers, with the default adoption of the “quantum (otherwise, classically)” secure hybrid key exchange in OpenSSH from April 2022. However, there is a lack of a comprehensive security analysis of this quantum-resistant version of SSH in the literature: related works either focus on the hybrid key exchange in isolation and do not consider security of the overall protocol, or analyze the protocol in security models which are not appropriate for SSH, especially in the “post-quantum” setting. In this paper, we remedy the state of affairs by providing a thorough post-quantum cryptographic analysis of SSH. We follow a “top-down” approach wherein we first prove security of SSH in a more appropriate model, namely, our post-quantum extension of the so-called authenticated and confidential channel establishment (ACCE) protocol security model; our extension which captures “harvest now, decrypt later” attacks could be of independent interest. Then we establish the cryptographic properties of SSH's underlying primitives, as concretely instantiated in practice, based on our protocol-level ACCE security analysis: for example, we prove relevant cryptographic properties of “Streamlined NTRU Prime”, a key encapsulation mechanism (KEM) which is used in recent versions of OpenSSH and TinySSH, in the quantum random oracle model, and address open problems related to its analysis in the literature. Notably, our ACCE security analysis of post-quantum SSH relies on the weaker notion of IND-CPA security of the ephemeral KEMs used in the hybrid key exchange. This is in contrast to prior works which rely on the stronger assumption of IND-CCA secure ephemeral KEMs. Hence we conclude the paper with a discussion on potentially replacing IND-CCA secure KEMs in current post-quantum implementations of SSH with simpler and faster IND-CPA secure counterparts, and also provide the corresponding benchmarks.
Classical public-key cryptographic algorithms like the RSA algorithm and Diffie-Hellman are used to secure internet communications. However, the emergence of quantum computers poses a significant threat to these established algorithms, potentially leading to security breaches and malicious attacks. If quantum computers can break these algorithms, the entire security infrastructure of the internet could be compromised. Post- quantum cryptography (PQC) is a cryptographic area dedicated to developing new encryption and key exchange mechanisms that can resist quantum attacks. Its goal is to create quantum-resistant cryptographic algorithms, with NIST playing a central role in standardization and collaboration to ensure information security in the era of quantum computing. This study conducts a survey of Key Encapsulation Mechanism (KEM) algorithms within the field of Post-Quantum Cryptography (PQC), along with their alternative counterparts. It provides a comprehensive analysis of performance metrics, including key sizes, execution times, and runtime performance, specifically focusing on Key Encapsulation Mechanism (KEM) algorithms.