]> ML-KEM Post-Quantum Key Agreement for TLS 1.3 Selkie Cryptography
durumcrustulum@gmail.com
Security Transport Layer Security kems tls This memo defines ML-KEM-512, ML-KEM-768, and ML-KEM-1024 as NamedGroups and registers IANA values in the TLS Supported Groups registry for use in TLS 1.3 to achieve post-quantum (PQ) key establishment. About This Document Status information for this document may be found at . Discussion of this document takes place on the Transport Layer Security Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .
Introduction ML-KEM is a FIPS standard for post-quantum key establishment via a lattice-based key encapsulation mechanism (KEM). This document defines key establishment options for TLS 1.3 via the existing supported_groups () and key_share () extensions.
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.
Key encapsulation mechanisms This document models key establishment as key encapsulation mechanisms (KEMs), which consist of three algorithms:
  • KeyGen() -> (pk, sk): A probabilistic key generation algorithm, which generates a public encapsulation key pk and a secret decapsulation key sk.
  • Encaps(pk) -> (ct, shared_secret): A probabilistic encapsulation algorithm, which takes as input a public encapsulation key pk and outputs a ciphertext ct and shared secret shared_secret.
  • Decaps(sk, ct) -> shared_secret: A decapsulation algorithm, which takes as input a secret decapsulation key sk and ciphertext ct and outputs a shared secret shared_secret.
ML-KEM-512, ML-KEM-768, and ML-KEM-1024 conform to this interface (Table 3 of ):
  • ML-KEM-512 has encapsulation keys of size 800 bytes, expanded decapsulation keys of 1632 bytes, decapsulation key seeds of size 64 bytes, ciphertext size of 768 bytes, and shared secrets of size 32 bytes.
  • ML-KEM-768 has encapsulation keys of size 1184 bytes, expanded decapsulation keys of 2400 bytes, decapsulation key seeds of size 64 bytes, ciphertext size of 1088 bytes, and shared secrets of size 32 bytes.
  • ML-KEM-1024 has encapsulation keys of size 1568 bytes, expanded decapsulation keys of 3168 bytes, decapsulation key seeds of size 64 bytes, ciphertext size of 1568 bytes, and shared secrets of size 32 bytes.
Construction The KEMs are defined as NamedGroups, sent in the supported_groups extension ().
Negotiation Each parameter set of ML-KEM is assigned an identifier, registered by IANA in the TLS Supported Groups registry:
Transmitting encapsulation keys and ciphertexts The public encapsulation key and ciphertext values are each directly encoded with fixed lengths as in . In TLS 1.3 a KEM public encapsulation key pk or ciphertext ct is represented as a KeyShareEntry as specified in . These are transmitted in the extension_data fields of KeyShareClientHello and KeyShareServerHello extensions. For the client's share, the key_exchange value contains the pk output of the corresponding ML-KEM parameter set's KeyGen algorithm. For the server's share, the key_exchange value contains the ct output of the corresponding ML-KEM parameter set's Encaps algorithm. For all parameter sets, the server MUST perform the encapsulation key check described in Section 7.2 of on the client's encapsulation key, and abort with an illegal_parameter alert if it fails. For all parameter sets, the client MUST check if the ciphertext length matches the selected parameter set, 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. Implementations MUST NOT reuse randomness in the generation of ML-KEM ciphertexts— it follows that ML-KEM ciphertexts also MUST NOT be reused. During encapsulation, ML-KEM draws the encapsulation randomness from a random bit generator; the peer holding the decapsulation key sk recovers this randomness exactly. Any information that this randomness provides about other outputs of the generator is therefore available to that peer.
Shared secret calculation The fixed-length shared secret output from the ML-KEM Encaps and Decaps algorithms over the appropriate keypair and ciphertext results in the same shared secret shared_secret as its peer, which is inserted into the TLS 1.3 key schedule in place of the (EC)DHE shared secret, as shown in .
Security Considerations This document defines standalone ML-KEM key establishment for TLS 1.3. Use of KEMs for key agreement in TLS 1.3 has been analyzed in multiple settings and security models ; ML-KEM's IND-CCA security (Indistinguishability under Chosen-Ciphertext Attack) exceeds the requirements for ephemeral key establishment . Multiple formal analyses, including pen-and-paper computational proofs and machine-checked symbolic analysis using ProVerif , demonstrate that replacing Diffie-Hellman with an IND-CCA-secure KEM preserves the security properties of the TLS handshake. Formal analysis has also shown that hybrid key establishment (e.g., , ) provides compositional security: the exchange remains secure as long as at least one of the component algorithms is unbroken . TLS 1.3's key schedule commits to the ML-KEM encapsulation key and the ciphertext as the key_exchange field of the key_share extension is populated with those values, which are included as part of the handshake messages. This provides resilience against re-encapsulation attacks against KEMs used for key establishment . includes guidelines and requirements for implementations on using KEMs securely. Implementers are encouraged to use implementations resistant to side-channel attacks, especially those that can be applied by remote attackers. Implementers must evaluate their specific security, performance, and operational constraints when deciding whether to deploy standalone ML-KEM or a hybrid construction. The recommended column in the IANA TLS Supported Groups registry contains the IETF's current guidance on the recommended use of these algorithms for general purposes. The disclosure of the output(s) of an insecure random number generator (RNG) when used in TLS and other protocols can be used in an attack to compromise the state of the insecure RNG itself as described in . The encapsulation randomness in ML-KEM is an additional place where raw RNG output may be disclosed, therefore it is important to follow the RNG guidance in and . Implementers can choose to implement mechanisms from for additional protection across sessions.
