Transport Layer Security K. Kwiatkowski Internet-Draft PQShield Intended status: Informational P. Kampanakis Expires: 19 April 2025 AWS B. E. Westerbaan Cloudflare D. Stebila University of Waterloo 16 October 2024 Post-quantum hybrid ECDHE-MLKEM Key Agreement for TLSv1.3 draft-kwiatkowski-tls-ecdhe-mlkem-latest Abstract This draft defines three hybrid key agreements for TLS 1.3: X25519MLKEM768, SecP256r1MLKEM768, and SecP384r1MLKEM1024 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/. 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 19 April 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 1.1. Motivation 2. Conventions and Definitions 3. Negotiated Groups 3.1. Construction 3.1.1. Client share 3.1.2. Server share 3.1.3. Shared secret 4. Security Considerations 5. IANA Considerations 5.1. SecP256r1MLKEM768 5.2. X25519MLKEM768 5.3. SecP384r1MLKEM1024 5.4. Obsoleted Supported Groups 6. References 6.1. Normative References 6.2. Informative References Appendix A. Change log Authors' Addresses 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. This document introduces three new supported groups for hybrid post- quantum key agreements in TLS 1.3: the X25519MLKEM768, SecP256r1MLKEM768, and SecP384r1MLKEM1024 which combine ML-KEM 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. The third one uses secp384r1 (NIST P-384) [ECDSA] [DSS]. The goal of this group is to provide support for high security environments that require use of FIPS-approved mechanisms. All 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 All 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 shared secret first in X25519MLKEM768, and the ECDH shared secret first in SecP256r1MLKEM768 and SecP384r1MLKEM1024. 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). 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). When the SecP384r1MLKEM1024 group is negotiated, the client's key_exchange value is the concatenation of the secp384r1 ephemeral share and the ML-KEM-1024 encapsulation key. The ECDH share is serialised value of the uncompressed ECDH point represenation as defined in Section 4.2.8.2 of [RFC8446]. The size of the client share is 1665 bytes (97 bytes for the secp384r1 and the 1568 for the 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). When the SecP384r1MLKEM1024 group is negotiated, the server's key exchange value is the concatenation of the server's ephemeral secp384r1 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 1665 bytes (1568 bytes for the ML-KEM part and 97 bytes for secp384r1) For all groups, the server MUST perform the encapsulation key check described in Section 7.2 of [FIPS203] on the client's encapsulation key, and abort with an illegal_parameter alert if it fails. For all groups, the client MUST check if the ciphertext length matches the selected group, and abort with an illegal_parameter alert if it fails. If ML-KEM decapsulation fails for any other reason, the connection MUST be aborted with an internal_error alert. For all groups, both client and server MUST process the ECDH part as described in Section 4.2.8.2 of [RFC8446], including all validity checks, 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). For SecP384r1MLKEM1024, 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 80 bytes (48 bytes for the ECDH part and 32 bytes for the ML-KEM part). For all groups, both client and server MUST calculate the ECDH part of the shared secret as described in Section 7.4.2 of [RFC8446], including the shared secret check as described in Section 5.7.1.2 of [SP56A] or the all-zero shared secret check (depending on the curve), and abort the connection with an illegal_parameter alert if it fails. 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. 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 three 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 5.3. SecP384r1MLKEM1024 Value: 4589 (0x11ED) Description: SecP384r1MLKEM1024 DTLS-OK: Y Recommended: N Reference: This document Comment: Combining secp384r1 ECDH with ML-KEM-1024 5.4. Obsoleted Supported Groups This document obsoletes 25497 and 25498 in the TLS Supported Groups registry. 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, . [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, . [SP56A] Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R. Davis, "Recommendation for pair-wise key-establishment schemes using discrete logarithm cryptography", National Institute of Standards and Technology, DOI 10.6028/nist.sp.800-56ar3, April 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-11, 7 October 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-03: - Adds P-384 combined with ML-KEM-1024 - Adds text that describes error-handling and outlines how the client and server must ensure the integrity of the key exchange process. * draft-kwiatkowski-tls-ecdhe-mlkem-02: - Adds section that mentions supported groups that this document obsoletes. - Fix a reference to encapsulation in the FIPS 203. * 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 Douglas Stebila University of Waterloo Email: dstebila@waterloo.ca