PQ/T Hybrid KEM: HPKE with JOSE/COSE

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Table of Contents

1. Introduction

The migration to Post-Quantum Cryptography (PQC) is unique in the history of modern digital cryptography in that neither the traditional algorithms nor the post-quantum algorithms are fully trusted to protect data for the required data lifetimes. The traditional algorithms, such as RSA and elliptic curve, will fall to quantum cryptalanysis, while the post-quantum algorithms face uncertainty about the underlying mathematics, compliance issues, unknown vulnerabilities, hardware and software implementations that have not had sufficient maturing time to rule out classical cryptanalytic attacks and implementation bugs.¶

During the transition from traditional to post-quantum algorithms, there is a desire or a requirement for protocols that use both algorithm types. Hybrid key exchange refers to using multiple key exchange algorithms simultaneously and combining the result with the goal of providing security even if all but one of the component algorithms is broken. It is motivated by transition to post-quantum cryptography.¶

HPKE offers a variant of public-key encryption of arbitrary-sized plaintexts for a recipient public key. The specifications for the use of HPKE with JOSE and COSE are described in [I-D.rha-jose-hpke-encrypt] and [I-D.ietf-cose-hpke], respectively. HPKE can be extended to support PQ/T Hybrid KEM as defined in [I-D.connolly-cfrg-xwing-kem]. This specification defines PQ/T Hybrid KEM in HPKE for use with JOSE and COSE.¶

2. Conventions and Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

This document makes use of the terms defined in [I-D.ietf-pquip-pqt-hybrid-terminology]. For the purposes of this document, it is helpful to be able to divide cryptographic algorithms into two classes:¶

"Traditional Algorithm": An asymmetric cryptographic algorithm based on integer factorisation, finite field discrete logarithms, elliptic curve discrete logarithms, or related mathematical problems. In the context of JOSE, examples of traditional key exchange algorithms include Elliptic Curve Diffie-Hellman Ephemeral Static [RFC6090] [RFC8037]. In the context of COSE, examples of traditional key exchange algorithms include Ephemeral-Static (ES) DH and Static-Static (SS) DH [RFC9052].¶

"Post-Quantum Algorithm": An asymmetric cryptographic algorithm that is believed to be secure against attacks using quantum computers as well as classical computers. Examples of PQC key exchange algorithms include ML-KEM.¶

"Post-Quantum Traditional (PQ/T) Hybrid Scheme": A multi-algorithm scheme where at least one component algorithm is a post-quantum algorithm and at least one is a traditional algorithm.¶

"PQ/T Hybrid Key Encapsulation Mechanism": A multi-algorithm KEM made up of two or more component KEM algorithms where at least one is a post-quantum algorithm and at least one is a traditional algorithm.¶

3. Construction

ML-KEM is a one-pass (store-and-forward) cryptographic mechanism for an originator to securely send keying material to a recipient using the recipient's ML-KEM public key. Three parameters sets for ML-KEMs are specified by [FIPS203]. In order of increasing security strength (and decreasing performance), these parameter sets are ML-KEM-512, ML-KEM-768, and ML-KEM-1024. [I-D.connolly-cfrg-xwing-kem] uses a multi-algorithm scheme, where one component algorithm is a post-quantum algorithm and another one is a traditional algorithm. The Combiner function defined in Section 5.3 of [I-D.connolly-cfrg-xwing-kem] combines the output of a post-quantum KEM and a traditional KEM to generate a single shared secret.¶

4. Ciphersuite Registration

This specification registers a number of PQ/T Hybrid KEMs for use with HPKE. A ciphersuite is thereby a combination of several algorithm configurations:¶

• KEM algorithm (Traditional Algorithm + PQ KEM, for example, X25519MLKEM768)¶

• KDF algorithm¶

• AEAD algorithm¶

The "KEM", "KDF", and "AEAD" values are conceptually taken from the HPKE IANA registry [HPKE-IANA]. Hence, JOSE and COSE cannot use an algorithm combination that is not already available with HPKE.¶

For readability the algorithm ciphersuites labels are built according to the following scheme:¶

   HPKE-<KEM>-<KDF>-<AEAD>

The HPKE PQ/T hybrid ciphersuites for JOSE and COSE are defined in Section 6. Note that the PQ/T Hybrid KEM in HPKE is not an authenticated KEM. Authenticated KEM is only possible when both parties contribute a PQC KEM public key and a traditional public key to the overall session key. The HPKE Base mode can only be supported with the PQ/T Hybrid KEM.¶

5. Security Considerations

The shared secrets computed in the hybrid key exchange should be computed in a way that achieves the "hybrid" property: the resulting secret is secure as long as at least one of the component key exchange algorithms is unbroken. PQC KEMs used in the manner described in this document MUST explicitly be designed to be secure in the event that the public key is reused, such as achieving IND-CCA2 security. ML-KEM has such security properties.¶

6. IANA Considerations

Acknowledgments

Thanks to Ilari Liusvaara and Orie Steele for the discussion and comments.¶

References

Authors' Addresses