Internet X.509 Public Key Infrastructure - Algorithm Identifiers for Kyber

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Table of Contents

1. Introduction

The US NIST PQC Project has selected the Kyber algorithm as winner of their PQC Project [PQCProj]. This algorithm is a Key Encapsulation Mechanism(KEM). NIST has also defined object identifiers for these algorithms (TODO insert reference).¶

This document specifies the use of the Kyber algorithm in X.509 public key certifiates, see [RFC5280]. It also specifies private key encoding. An ASN.1 module is included for reference purposes.¶

These certificates could be used as Issuers in CMS where the public key is used to encapsulate a shared secret used to derive a symmetric key used to encrypt content in CMS [EDNOTE: Add reference draft-perret-prat-lamps-cms-pq-kem]. To be used in TLS, these certificates could only be used as end-entity identity certificates and would require significant updates to the protocol [EDNOTE: Add reference draft-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  ::=  SEQUENCE  {
       algorithm   OBJECT IDENTIFIER,
       parameters  ANY DEFINED BY algorithm OPTIONAL
   }

The fields in AlgorithmIdentifier have the following meanings:¶

• algorithm identifies the cryptographic algorithm with an object identifier. XXX such OIDs are defined in Sections Section 4.¶
• parameters, which are optional, are the associated parameters for the algorithm identifier in the algorithm field.¶

In this document, TODO (specify number) new OIDs for identifying the different algorithm and parameter pairs. For all of the object identifiers, the parameters MUST be absent.¶

It is possible to find systems that require the parameters to be present. This can be due to either a defect in the original 1997 syntax or a programming error where developers never got input where this was not true. The optimal solution is to fix these systems; where this is not possible, the problem needs to be restricted to that subsystem and not propagated to the Internet.¶

4. Candidate TBD1

TODO insert object-identifiers for Kyber-768, Kyber-1024 etc.¶

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  ::=  SEQUENCE  {
      algorithm         AlgorithmIdentifier,
      subjectPublicKey  BIT STRING
  }

The fields in SubjectPublicKeyInfo have the following meanings:¶

• 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 Kyber TBD1 public key encoded using the textual encoding defined in [RFC7468].¶

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

6. Key Usage Bits

The intended application for the key is indicated in the keyUsage certificate extension; see [RFC5280, section 4.2.1.3].¶

If the keyUsage extension is present in a certificate that indicates Kyber TBD1 in SubjectPublicKeyInfo, then the following MUST be present:¶

  keyEncipherment;

7. 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      PrivateKeyAlgorithmIdentifier,
      privateKey               PrivateKey,
      attributes           [0] IMPLICIT Attributes OPTIONAL,
      ...,
      [[2: publicKey       [1] IMPLICIT PublicKey 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 Kyber TBD 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 Kyber TBD 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-------

8. ASN.1 Module

TODO ASN.1 Module¶

9. Security Considerations

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

[EDNOTE: Discuss side-channels for Kyber TBD1.]¶

10. IANA Considerations

This document will have some IANA actions.¶

Acknowledgments

TODO acknowledge.¶

Authors' Addresses