| Internet-Draft | ietf-tls-mlkem | November 2025 |
| Connolly | Expires 7 May 2026 | [Page] |
This memo defines ML-KEM-512, ML-KEM-768, and ML-KEM-1024 as NamedGroups
and and registers IANA values in the TLS Supported Groups registry for use
in TLS 1.3 to achieve post-quantum (PQ) key establishment.¶
This note is to be removed before publishing as an RFC.¶
Status information for this document may be found at https://datatracker.ietf.org/doc/draft-ietf-tls-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/tlswg/draft-ietf-tls-mlkem.¶
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Copyright (c) 2025 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.¶
FIPS 203 (ML-KEM) [FIPS203] is a FIPS standard for post-quantum [RFC9794] key establishment via lattice-based key establishment mechanism (KEM). Having a purely post-quantum (not hybrid) key establishment option for TLS 1.3 is necessary for migrating beyond hybrids and for users that want or need post-quantum security without hybrids.¶
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.¶
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:¶
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¶
The KEMs are defined as NamedGroups, sent in the supported_groups
extension. Section 4.2.7 of [RFC8446]¶
Each parameter set of ML-KEM is assigned an identifier, registered by IANA in the TLS Supported Groups registry:¶
enum {
...,
/* ML-KEM Key Establishment Methods */
mlkem512(0x0200),
mlkem768(0x0201),
mlkem1024(0x0202)
...,
} NamedGroup;
¶
The public encapsulation key and ciphertext values are each directly encoded with fixed lengths as in [FIPS203].¶
In TLS 1.3 a KEM public encapsulation key pk or ciphertext ct is
represented as a KeyShareEntry Section 4.2.8 of [RFC8446]:¶
struct {
NamedGroup group;
opaque key_exchange<1..2^16-1>;
} KeyShareEntry;
¶
These are transmitted in the extension_data fields of
KeyShareClientHello and KeyShareServerHello extensions:¶
struct {
KeyShareEntry client_shares<0..2^16-1>;
} KeyShareClientHello;
struct {
KeyShareEntry server_share;
} KeyShareServerHello;
¶
The KeyShareClientHello includes a list of KeyShareEntry structs that
represent the key establishment algorithms the client supports. For each
parameter of ML-KEM the client supports, the corresponding KeyShareEntry
consists of a NamedGroup that indicates the appropriate parameter, and a
key_exchange value that is the pk output of the KeyGen algorithm.¶
For the client's share, the key_exchange value contains the pk
output of the corresponding KEM NamedGroup's KeyGen algorithm.¶
For the server's share, the key_exchange value contains the ct
output of the corresponding KEM NamedGroup's Encaps algorithm.¶
For all parameter sets, 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 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.¶
The main security property for KEMs is indistinguishability under adaptive chosen ciphertext attack (IND-CCA), which means that shared secret values should be indistinguishable from random strings even given the ability to have other arbitrary ciphertexts decapsulated. IND-CCA corresponds to security against an active attacker, and the public key / secret key pair can be treated as a long-term key or reused. ML-KEM satisfies IND-CCA security in the random oracle model [KYBERV].¶
TLS 1.3 does not prohibit key re-use; some implementations may use the same ephemeral public key for more than one key establishment at the cost of limited forward secrecy. Care must be taken to ensure that keys are only re-used if the algorithms from which they are derived are designed to be secure under key-reuse. ML-KEM's IND-CCA security satisfies this requirement such that the public key/secret key pair can be used long-term or re-used without compromising the security of the keys. However, it is still recommended that implementations avoid re-use of any keys (including ML-KEM keys) to ensure perfect forward secrecy.¶
Implementations MUST NOT reuse randomness in the generation of ML-KEM ciphertexts.¶
TLS 1.3's key schedule commits to the the ML-KEM encapsulation key and the
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 establishment [CDM23].¶
This document requests/registers three new entries to the TLS Named Group (or Supported Group) registry, according to the procedures in Section 6 of [tlsiana].¶
0x0200¶
MLKEM512¶
Y¶
N¶
This document¶
FIPS 203 version of ML-KEM-512¶
0x0201¶
MLKEM768¶
Y¶
N¶
This document¶
FIPS 203 version of ML-KEM-768¶
0x0202¶
MLKEM1024¶
Y¶
N¶
This document¶
FIPS 203 version of ML-KEM-1024¶
Thanks to Douglas Stebila for consultation on the draft-ietf-tls-hybrid-design design, and to Scott Fluhrer, Eric Rescorla, and Rebecca Guthrie for reviews.¶