Internet X.509 Public Key Infrastructure - Algorithm Identifiers for the Module-Lattice-Based Key-Encapsulation Mechanism (ML-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."¶

This Internet-Draft will expire on 24 April 2025.¶

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. 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.¶

Table of Contents

1. Introduction

The Module-Lattice-Based Key-Encapsulation Mechanism (ML-KEM) standardized in [FIPS203] is a quantum-resistant key-encapsulation mechanism (KEM) standardized by the US National Institute of Standards and Technology (NIST) PQC Project [NIST-PQC]. Prior to standardization, the mechanism was known as Kyber. ML-KEM and Kyber are not compatible. This document specifies the use of ML-KEM in Public Key Infrastructure X.509 (PKIX) certificates [RFC5280] at three security levels: ML-KEM-512, ML-KEM-768, and ML-KEM-1024, using object identifiers assigned by NIST. The private key format is also specified.¶

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

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

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

The fields in AlgorithmIdentifier have the following meanings:¶

• algorithm identifies the cryptographic algorithm with an object identifier.¶

• parameters, which are optional, are the associated parameters for the algorithm identifier in the algorithm field.¶

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 4.2.1.3 of [RFC5280].¶

  nistAlgorithms OBJECT IDENTIFIER ::= { joint-iso-ccitt(2)
    country(16) us(840) organization(1) gov(101) csor(3)
    nistAlgorithm(4) }

  kems OBJECT IDENTIFIER ::= { nistAlgorithms 4 }

  id-alg-ml-kem-512 OBJECT IDENTIFIER ::= { kems 1 }

  id-alg-ml-kem-768 OBJECT IDENTIFIER ::= { kems 2 }

  id-alg-ml-kem-1024 OBJECT IDENTIFIER ::= { kems 3 }

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

    ML-KEM-PublicKey ::= OCTET STRING

    ML-KEM-PrivateKey ::= OCTET STRING

No additional encoding of the ML-KEM public key value is applied in the SubjectPublicKeyInfo field of an X.509 certificate [RFC5280]. However, whenever the ML-KEM public key value appears outside of a certificate, it MAY be encoded as an OCTET STRING.¶

No additional encoding of the ML-KEM private key value is applied in the PrivateKeyInfo field of an Asymmetric Key Package [RFC5958]. However, whenever the ML-KEM private key value appears outside of a Asymmetric Key Package, it MAY be encoded as an OCTET STRING.¶

4. 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:¶

• algorithm is the algorithm identifier and parameters for the public key (see above).¶

• subjectPublicKey contains the byte stream of the public key.¶

Appendix B.1 contains an example of an id-alg-ml-kem-768 public key encoded using the textual encoding defined in [RFC7468].¶

5. 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}
                               {@privateKeyAlgorithm.algorithm})
                                  OPTIONAL}
    privateKey               OCTET STRING (CONTAINING
                               PUBLIC-KEY.&PrivateKey({PublicKeySet}
                                 {@privateKeyAlgorithm.algorithm})),
    attributes           [0] Attributes OPTIONAL,
    ...,
    [[2: publicKey       [1] BIT STRING (CONTAINING
                               PUBLIC-KEY.&Params({PublicKeySet}
                                 {@privateKeyAlgorithm.algorithm})
                                 OPTIONAL,
    ...
  }

When used in a OneAsymmetricKey type, the privateKey OCTET STRING contains the raw octet string encoding of the private key.¶

When an ML-KEM public key is included in a OneAsymmetricKey type, it is encoded in the same manner as in a SubjectPublicKeyInfo type. That is, the publicKey BIT STRING contains the raw octet string encoding of the public key.¶

Appendix B.2 contains an example of an id-alg-ml-kem-768 private key encoded using the textual encoding defined in [RFC7468].¶

6. Security Considerations

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

7. IANA Considerations

For the ASN.1 Module in Appendix A, IANA is requested to assign an object identifier (OID) for the module identifier (TBD2) with a Description of "id-mod-x509-ml-kem-2024". The OID for the module should be allocated in the "SMI Security for PKIX Module Identifier" registry (1.3.6.1.5.5.7.0).¶

8. References

Appendix A. ASN.1 Module

This appendix includes the ASN.1 module [X680] for the ML-KEM. Note that as per [RFC5280], certificates use the Distinguished Encoding Rules; see [X690]. This module imports objects from [RFC5912] and [RFC9629].¶

<CODE BEGINS>
X509-ML-KEM-2024
{ iso(1) identified-organization(3) dod(6)
  internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
  id-mod-x509-ml-kem-2024(TBD2) }

DEFINITIONS IMPLICIT TAGS ::= BEGIN

EXPORTS ALL;

IMPORTS
  PUBLIC-KEY
    FROM AlgorithmInformation-2009  -- [RFC 5912]
      { iso(1) identified-organization(3) dod(6) internet(1)
        security(5) mechanisms(5) pkix(7) id-mod(0)
        id-mod-algorithmInformation-02(58) }

  KEM-ALGORITHM
    FROM KEMAlgorithmInformation-2023  -- [RFC 9629]
      { iso(1) identified-organization(3) dod(6) internet(1)
        security(5) mechanisms(5) pkix(7) id-mod(0)
        id-mod-kemAlgorithmInformation-2023(109) };

--
-- ML-KEM Identifiers
--

nistAlgorithms OBJECT IDENTIFIER ::= { joint-iso-ccitt(2)
  country(16) us(840) organization(1) gov(101) csor(3)
  nistAlgorithm(4) }

kems OBJECT IDENTIFIER ::= { nistAlgorithms 4 }

id-alg-ml-kem-512 OBJECT IDENTIFIER ::= { kems 1 }

id-alg-ml-kem-768 OBJECT IDENTIFIER ::= { kems 2 }

id-alg-ml-kem-1024 OBJECT IDENTIFIER ::= { kems 3 }

  --
  -- Public Key Algorithms
  --
  -- To use the following with the PKIX1Explicit-2009 [RFC5912], replace
  -- the PublicKeyAlgorithms therein with the following:
  --
  -- PublicKeyAlgorithms PUBLIC-KEY ::= {
  --   PKIXAlgs-2009.PublicKeys,
  --   ...,
  --   PKIX1-PSS-OAEP-Algorithms-2009.PublicKeys,
  --   X509-ML-KEM-2024.PublicKeys }

  --
  -- Public Key (pk-) Algorithms
  --

PublicKeys PUBLIC-KEY ::= {
  -- This expands PublicKeys from RFC 5912
  pk-ml-kem-512 |
  pk-ml-kem-768 |
  pk-ml-kem-1024,
  ...
  }

--
-- ML-KEM Public Keys
--

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

END

<CODE ENDS>

Appendix B. Examples

This appendix contains examples of ML-KEM public keys, private keys and certificates.¶

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

  -----BEGIN PRIVATE KEY-----
  TODO insert example private key
  -----END PRIVATE KEY-------

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

  TODO insert ASN.1 Pretty Print

  -----BEGIN CERTIFICATE-----
  TODO Certificate
  -----END CERTIFICATE-------

Acknowledgments

TODO acknowledge.¶

Authors' Addresses