Crypto Forum D. Connolly Internet-Draft SandboxAQ Intended status: Informational 28 February 2024 Expires: 31 August 2024 ML-KEM for HPKE draft-connolly-cfrg-hpke-mlkem-00 Abstract This memo defines the ML-KEM-768-based and ML-KEM-1024-based ciphersuites for HPKE (RFC9180). ML-KEM is believed to be secure even against adversaries who possess a quantum computer. 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://dconnolly.github.io/draft-connolly-cfrg-xwing-kem/draft- connolly-cfrg-xwing-kem.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-connolly-cfrg- hpke-mlkem/. Discussion of this document takes place on the Crypto Forum Research Group mailing list (mailto:cfrg@ietf.org), which is archived at https://mailarchive.ietf.org/arch/search/?email_list=cfrg. Subscribe at https://www.ietf.org/mailman/listinfo/cfrg/. Source for this draft and an issue tracker can be found at https://github.com/dconnolly/draft-connolly-cfrg-xwing-kem. 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." Connolly Expires 31 August 2024 [Page 1] Internet-Draft hpke-mlkem February 2024 This Internet-Draft will expire on 31 August 2024. 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 2 1.2. Not an authenticated KEM . . . . . . . . . . . . . . . . 3 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3 3. Construction . . . . . . . . . . . . . . . . . . . . . . . . 3 3.1. Encap and Decap . . . . . . . . . . . . . . . . . . . . . 3 3.2. AuthEncap and AuthDecap . . . . . . . . . . . . . . . . . 3 4. Security Considerations . . . . . . . . . . . . . . . . . . . 4 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 6.1. Normative References . . . . . . . . . . . . . . . . . . 5 6.2. Informative References . . . . . . . . . . . . . . . . . 6 Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 6 Appendix B. Change log . . . . . . . . . . . . . . . . . . . . . 6 B.1. Since draft-connolly-cfrg-hpke-mlkem-00 . . . . . . . . . 6 Appendix C. Test Vectors . . . . . . . . . . . . . . . . . . . . 6 C.1. ML-KEM-768, HKDF-SHA256, AES-128-GCM . . . . . . . . . . 6 C.2. ML-KEM-1024, HKDF-SHA512, AES-256-GCM . . . . . . . . . . 6 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6 1. Introduction 1.1. Motivation The final draft for ML-KEM is expected in 2024. For parties that must move to exclusively post-quantum algorithms, having a pure PQ choice for public-key hybrid encryption is desireable. HPKE is the leading modern protocol for public-key encryption, and ML-KEM as a post-quantum IND-CCA2-secure KEM fits nicely into HPKE's design. Supporting multiple security levels for ML-KEM allow a spectrum of use cases including settings where NIST PQ security category 5 is required. Connolly Expires 31 August 2024 [Page 2] Internet-Draft hpke-mlkem February 2024 1.2. Not an authenticated KEM ML-KEM is a plain KEM that does not support the static-ephemeral key exchange that allows HPKE based on Diffie-Hellman based KEMs its (optional) authenticated modes. 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 construct 'wrapper' KEMs based on ML-KEM to bind the encapsulated shared secret ciphertext into the shared secret value, such that the final KEM has similar binding security properties as the original DHKEM HPKE was designed around. The encapsulation and decapsulation keys are computed, serialized, and deserialized the same as in [FIPS203]. We use HKDF-SHA256 and HKDF-SHA512 as the HPKE KDFs and AES-128-GCM and AES-256-GCM as the AEADs for ML-KEM-768 and ML-KEM-1024 respectively. 