IANA Considerations This document registers three new entries to the TLS Supported Groups registry, according to the procedures in .
Value Description DTLS-OK Recommended Reference Comment
512 (0x0200) MLKEM512 Y N This document FIPS 203 version of ML-KEM-512
513 (0x0201) MLKEM768 Y N This document FIPS 203 version of ML-KEM-768
514 (0x0202) MLKEM1024 Y N This document FIPS 203 version of ML-KEM-1024
As defined in , the value N:
Indicates that the item has not been evaluated by the IETF and that the IETF has made no statement about the suitability of the associated mechanism. This does not necessarily mean that the mechanism is flawed, only that no consensus exists. The IETF might have consensus to leave an item marked as "N" on the basis of the item having limited applicability or usage constraints.
References Normative References Module-lattice-based key-encapsulation mechanism standard National Institute of Standards and Technology (U.S.) The Transport Layer Security (TLS) Protocol Version 1.3 This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document obsoletes RFC 8446, which specified TLS 1.3. This document obsoletes RFC 5246 (specifying TLS 1.2) and RFCs 5077, 6961, 7627, and 8422, all of which pertain to TLS 1.2 or earlier, and updates RFCs 5705 and 6066. This document also specifies new requirements for TLS 1.2 implementations. Key words for use in RFCs to Indicate Requirement Levels In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References Post-Quantum Sound CryptoVerif and Verification of Hybrid TLS and SSH Key-Exchanges Keeping Up with the KEMs: Stronger Security Notions for KEMs and automated analysis of KEM-based protocols CISPA Helmholtz Center for Information Security CISPA Helmholtz Center for Information Security CISPA Helmholtz Center for Information Security A Tale of Two Models: Formal Verification of KEMTLS via Tamarin On the Security and Efficiency of TLS 1.3 Handshake with Hybrid Key Exchange from CPA-Secure KEMs Post-Quantum {TLS} 1.3 Handshake from {CPA}-Secure {KEMs} with Tighter Reductions A Cryptographic Analysis of the TLS 1.3 Handshake Protocol On the Practical Exploitability of Dual EC in TLS Implementations On The Multi-target Security of Post-Quantum Key Encapsulation Mechanisms On IND-qCCA Security in the ROM and Its Applications - CPA Security Is Sufficient for TLS 1.3 n.d. FATT Chance: On the Robustness of Standalone and Hybrid ML-KEM Key Exchange in TLS 1.3 n.d. Post-Quantum TLS Without Handshake Signatures Max Planck Institute for Security and Privacy & Radboud University, Bochum, Germany University of Waterloo, Waterloo, ON, Canada Radboud University, Nijmegen, Netherlands ACM Recommendations for key-encapsulation mechanisms National Institute of Standards and Technology (U.S.) Randomness Improvements for Security Protocols Randomness is a crucial ingredient for Transport Layer Security (TLS) and related security protocols. Weak or predictable "cryptographically secure" pseudorandom number generators (CSPRNGs) can be abused or exploited for malicious purposes. An initial entropy source that seeds a CSPRNG might be weak or broken as well, which can also lead to critical and systemic security problems. This document describes a way for security protocol implementations to augment their CSPRNGs using long-term private keys. This improves randomness from broken or otherwise subverted CSPRNGs. This document is a product of the Crypto Forum Research Group (CFRG) in the IRTF. Terminology for Post-Quantum Traditional Hybrid Schemes One aspect of the transition to post-quantum algorithms in cryptographic protocols is the development of hybrid schemes that incorporate both post-quantum and traditional asymmetric algorithms. This document defines terminology for such schemes. It is intended to be used as a reference and, hopefully, to ensure consistency and clarity across different protocols, standards, and organisations. IANA Registry Updates for TLS and DTLS This document updates the changes to the TLS and DTLS IANA registries made in RFC 8447. It adds a new value, "D" for discouraged, to the "Recommended" column of the selected TLS registries and adds a "Comment" column to all active registries that do not already have a "Comment" column. Finally, it updates the registration request instructions. This document updates RFC 8447. Hybrid Key Exchange in TLS 1.3 Hybrid key exchange refers to using multiple key exchange algorithms simultaneously and combining the result with the goal of providing security even if a way is found to defeat the encryption for all but one of the component algorithms. It is motivated by the transition to post-quantum cryptography. This document provides a construction for hybrid key exchange in the Transport Layer Security (TLS) protocol version 1.3. Post-Quantum Traditional (PQ/T) Hybrid Key Agreement Mechanisms for TLS 1.3 This document defines three hybrid key agreement mechanisms for TLS 1.3 -- X25519MLKEM768, SecP256r1MLKEM768, and SecP384r1MLKEM1024 -- that combine the post-quantum ML-KEM (Module-Lattice-Based Key Encapsulation Mechanism) with an ECDHE (Ephemeral Elliptic Curve Diffie-Hellman) exchange. CPA-Secure KEMs are also Sufficient for Post-quantum TLS 1.3 n.d.
Acknowledgments Thanks to Douglas Stebila for consultation on RFC 9954's design, and to Scott Fluhrer, Eric Rescorla, John Preuß Mattsson, Martin Thomson, and Rebecca Guthrie for reviews.