Internet-Draft ML-KEM in Certificates March 2024
Turner, et al. Expires 5 September 2024 [Page]
Intended Status:
Standards Track
S. Turner
P. Kampanakis
J. Massimo
B. Westerbaan

Internet X.509 Public Key Infrastructure - Algorithm Identifiers for Module-Lattice-Based Key-Encapsulation Mechanism (ML-KEM)


Module-Lattice-Based Key-Encapsulation Mechanism (ML-KEM), also known as Kyber, is a key-encapsulation mechanism (KEM). This document specifies algorithm identifiers and ASN.1 encoding format for ML-KEM in public key certificates. The encoding for public and private keys are also provided.

[EDNOTE: This document is not expected to be finalized before the NIST PQC Project has standardized PQ algorithms. This specification will use object identifiers for the new algorithms that are assigned by NIST, and will use placeholders until these are released.]

About This Document

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Table of Contents

1. Introduction

Module-Lattice-Based Key-Encapsulation Mechanism (ML-KEM), also known as Kyber, is a key-encapsulation mechanism (KEM) standardized by the US NIST PQC Project [DRAFTFIPS203]. This document specifies the use of the ML-KEM algorithm at three security levels: ML-KEM-512, ML-KEM-768, and ML-KEM-1024, in X.509 public key certificates; see [RFC5280]. Public and private key encodings are also specified.

1.1. ASN.1 and ML-KEM Identifiers

An ASN.1 module [X680] is included for reference purposes. Note that as per [RFC5280], certificates use the Distinguished Encoding Rules; see [X690]. Also note that NIST defined the object identifiers for the ML-KEM algorithms in an ASN.1 modulle; see (TODO insert reference).

1.2. Applicability Statement

ML-KEM certificates are used in protocols where the public key is used to generate and encapsulate a shared secret used to derive a symmetric key used to encrypt a payload; see [I-D.ietf-lamps-kyber]. To be used in TLS, ML-KEM certificates could only be used as end-entity identity certificates and would require significant updates to the protocol; see [I-D.celi-wiggers-tls-authkem].

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. Algorithm Identifiers

Certificates conforming to [RFC5280] can convey a public key for any public key algorithm. The certificate indicates the algorithm through an algorithm identifier. An algorithm identifier consists of an object identifier and optional parameters.

The AlgorithmIdentifier type, which is included herein for convenience, is defined as follows:

  AlgorithmIdentifier{ALGORITHM-TYPE, ALGORITHM-TYPE:AlgorithmSet} ::=
      algorithm   ALGORITHM-TYPE.&id({AlgorithmSet}),
      parameters  ALGORITHM-TYPE.
                    &Params({AlgorithmSet}{@algorithm}) OPTIONAL

The fields in AlgorithmIdentifier have the following meanings:

Section 4 includes object identifiers for ML-KEM-512, ML-KEM-768, and ML-KEM-1024. For all of these OIDs, the parameters MUST be absent.

4. ML-KEM Public Key Identifiers

The AlgorithmIdentifier for a ML-KEM public key MUST use one of the id-alg-ml-kem object identifiers listed below, based on the security level. The parameters field of the AlgorithmIdentifier for the ML-KEM public key MUST be absent.

When any of the ML-KEM AlgorithmIdentifier appears in the SubjectPublicKeyInfo field of an X.509 certificate, the key usage certificate extension MUST only contain keyEncipherment Section of [RFC5280].

  pk-ml-kem-512 PUBLIC-KEY ::= {
    IDENTIFIER id-alg-ml-kem-512
    -- KEY no ASN.1 wrapping --
    PARAMS ARE absent
      { keyEncipherment }
    --- PRIVATE-KEY no ASN.1 wrapping --

  pk-ml-kem-768 PUBLIC-KEY ::= {
    IDENTIFIER id-alg-ml-kem-768
    -- KEY no ASN.1 wrapping --
    PARAMS ARE absent
      { keyEncipherment }
    --- PRIVATE-KEY no ASN.1 wrapping --

  pk-ml-kem-1024 PUBLIC-KEY ::= {
    IDENTIFIER id-alg-ml-kem-1024
    -- KEY no ASN.1 wrapping --
    PARAMS ARE absent
      { keyEncipherment }
    --- PRIVATE-KEY no ASN.1 wrapping --

