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

Module-Lattice-Based Key-Encapsulation Mechanism (ML-KEM), previously known as known as Kyber, is a quantum-resistant key-encapsulation mechanism (KEM) standardized by the US NIST PQC Project [NIST-PQC] in [DRAFTFIPS203]. 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 --
    }

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?.¶

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-------

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,
    ...
  }

  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]:¶

  -----BEGIN PRIVATE KEY-----
  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:¶

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

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

Acknowledgments

TODO acknowledge.¶

Authors' Addresses