3.1. Encap and Decap ~~ def Encap(pkR): ss, ct = MLKEM.Encaps(pkR) shared_secret = ExtractAndExpand(ss, ct) return shared_secret, ct ~~ ~~ def Decap(enc, skR): ss, ct = MLKEM.Decaps(enc, skR) shared_secret = ExtractAndExpand(ss, ct) return shared_secret, ct ~~ 3.2. AuthEncap and AuthDecap HPKE-ML-KEM is not an authenticeted KEM and does not support AuthEncap() or AuthDecap(), see Section 1.2. Connolly Expires 31 August 2024 [Page 3] Internet-Draft hpke-mlkem February 2024 4. Security Considerations HPKE's IND-CCA2 security relies upon the IND-CCA2 security of the underlying KEM and AEAD schemes. ML-KEM is believed to be IND-CCA secure via multiple analyses. The HPKE key schedule is independent of the encapsulated KEM shared secret ciphertext of the ciphersuite KEM, and dependent on the shared secret produced by the KEM. If HPKE had committed to the encapsulated shared secret ciphertext, we wouldn't have to worry about the binding properties of the ciphersuite KEM's X-BIND-K-CT properties. These computational binding properties for KEMs were formalized in [CDM23]. ML-KEM, unlike DHKEM, is also an implicitly-rejecting instantiation of the Fujisaki-Okamoto transform, meaning the ML-KEM output shared secret may be computed differently in case of decryption failure, that reults in different binding properties, such as the lack of X- BIND-CT-PK and X-BIND-CT-K completely. The DHKEM construction in HPKE provides MAL-BIND-K-PK and MAL-BIND- K-CT security (the shared secret 'binds' or uniquely determines the encapsulation key and the encapsualted shared secret ciphertext), where the adversary is able to create the key pairs any way they like in addition to the key generation. ML-KEM as specified provides LEAK-BIND-PK,K-CT security, where the involved key pairs are output by the key generation algorithm of the KEM and then leaked to the adversary. LEAK-BIND-PK,K-CT is a weaker property than the DHKEM properties as it is not resistant in the presence of an actively malicious adversary, and requires both the shared secret _and_the public key together to uniquely bind the ciphertext, so its shared secret alone is insufficient. This results in a wrapper construction around ML-KEM to bind to the encapsulated shared secret ciphertext as the kem_context provided to ExtractAndExpand(). This binds the final shared_secret (K) to the encapsulated shared secret ciphertext (CT), achieving MAL-BIND-K-CT. ML-KEM already is MAL-BIND-K-PK as the hash of the encapsulation key (PK) is an input the computation of the shared secret (K). Together this wrapper KEM matches the binding properties of HPKE's default KEM construction DHKEM in being MAL-BIND-K-CT and MAL-BIND-K-PK. 5. IANA Considerations This document requests/registers two new entries to the "HPKE KEM Identifiers" registry. Value: 0x0070 (please) Connolly Expires 31 August 2024 [Page 4] Internet-Draft hpke-mlkem February 2024 KEM: ML-KEM-768 Nsecret: 32 Nenc: 1088 Npk: 1184 Nsk: 2400 Auth: no Reference: This document Value: 0x0080 (please) KEM: ML-KEM-1024 Nsecret: 32 Nenc: 1568 Npk: 1568 Nsk: 3168 Auth: no Reference: This document 6. References 6.1. Normative References [FIPS203] "*** BROKEN REFERENCE ***". [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC9180] Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180, February 2022, . Connolly Expires 31 August 2024 [Page 5] Internet-Draft hpke-mlkem February 2024 6.2. Informative References [CDM23] Cremers, C., Dax, A., and N. Medinger, "Keeping Up with the KEMs: Stronger Security Notions for KEMs and automated analysis of KEM-based protocols", 2023, . Appendix A. Acknowledgments The authors would like to thank Cas Cremers for their input. Appendix B. Change log *RFC Editor's Note:* Please remove this section prior to publication of a final version of this document. TODO B.1. Since draft-connolly-cfrg-hpke-mlkem-00 TODO Appendix C. Test Vectors This section contains test vectors formatted similary to that which are found in [RFC9180], with two changes. First, we only provide vectors for the non-authenticated modes of operation. Secondly, as ML-KEM encapsulation does not involve an ephemeral keypair, we omit the ikmE, skEm, pkEm entries and provide an ier entry instead. The value of ier is the randomness used to encapsulate, so ier[0:32] is the seed that is fed to H in the first step of ML-KEM encapsulation in [FIPS203]. C.1. ML-KEM-768, HKDF-SHA256, AES-128-GCM TODO C.2. ML-KEM-1024, HKDF-SHA512, AES-256-GCM TODO Author's Address Deirdre Connolly SandboxAQ Email: durumcrustulum@gmail.com Connolly Expires 31 August 2024 [Page 6]