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Security Transport Layer Security mlkem tls post-quantum This memo defines ML-KEM-768 and ML-KEM-1024 as a standalone NamedGroup for use in TLS 1.3 to achieve post-quantum key agreement. 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

Motivation FIPS 203 standard (ML-KEM) is a new FIPS standard for post-quantum key agreement via lattice-based key establishment mechanism (KEM). Having a fully post-quantum (not hybrid) key agreement option for TLS 1.3 is necessary for migrating beyond hybrids and for users that need to be fully post-quantum.

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 agreement 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-768 and ML-KEM-1024 conform to this API:

• ML-KEM-768 has encapsulation keys of size 1184 bytes, decapsulation keys of 2400 bytes, ciphertext size of 1088 bytes, and shared secrets of size 32 bytes
• ML-KEM-1024 has encapsulation keys of size 1568 bytes, decapsulation keys of 3168 bytes, ciphertext size of 1568 bytes, and shared secrets of size 32 bytes

Construction We define the KEMs as NamedGroups and sent in the supported_groups extension.

Negotiation Each method is its own solely post-quantum key agreement method, which are assigned their own identifiers, registered by IANA in the TLS Supported Groups registry:

Transmitting encapsulation keys and ciphertexts The encapsulation key and ciphertext values are directly encoded with fixed lengths as in ; the representation and length of elements MUST be fixed once the algorithm is fixed. In TLS 1.3 a KEM encapsulation key or KEM ciphertext is represented as a KeyShareEntry: ; } KeyShareEntry; ]]> These are transmitted in the extension_data fields of KeyShareClientHello and KeyShareServerHello extensions: ; } KeyShareClientHello; struct { KeyShareEntry server_share; } KeyShareServerHello; ]]> The client's shares are listed in descending order of client preference; the server selects one algorithm and sends its corresponding share. For the client's share, the key_exchange value contains the pk output of the corresponding ML-KEM NamedGroup's KeyGen algorithm. For the server's share, the key_exchange value contains the ct output of the corresponding ML-KEM NamedGroup's Encaps algorithm.

Shared secret calculation The 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, which is inserted into the TLS 1.3 key schedule in place of the (EC)DHE shared secret, as shown in .

Key schedule for key agreement HKDF-Extract = Early Secret | +-----> Derive-Secret(...) +-----> Derive-Secret(...) +-----> Derive-Secret(...) | v Derive-Secret(., "derived", "") | v shared_secret -> HKDF-Extract = Handshake Secret ^^^^^^^^^^^^^ | +-----> Derive-Secret(...) +-----> Derive-Secret(...) | v Derive-Secret(., "derived", "") | v 0 -> HKDF-Extract = Master Secret | +-----> Derive-Secret(...) +-----> Derive-Secret(...) +-----> Derive-Secret(...) +-----> Derive-Secret(...) ]]>

Discussion Larger encapsulation keys and/or ciphertexts The HybridKeyExchange struct in limits public keys and ciphertexts to 2^16-1 bytes; this is bounded by the same (2^16-1)-byte limit on the key_exchange field in the KeyShareEntry struct. All defined parameter sets for ML-KEM have encapsulation keys and ciphertexts that fall within the TLS constraints. Failures Some post-quantum key exchange algorithms, including ML-KEM, have non-zero probability of failure, meaning two honest parties may derive different shared secrets. This would cause a handshake failure. ML-KEM has a cryptographically small failure rate; implementers should be aware of the potential of handshake failure. Clients can retry if a failure is encountered.

