Ed25519 Key Usage: Clarifications for Ed25519, Ed448, X25519, and X448 Algorithm Identifiers

Internet Engineering Task Force (IETF)                         S. Turner
Request for Comments: 9295                                         sn3rd
Updates: 8410                                               S. Josefsson
Category: Standards Track                                         SJD AB
ISSN: 2070-1721                                              D. McCarney
                                                             Square Inc.
                                                                  T. Ito
                                                         SECOM CO., LTD.
                                                          September 2022


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

Abstract

   This document updates RFC 8410 to clarify existing semantics, and
   specify missing semantics, for key usage bits when used in
   certificates that support the Ed25519, Ed448, X25519, and X448
   Elliptic Curve Cryptography algorithms.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9295.

Copyright Notice

   Copyright (c) 2022 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
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   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
   3.  New Section 5 for RFC 8410
   4.  Security Considerations
   5.  IANA Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Acknowledgments
   Authors' Addresses

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 keyUsage extension [RFC5280].  [RFC8410] did
   not define what values are not permissible, nor did it refer to
   keyEncipherment or dataEncipherment.  [Err5696] 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
     decipherOnly

   and any of the following MUST NOT be present:

     digitalSignature
     nonRepudiation
     keyEncipherment
     dataEncipherment
     keyCertSign
     cRLSign

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

     nonRepudiation
     digitalSignature
     cRLSign

   and any of the following MUST NOT be present:

     keyEncipherment
     dataEncipherment
     keyAgreement
     keyCertSign
     encipherOnly
     decipherOnly

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

     cRLSign

   and zero or more of the following:

     nonRepudiation
     digitalSignature

   and any of the following MUST NOT be present:

     keyEncipherment
     dataEncipherment
     keyAgreement
     encipherOnly
     decipherOnly

   and if the CRL issuer is also a certification authority, then the
   keyUsage extension MUST also contain:

     keyCertSign

   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
     cRLSign

   and any of the following MUST NOT be present:

     keyEncipherment
     dataEncipherment
     keyAgreement
     encipherOnly
     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.

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8410]  Josefsson, S. and J. Schaad, "Algorithm Identifiers for
              Ed25519, Ed448, X25519, and X448 for Use in the Internet
              X.509 Public Key Infrastructure", RFC 8410,
              DOI 10.17487/RFC8410, August 2018,
              <https://www.rfc-editor.org/info/rfc8410>.

6.2.  Informative References

   [Err5696]  RFC Errata, Erratum ID 5696, RFC 8410,
              <https://www.rfc-editor.org/errata/eid5696>.

Acknowledgments

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

Authors' Addresses

   Sean Turner
   sn3rd
   Email: sean@sn3rd.com


   Simon Josefsson
   SJD AB
   Email: simon@josefsson.org


   Daniel McCarney
   Square Inc.
   Email: daniel@binaryparadox.net


   Tadahiko Ito
   SECOM CO., LTD.
   Email: tadahiko.ito.public@gmail.com