Post-quantum hybrid ECDHE-Kyber Key Agreement for TLSv1.3

Introduction

Motivation Kyber is a key encapsulation method (KEM) designed to be resistant to cryptanalytic attacks with quantum computers. Standardization of Kyber KEM is expected to be finalized in 2024. Experimentation and early deployments are crucial part of the migration to post-quantum cryptography. To promote interoperability of those deployments this document provides specification of preliminary hybrid post-quantum key agreement to be used in TLS 1.3 protocol.

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 when, and only when, they appear in all capitals, as shown here.

Negotiated Groups This document defines an additional supported group which can be used for hybrid post-quantum key agreements. The hybrid key agreement for TLS 1.3 is detailed in the draft. We compose the hybrid scheme with the Kyber KEM as defined in draft, and the ECDHE scheme parametrized with elliptic curves defined in ANSI X9.62 and NIST SP 800-186 . The new group allows deriving TLS session keys by using FIPS-approved schemes. NIST's special publication 800-56Cr2 approves the usage of HKDF with two distinct shared secrets as long as the first one is computed by a FIPS-approved key-establishment scheme. Both ECDHE and a curve secp256r1 (NIST P-256) are FIPS-approved by NIST SP 800-56Ar3 and NIST SP 800-186 correspondingly.

Construction The name of the new supported hybrid post-quantum group is secp256r1_kyber768_d00. When this group is negotiated, the client's share is a fixed-size concatenation of the ECDHE share and Kyber's public key. The ECDHE share is the serialized value of the uncompressed ECDH point representation as defined in Section 4.2.8.2 of . The Kyber's ephemeral share is the public key of the KeyGen step (see ) represented as an octet string. The size of client share is 1248 bytes. The server's share is a fixed-size concatenation of ECDHE share and Kyber's ciphertext returned from encapsulation (see ). The server ECDHE share is the serialized value of the uncompressed ECDH point representation UncompressedPointRepresentation as defined in Section 4.2.8.2 of . The server share is the Kyber's ciphertext returned from the Encapsulate step (see ) represented as an octet string. The size of server's share is 1152 bytes. Finally, the shared secret is a concatenation of the ECDHE and the Kyber shared secrets. The ECDHE shared secret is the x-coordinate of the ECDH shared secret elliptic curve point represented as an octet string as defined in Section 7.4.2 of . The Kyber shared secret is the value returned from either encapsulation (on the server side) or decapsulation (on the client side) represented as an octet string. The size of a shared secret is 64 bytes.

Security Considerations The same security considerations as those described in apply to the approach used by this document. Implementers are encouraged to use implementations resistant to side-channel attacks, especially those that can be applied by remote attackers.

IANA Considerations This document requests/registers a new entry to the TLS Named Group (or Supported Group) registry, according to the procedures in . These identifiers are to be used with the point-in-time specified versions of Kyber in the third round of NIST's Post-quantum Project which is specified in . The identifiers used with the final, ratified by NIST, version of Kyber will be specified later with in a different draft. [ EDNOTE: The identifiers for the final, ratified version of Kyber should preferrably by different that the commonly used OQS codepoints ]

Value:: 0x6400
Description:: secp256r1_kyber768_d00
DTLS-OK:: Y
Recommended:: N
Reference:: This document
Comment:: Combining secp256r1 ECDH with pre-standards version of Kyber768