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

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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.¶

This specification includes conventions for the subjectPublicKeyInfo field within Internet X.509 certificates [RFC5280], like [RFC3279] did for classic cryptography and [RFC5480] did for elliptic curve cryptography. 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. 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} ::=
    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].¶

  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. The algorithms defined in this document always encode the public key as TODO pick format e.g., exact multiple of 8 bits?.¶

Appendix A.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

An ML-KEM private key is encoded by storing its 64-octet seed in the privateKey field as follows.¶

[FIPS203] specifies two formats for an ML-KEM private key: a 64-octet seed and an (expanded) private key, which is referred to as the decapsulation key. The expanded private key (and public key) is computed from the seed using ML-KEM.KeyGen_internal(d,z) (algorithm 16) using the first 32 octets as d and the remaining 32 octets as z.¶

"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 64-octet seed.¶

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

6. ASN.1 Module

TODO ASN.1 Module¶

7. Security Considerations

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

8. IANA Considerations

This document will have some IANA actions.¶

9. References

Appendix A. 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