Clarifications for Ed25519, Ed448, X25519, and X448 Algorithm Identifiers

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Table of Contents

1. Introduction

[RFC8410] specifies the syntax and semantics for the Subject Public Key Information field in certificates that support Ed25519, Ed448, X25519, and X448 Elliptic Curve Cryptography (ECC) algorithms. As part of these semantics, it defines what combinations are permissible for the values of the key usage extension [RFC5280]. [RFC8410] did not define what values are not permissible nor did it refer to keyEncipherment or dataEncipherment. [ERRATA] has also been submitted to clarify that keyCertSign is always set in certification authority certificates. To address these changes, this document replaces Section 5 of [RFC8410] with Section 3.¶

2. Terminology

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. New Section 5 for RFC 8410

The intended application for the key is indicated in the keyUsage certificate extension.¶

If the keyUsage extension is present in a certificate that indicates id-X25519 or id-X448 in SubjectPublicKeyInfo, then the following MUST be present:¶

  keyAgreement;

one of the following MAY also be present:¶

  encipherOnly; or
  decipherOnly;

and the following MUST NOT be present:¶

  digitalSignature;
  nonRepudiation;
  keyEncipherment;
  dataEncipherment;
  keyCertSign; and
  cRLSign.

If the keyUsage extension is present in an end-entity or CRL issuer certificate that indicates id-Ed25519 or id-Ed448 in SubjectPublicKeyInfo, then the keyUsage extension MUST contain at least one of the following:¶

  nonRepudiation;
  digitalSignature; and
  cRLSign;

and the following MUST NOT be present:¶

  keyEncipherment;
  dataEncipherment;
  keyAgreement;
  keyCertSign;
  encipherOnly; and
  decipherOnly.

If the keyUsage extension is present in a certification authority certificate that indicates id-Ed25519 or id-Ed448 in SubjectPublicKeyInfo, then the keyUsage extension MUST contain:¶

  keyCertSign

and zero or more of the following:¶

  nonRepudiation;
  digitalSignature; and
  cRLSign;

and the following MUST NOT be present:¶

  keyEncipherment;
  dataEncipherment;
  keyAgreement;
  encipherOnly; and
  decipherOnly.

4. Security Considerations

This document introduces no new security considerations beyond those found in [RFC8410].¶

5. IANA Considerations

This document has no IANA actions.¶

Acknowledgments

We would like to thank Russ Housley, Mike Jenkins, and Corey Bonnell for the comments.¶

Authors' Addresses