Security Considerations IND-CCA The main security property for KEMs is indistinguishability under adaptive chosen ciphertext attack (IND-CCA2), which means that shared secret values should be indistinguishable from random strings even given the ability to have other arbitrary ciphertexts decapsulated. IND-CCA2 corresponds to security against an active attacker, and the public key / secret key pair can be treated as a long-term key or reused. A common design pattern for obtaining security under key reuse is to apply the Fujisaki-Okamoto (FO) transform or a variant thereof . Key exchange in TLS 1.3 is phrased in terms of Diffie-Hellman key exchange in a group. DH key exchange can be modeled as a KEM, with KeyGen corresponding to selecting an exponent x as the secret key and computing the public key g^x; encapsulation corresponding to selecting an exponent y, computing the ciphertext g^y and the shared secret g^(xy), and decapsulation as computing the shared secret g^(xy). See for more details of such Diffie-Hellman-based key encapsulation mechanisms. Diffie-Hellman key exchange, when viewed as a KEM, does not formally satisfy IND-CCA2 security, but is still safe to use for ephemeral key exchange in TLS 1.3, see e.g. . TLS 1.3 does not require that ephemeral public keys be used only in a single key exchange session; some implementations may reuse them, at the cost of limited forward secrecy. As a result, any KEM used in the manner described in this document MUST explicitly be designed to be secure in the event that the public key is reused. Finite-field and elliptic-curve Diffie-Hellman key exchange methods used in TLS 1.3 satisfy this criteria. For generic KEMs, this means satisfying IND-CCA2 security or having a transform like the Fujisaki-Okamoto transform applied. While it is recommended that implementations avoid reuse of KEM public keys, implementations that do reuse KEM public keys MUST ensure that the number of reuses of a KEM public key abides by any bounds in the specification of the KEM or subsequent security analyses. Implementations MUST NOT reuse randomness in the generation of KEM ciphertexts. Binding properties TLS 1.3's key schedule commits to the the ML-KEM encapsulation key and the encapsulated shared secret ciphertext as the key_exchange field as part of the key_share extension are populated with those values are included as part of the handshake messages, providing resilience against re-encapsulation attacks against KEMs used for key agreement. ML-KEM is MAL-BIND-K-PK-secure but only LEAK-BIND-K-CT and LEAK-BIND-K,PK-CT-secure, but because of the inclusion of the ML-KEM ciphertext in the TLS 1.3 key schedule there is no concern of malicious tampering (MAL) adversaries, not just honestly-generated but leaked key pairs (LEAK adversaries). The same is true of other KEMs with weaker binding properties, even if they were to have more constraints for secure use in contexts outside of TLS 1.3 handshake key agreement.These computational binding properties for KEMs were formalized in .

IANA Considerations This document requests/registers two new entries to the TLS Supported Groups registry, according to the procedures in .

Value:

0x0768 (please)

Description:

MLKEM768

DTLS-OK:

Y

Recommended:

N

Reference:

This document

Comment:

FIPS 203 version of ML-KEM-768

Value:

0x1024 (please)

Description:

MLKEM1024

DTLS-OK:

Y

Recommended:

N

Reference:

This document

Comment:

FIPS 203 version of ML-KEM-1024

References Normative References This document describes a scheme for hybrid public key encryption (HPKE). This scheme provides a variant of public key encryption of arbitrary-sized plaintexts for a recipient public key. It also includes three authenticated variants, including one that authenticates possession of a pre-shared key and two optional ones that authenticate possession of a key encapsulation mechanism (KEM) private key. HPKE works for any combination of an asymmetric KEM, key derivation function (KDF), and authenticated encryption with additional data (AEAD) encryption function. Some authenticated variants may not be supported by all KEMs. We provide instantiations of the scheme using widely used and efficient primitives, such as Elliptic Curve Diffie-Hellman (ECDH) key agreement, HMAC-based key derivation function (HKDF), and SHA2. This document is a product of the Crypto Forum Research Group (CFRG) in the IRTF. 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. 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 CISPA Helmholtz Center for Information Security CISPA Helmholtz Center for Information Security CISPA Helmholtz Center for Information Security Springer Science and Business Media LLC Springer Science and Business Media LLC Springer International Publishing This document describes a scheme for hybrid public key encryption (HPKE). This scheme provides a variant of public key encryption of arbitrary-sized plaintexts for a recipient public key. It also includes three authenticated variants, including one that authenticates possession of a pre-shared key and two optional ones that authenticate possession of a key encapsulation mechanism (KEM) private key. HPKE works for any combination of an asymmetric KEM, key derivation function (KDF), and authenticated encryption with additional data (AEAD) encryption function. Some authenticated variants may not be supported by all KEMs. We provide instantiations of the scheme using widely used and efficient primitives, such as Elliptic Curve Diffie-Hellman (ECDH) key agreement, HMAC-based key derivation function (HKDF), and SHA2. This document is a product of the Crypto Forum Research Group (CFRG) in the IRTF. University of Waterloo Cisco Systems University of Haifa Hybrid key exchange refers to using multiple key exchange algorithms simultaneously and combining the result with the goal of providing security even if all but one of the component algorithms is broken. It is motivated by transition to post-quantum cryptography. This document provides a construction for hybrid key exchange in the Transport Layer Security (TLS) protocol version 1.3. Discussion of this work is encouraged to happen on the TLS IETF mailing list tls@ietf.org or on the GitHub repository which contains the draft: https://github.com/dstebila/draft-ietf-tls-hybrid-design. Venafi sn3rd This document updates the changes to 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. This document updates the following RFCs: 3749, 5077, 4680, 5246, 5705, 5878, 6520, 7301, and 8447.

Acknowledgments Thanks to Douglas Stebila for consultation on the draft-ietf-tls-hybrid-design design.