Internet-Draft xyber768d00 September 2023
Westerbaan & Stebila Expires 27 March 2024 [Page]
Intended Status:
B. E. Westerbaan
D. Stebila
University of Waterloo

X25519Kyber768Draft00 hybrid post-quantum key agreement


This memo defines X25519Kyber768Draft00, a hybrid post-quantum key exchange for TLS 1.3.

About This Document

This note is to be removed before publishing as an RFC.

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Status of This Memo

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This Internet-Draft will expire on 27 March 2024.

1. Introduction

1.1. Motivation

The final draft for Kyber is expected in 2024. There are already early deployments of post-quantum key agreement, with more to come before Kyber is standardised. To promote interoperability of early implementations, this document specifies a preliminary hybrid post-quantum key agreement.

1.2. Warning: relation with X25519Kyber768Draft00 for HPKE

In [hpkexyber] a hybrid KEM with the same name is defined for use in HPKE. It differs from the hybrid KEM implicit in this document: here we use the X25519 shared secret directly, whereas in [hpkexyber], the ephemeral X25519 public key (ciphertext) is mixed in. For use in HPKE this is required to be IND-CCA2 robust. This is not required for use in TLS 1.3, thanks to the inclusion of the keyshare in the message transcript.

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.

3. Construction

We instantiate draft-ietf-tls-hybrid-design-06 with X25519 [rfc7748] and Kyber768Draft00 [kyber]. The latter is Kyber as submitted to round 3 of the NIST PQC process [KyberV302].

For the client's share, the key_exchange value contains the concatenation of the client's X25519 ephemeral share (32 bytes) and the client's Kyber768Draft00 public key (1184 bytes). The resulting key_exchange value is 1216 bytes in length.

For the server's share, the key_exchange value contains the concatenation of the server's X25519 ephemeral share (32 bytes) and the Kyber768Draft00 ciphertext (1088 bytes) returned from encapsulation for the client's public key. The resulting key_exchange value is 1120 bytes in length.

The shared secret is calculated as the concatenation of the X25519 shared secret (32 bytes) and the Kyber768Draft00 shared secret (32 bytes). The resulting shared secret value is 64 bytes in length.

4. Security Considerations

For TLS 1.3, this concatenation approach provides a secure key exchange if either component key exchange methods (X25519 or Kyber768Draft00) are secure [hybrid].

5. IANA Considerations

This document requests/registers a new entry to the TLS Named Group (or Supported Group) registry, according to the procedures in Section 6 of [tlsiana].


0x6399 (please)








This document


Pre-standards version of Kyber768

6. References

6.1. Normative References

Schwabe, P. and B. Westerbaan, "Kyber Post-Quantum KEM", Work in Progress, Internet-Draft, draft-cfrg-schwabe-kyber-02, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves for Security", RFC 7748, DOI 10.17487/RFC7748, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.

6.2. Informative References

Westerbaan, B. and C. A. Wood, "X25519Kyber768Draft00 hybrid post-quantum KEM for HPKE", Work in Progress, Internet-Draft, draft-westerbaan-cfrg-hpke-xyber768d00-02, , <>.
Stebila, D., Fluhrer, S., and S. Gueron, "Hybrid key exchange in TLS 1.3", Work in Progress, Internet-Draft, draft-ietf-tls-hybrid-design-09, , <>.
Avanzi, R., Bos, J., Ducas, L., Kiltz, E., Lepoint, T., Lyubashevsky, V., Schanck, J., Schwabe, P., Seiler, G., and D. Stehle, "CRYSTALS-Kyber, Algorithm Specification And Supporting Documentation (version 3.02)", , <>.
Salowey, J. A. and S. Turner, "IANA Registry Updates for TLS and DTLS", Work in Progress, Internet-Draft, draft-ietf-tls-rfc8447bis-04, , <>.

Appendix A. Change log

  • RFC Editor's Note: Please remove this section prior to publication of a final version of this document.

A.1. Since draft-tls-westerbaan-xyber768d00-02

  • Explain relation with HPKE hybrid

A.3. Since draft-tls-westerbaan-xyber768d00-00

  • Set working group to None.
  • Bump to cfrg-schwabe-kyber-02

Authors' Addresses

Bas Westerbaan
Douglas Stebila
University of Waterloo