Transport Layer Security K. Kwiatkowski Internet-Draft PQShield Intended status: Informational P. Kampanakis Expires: 27 February 2025 AWS B. E. Westerbaan Cloudflare D. Stebila University of Waterloo 26 August 2024 Post-quantum hybrid ECDHE-MLKEM Key Agreement for TLSv1.3 draft-kwiatkowski-tls-ecdhe-mlkem-01 Abstract This draft defines two hybrid key agreements for TLS 1.3: X25519MLKEM768 and SecP256r1MLKEM768, which combine a post-quantum KEM with an elliptic curve Diffie-Hellman (ECDHE). About This Document This note is to be removed before publishing as an RFC. The latest revision of this draft can be found at https://post- quantum-cryptography.github.io/draft-kwiatkowski-tls-ecdhe-mlkem/. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-kwiatkowski-tls-ecdhe-mlkem/. Discussion of this document takes place on the Transport Layer Security Working Group mailing list (mailto:tls@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/tls/. Subscribe at https://www.ietf.org/mailman/listinfo/tls/. Source for this draft and an issue tracker can be found at https://github.com/post-quantum-cryptography/draft-kwiatkowski-tls- ecdhe-mlkem. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Kwiatkowski, et al. Expires 27 February 2025 [Page 1] Internet-Draft ECDHE-MLKEM August 2024 Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 27 February 2025. Copyright Notice Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3 3. Negotiated Groups . . . . . . . . . . . . . . . . . . . . . . 3 3.1. Construction . . . . . . . . . . . . . . . . . . . . . . 3 3.1.1. Client share . . . . . . . . . . . . . . . . . . . . 3 3.1.2. Server share . . . . . . . . . . . . . . . . . . . . 4 3.1.3. Shared secret . . . . . . . . . . . . . . . . . . . . 4 4. Security Considerations . . . . . . . . . . . . . . . . . . . 4 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 5.1. SecP256r1MLKEM768 . . . . . . . . . . . . . . . . . . . . 5 5.2. X25519MLKEM768 . . . . . . . . . . . . . . . . . . . . . 5 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 6.1. Normative References . . . . . . . . . . . . . . . . . . 5 6.2. Informative References . . . . . . . . . . . . . . . . . 6 Appendix A. Change log . . . . . . . . . . . . . . . . . . . . . 7 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 1. Introduction 1.1. Motivation ML-KEM is a key encapsulation method (KEM) defined in the [FIPS203]. It is designed to withstand cryptanalytic attacks from quantum computers. Kwiatkowski, et al. Expires 27 February 2025 [Page 2] Internet-Draft ECDHE-MLKEM August 2024 This document introduces two new supported groups for hybrid post- quantum key agreements in TLS 1.3: X25519MLKEM768 and SecP256r1MLKEM768. Both combine ML-KEM-768 with ECDH in the manner of [hybrid]. The first one uses X25519 [rfc7748] and is an update to X25519Kyber768Draft00 [xyber], the most widely deployed PQ/T hybrid combiner for TLS v1.3 deployed in 2024. The second one uses secp256r1 (NIST P-256) [ECDSA] [DSS]. The goal of this group is to support a use case that requires both shared secrets to be generated by FIPS-approved mechanisms. Both constructions aim to provide a FIPS-approved key-establishment scheme (as per [SP56C]). 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. Negotiated Groups Both groups enable the derivation of TLS session keys using FIPS- approved schemes. NIST's special publication 800-56Cr2 [SP56C] approves the usage of HKDF [HKDF] with two distinct shared secrets, with the condition that the first one is computed by a FIPS-approved key-establishment scheme. FIPS also requires a certified implementation of the scheme, which will remain more ubiqutous for secp256r1 in the coming years. For this reason we put the ML-KEM-768 shared secret first in X25519MLKEM768, and the secp256r1 shared secret first in SecP256r1MLKEM768. 3.1. Construction 3.1.1. Client share When the X25519MLKEM768 group is negotiated, the client's key_exchange value is the concatenation of the client's ML-KEM-768 encapsulation key and the client's X25519 ephemeral share. The size of the client share is 1216 bytes (1184 bytes for the ML-KEM part and 32 bytes for X25519). Kwiatkowski, et al. Expires 27 February 2025 [Page 3] Internet-Draft ECDHE-MLKEM August 2024 When the SecP256r1MLKEM768 group is negotiated, the client's key_exchange value is the concatenation of the secp256r1 ephemeral share and ML-KEM-768 encapsulation key. The ECDHE share is the serialized value of the uncompressed ECDH point representation as defined in Section 4.2.8.2 of [RFC8446]. The size of the client share is 1249 bytes (65 bytes for the secp256r1 part and 1184 bytes for ML-KEM). 