5. Subject Public Key Fields

In the X.509 certificate, the subjectPublicKeyInfo field has the SubjectPublicKeyInfo type, which has the following ASN.1 syntax:

  SubjectPublicKeyInfo {PUBLIC-KEY: IOSet} ::= SEQUENCE {
      algorithm        AlgorithmIdentifier {PUBLIC-KEY, {IOSet}},
      subjectPublicKey BIT STRING

The fields in SubjectPublicKeyInfo have the following meaning:

The following is an example of a ML-KEM-512 public key encoded using the textual encoding defined in [RFC7468]:

  -----BEGIN PUBLIC KEY-----
  TODO insert example public key
  -----END PUBLIC KEY-------

6. Private Key Format

"Asymmetric Key Packages" [RFC5958] describes how to encode a private key in a structure that both identifies what algorithm the private key is for and allows for the public key and additional attributes about the key to be included as well. For illustration, the ASN.1 structure OneAsymmetricKey is replicated below. The algorithm-specific details of how a private key is encoded are left for the document describing the algorithm itself.

  OneAsymmetricKey ::= SEQUENCE {
    version                  Version,
    privateKeyAlgorithm      SEQUENCE {
    algorithm                PUBLIC-KEY.&id({PublicKeySet}),
    parameters               PUBLIC-KEY.&Params({PublicKeySet}
    privateKey               OCTET STRING (CONTAINING
    attributes           [0] Attributes OPTIONAL,
    [[2: publicKey       [1] BIT STRING (CONTAINING

  PrivateKey ::= OCTET STRING

  PublicKey ::= BIT STRING

For the keys defined in this document, the private key is always an opaque byte sequence. The ASN.1 type PqckemPrivateKey is defined in this document to hold the byte sequence. Thus, when encoding a OneAsymmetricKey object, the private key is wrapped in a PqckemPrivateKey object and wrapped by the OCTET STRING of the "privateKey" field.

  PqckemPrivateKey ::= OCTET STRING

The following is an example of a ML-KEM-512 private key encoded using the textual encoding defined in [RFC7468]:

  TODO iser example private key
  -----END PRIVATE KEY-------

The following example, in addition to encoding the ML-KEM-512 private key, has an attribute included as well as the public key. As with the prior example, the textual encoding defined in [RFC7468] is used:

  TODO insert example private key with attribute
  -----END PRIVATE KEY-------

7. ASN.1 Module

TODO ASN.1 Module

8. Security Considerations

The Security Considerations section of [RFC5280] applies to this specification as well.

9. IANA Considerations

This document will have some IANA actions.

10. References

10.1. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, , <>.
Hoffman, P. and J. Schaad, "New ASN.1 Modules for the Public Key Infrastructure Using X.509 (PKIX)", RFC 5912, DOI 10.17487/RFC5912, , <>.
Turner, S., "Asymmetric Key Packages", RFC 5958, DOI 10.17487/RFC5958, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
ITU-T, "Information technology - Abstract Syntax Notation One (ASN.1): Specification of basic notation", ITU-T Recommendation X.680, ISO/IEC 8824-1:2021, , <>.
ITU-T, "Information technology - Abstract Syntax Notation One (ASN.1): ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)", ITU-T Recommendation X.690, ISO/IEC 8825-1:2021, , <>.

10.2. Informative References

National Institute of Standards and Technology (NIST), "DRAFT Module-Lattice-based Key-Encapsulation Mechanism Standard", FIPS PUB 203, , <>.
Wiggers, T., Celi, S., Schwabe, P., Stebila, D., and N. Sullivan, "KEM-based Authentication for TLS 1.3", Work in Progress, Internet-Draft, draft-celi-wiggers-tls-authkem-02, , <>.
Prat, J. and M. Ounsworth, "Use of KYBER in the Cryptographic Message Syntax (CMS)", Work in Progress, Internet-Draft, draft-ietf-lamps-kyber-00, , <>.
Josefsson, S. and S. Leonard, "Textual Encodings of PKIX, PKCS, and CMS Structures", RFC 7468, DOI 10.17487/RFC7468, , <>.


TODO acknowledge.

Authors' Addresses

Sean Turner
Panos Kampanakis
Jake Massimo
Bas Westerbaan