Ed25519 Key Usage: Clarifications for Ed25519, Ed448, X25519, and X448 Algorithm Identifiers
Clarifies key usage bits for modern elliptic curve algorithms in certificates.
Turner, S., Josefsson, S., McCarney, D., and T. Ito, "Clarifications for Ed25519, Ed448, X25519, and X448 Algorithm Identifiers", RFC 9295, DOI 10.17487/RFC9295, September 2022, <https://www.rfc-editor.org/info/rfc9295>.
@misc{rfc9295,
series = {Request for Comments},
number = 9295,
howpublished = {RFC 9295},
publisher = {RFC Editor},
doi = {10.17487/RFC9295},
url = {https://www.rfc-editor.org/info/rfc9295},
author = {S. Turner and S. Josefsson and D. McCarney and T. Ito},
title = {{Clarifications for Ed25519, Ed448, X25519, and X448 Algorithm Identifiers}},
pagetotal = 5,
year = 2022,
month = sep,
} RFC text
Contents
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
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
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
AI ELI5
On this page
§1Modern Elliptic Curve Keys
RFC 8410 introduced four elliptic curve cryptography algorithms: Ed25519 and Ed448 for signing, and X25519 and X448 for key agreement (secure handshakes). These are now widely used in certificates, but RFC 8410 was unclear about which key usage bits should appear on certificates for each algorithm.
Key usage bits are a standard way to label what a key is allowed to do: sign documents, verify other certificates, decrypt data, or agree on shared secrets. Without clear rules, implementations disagreed.
Keys have permission labels. Modern keys can sign, or they can agree on secrets. The old rules didn't clearly say which labels each key type needs.
§2Key Usage Rules
For X25519 and X448 (key agreement): the keyAgreement bit MUST be present. The bits encipherOnly and decipherOnly MAY also appear, but signing-related bits like digitalSignature and keyCertSign MUST NOT.
For Ed25519 and Ed448 signing keys, the rules depend on the certificate's role. End-entity certificates MUST have at least one of nonRepudiation, digitalSignature, or cRLSign, and MUST NOT have key-agreement bits. Certification authority certificates MUST have keyCertSign. CRL issuer certificates (which sign revocation lists) MUST have cRLSign.
If a key does key agreement, it must be labeled for that and nothing else. If a key signs, it gets signing labels, and the exact labels depend on what role the certificate plays.
§3Security Implications
These clarifications prevent misuse of keys. By enforcing that a key agreement key cannot also be used for signing, and vice versa, the RFC reduces the risk of a key being used for an unintended purpose. The separation between signing keys and key-agreement keys is fundamental to modern cryptography.
Keeping key permissions simple and clear prevents accidents. A key that does one job well is safer than a key that tries to do everything.