3.1.2. Server share When the X25519MLKEM768 group is negotiated, the server's key exchange value is the concatenation of an ML-KEM ciphertext returned from encapsulation to the client's encapsulation key, and the server's ephemeral X25519 share. The size of the server share is 1120 bytes (1088 bytes for the ML-KEM part and 32 bytes for X25519). When the SecP256r1MLKEM768 group is negotiated, the server's key exchange value is the concatenation of the server's ephemeral secp256r1 share encoded in the same way as the client share and an ML-KEM ciphertext returned from encapsulation to the client's encapsulation key. The size of the server share is 1153 bytes (1088 bytes for the ML-KEM part and 65 bytes for secp256r1). For both groups, the server MUST perform the encapsulation key check described in Section 7.3 of [FIPS203] on the client's encapsulation key, and abort with an illegal_parameter alert if it fails. 3.1.3. Shared secret For X25519MLKEM768, the shared secret is the concatenation of the ML- KEM shared secret and the X25519 shared secret. The shared secret is 64 bytes (32 bytes for each part). For SecP256r1MLKEM768, the shared secret is the concatenation of the ECDHE and ML-KEM shared secret. 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 [RFC8446]. The size of the shared secret is 64 bytes (32 bytes for each part). 4. Security Considerations The same security considerations as those described in [hybrid] apply to the approach used by this document. The security analysis relies crucially on the TLS 1.3 message transcript, and one cannot assume a similar hybridisation is secure in other protocols. Kwiatkowski, et al. Expires 27 February 2025 [Page 4] Internet-Draft ECDHE-MLKEM August 2024 Implementers are encouraged to use implementations resistant to side- channel attacks, especially those that can be applied by remote attackers. 5. IANA Considerations This document requests/registers two new entries to the TLS Supported Groups registry, according to the procedures in Section 6 of [tlsiana]. These identifiers are to be used with the final, ratified by NIST, version of ML-KEM which is specified in [FIPS203]. 5.1. SecP256r1MLKEM768 Value: 4587 (0x11EB) Description: SecP256r1MLKEM768 DTLS-OK: Y Recommended: N Reference: This document Comment: Combining secp256r1 ECDH with ML-KEM-768 5.2. X25519MLKEM768 Value: 4588 (0x11EC) Description: X25519MLKEM768 DTLS-OK: Y Recommended: N Reference: This document Comment: Combining X25519 ECDH with ML-KEM-768 6. References 6.1. Normative References [FIPS203] "Module-Lattice-Based Key-Encapsulation Mechanism Standard", National Institute of Standards and Technology, DOI 10.6028/nist.fips.203, August 2024, . Kwiatkowski, et al. Expires 27 February 2025 [Page 5] Internet-Draft ECDHE-MLKEM August 2024 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [rfc7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves for Security", RFC 7748, DOI 10.17487/RFC7748, January 2016, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, . [SP56C] Barker, E., Chen, L., and R. Davis, "Recommendation for Key-Derivation Methods in Key-Establishment Schemes", National Institute of Standards and Technology, DOI 10.6028/nist.sp.800-56cr2, August 2020, . 6.2. Informative References [DSS] Chen, L., Moody, D., Regenscheid, A., Robinson, A., and K. Randall, "Recommendations for Discrete Logarithm-based Cryptography:: Elliptic Curve Domain Parameters", National Institute of Standards and Technology, DOI 10.6028/nist.sp.800-186, February 2023, . [ECDSA] American National Standards Institute, "Public Key Cryptography for the Financial Services Industry: The Elliptic Curve Digital Signature Algorithm (ECDSA)", ANSI ANS X9.62-2005, November 2005. [HKDF] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand Key Derivation Function (HKDF)", RFC Editor, DOI 10.17487/rfc5869, May 2010, . [hybrid] Stebila, D., Fluhrer, S., and S. Gueron, "Hybrid key exchange in TLS 1.3", Work in Progress, Internet-Draft, draft-ietf-tls-hybrid-design-10, 5 April 2024, . Kwiatkowski, et al. Expires 27 February 2025 [Page 6] Internet-Draft ECDHE-MLKEM August 2024 [tlsiana] Salowey, J. A. and S. Turner, "IANA Registry Updates for TLS and DTLS", Work in Progress, Internet-Draft, draft- ietf-tls-rfc8447bis-09, 30 April 2024, . [xyber] Westerbaan, B. and D. Stebila, "X25519Kyber768Draft00 hybrid post-quantum key agreement", Work in Progress, Internet-Draft, draft-tls-westerbaan-xyber768d00-03, 24 September 2023, . Appendix A. Change log * draft-kwiatkowski-tls-ecdhe-mlkem-01: - Add X25519MLKEM768 * draft-kwiatkowski-tls-ecdhe-mlkem-00: - Change Kyber name to ML-KEM - Swap reference to I-D.cfrg-schwabe-kyber with FIPS-203 - Change codepoint. New value is equal to old value + 1. * draft-kwiatkowski-tls-ecdhe-kyber-01: Fix size of key shares generated by the client and the server * draft-kwiatkowski-tls-ecdhe-kyber-00: updates following IANA review Authors' Addresses Kris Kwiatkowski PQShield Email: kris@amongbytes.com Panos Kampanakis AWS Email: kpanos@amazon.com Bas Westerbaan Cloudflare Email: bas@cloudflare.com Kwiatkowski, et al. Expires 27 February 2025 [Page 7] Internet-Draft ECDHE-MLKEM August 2024 Douglas Stebila University of Waterloo Email: dstebila@waterloo.ca Kwiatkowski, et al. Expires 27 February 2025 [Page 8]