Internet Engineering Task Force (IETF) T. Ito

Internet Engineering Task Force (IETF) T. Ito
Request for Comments: 9336 SECOM CO., LTD.
Category: Standards Track T. Okubo
ISSN: 2070-1721 DigiCert, Inc.
S. Turner
sn3rd
December 2022

X.509 Certificate General-Purpose Extended Key Usage (EKU) for Document
Signing

Abstract

RFC 5280 specifies several extended key purpose identifiers
(KeyPurposeIds) for X.509 certificates. This document defines a
general-purpose Document-Signing KeyPurposeId for inclusion in the
Extended Key Usage (EKU) extension of X.509 public key certificates.
Document-Signing applications may require that the EKU extension be
present and that a Document-Signing KeyPurposeId be indicated in
order for the certificate to be acceptable to that Document-Signing
application.

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/rfc9336.

Copyright Notice

Copyright (c) 2022 IETF Trust and the persons identified as the
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Table of Contents

1. Introduction
2. Conventions and Definitions
3. Extended Key Purpose for Document Signing
3.1. Including the Extended Key Purpose for Document Signing in
Certificates
4. Using the Extended Key Purpose for Document Signing in a
Certificate
5. Implications for a Certification Authority
6. Security Considerations
7. IANA Considerations
8. References
8.1. Normative References
8.2. Informative References
Appendix A. ASN.1 Module
Acknowledgments
Authors' Addresses

1. Introduction

[RFC5280] specifies several extended key purpose identifiers
(KeyPurposeIds) for X.509 certificates. In addition, the IANA
repository "SMI Security for PKIX Extended Key Purpose" [RFC7299]
includes a number of KeyPurposeIds. While usage of the
anyExtendedKeyUsage KeyPurposeId is bad practice for publicly trusted
certificates, there is no public and general KeyPurposeId explicitly
assigned for Document Signing. The current practice is to use id-kp-
emailProtection, id-kp-codeSigning, or a vendor-defined KeyPurposeId
for general Document-Signing purposes.

In circumstances where code signing and S/MIME certificates are also
used for Document Signing, technical or policy changes made to the
code signing and S/MIME ecosystem may cause unexpected behaviors or
have an adverse impact such as decreased cryptographic agility on the
Document-Signing ecosystem and vice versa.

Vendor-defined KeyPurposeIds that are used in a PKI governed by the
vendor or a group of vendors pose no interoperability concern.
Appropriating, or misappropriating as the case may be, KeyPurposeIDs
for use outside of their originally intended vendor or group of
vendors controlled environment can introduce problems, the impact of
which is difficult to determine.

Therefore, it is not favorable to use a vendor-defined KeyPurposeId
for signing a document that is not governed by the vendor.

This document defines an extended key purpose identifier for Document
Signing.

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. Extended Key Purpose for Document Signing

This specification defines the KeyPurposeId id-kp-documentSigning.

As described in [RFC5280], "[i]f the [Extended Key Usage] extension
is present, then the certificate MUST only be used for one of the
purposes indicated." [RFC5280] also notes that "[i]f multiple [key]
purposes are indicated the application need not recognize all
purposes indicated, as long as the intended purpose is present."

Document-Signing applications MAY require that the EKU extension be
present and that the id-kp-documentSigning be indicated in order for
the certificate to be acceptable to that Document-Signing
application.

The term "Document Signing" in this document refers to digitally
signing contents that are consumed by people. To be more precise,
contents are intended to be shown to a person in a printable or
displayable form by means of services or software, rather than
processed by machines.

3.1. Including the Extended Key Purpose for Document Signing in
Certificates

[RFC5280] specifies the EKU X.509 certificate extension for use on
the Internet. The extension indicates one or more purposes for which
the certified public key is valid. The EKU extension can be used in
conjunction with the key usage extension, which indicates the set of
basic cryptographic operations for which the certified key may be
used.

The EKU extension syntax is repeated here for convenience:

ExtKeyUsageSyntax ::= SEQUENCE SIZE (1..MAX) OF KeyPurposeId

KeyPurposeId ::= OBJECT IDENTIFIER

As described in [RFC5280], the EKU extension may, at the option of
the certificate issuer, be either critical or non-critical.

This specification defines the KeyPurposeId id-kp-documentSigning.
Inclusion of this KeyPurposeId in a certificate indicates that the
public key encoded in the certificate has been certified to be used
for cryptographic operations on contents that are consumed by people.

id-kp OBJECT IDENTIFIER ::=
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) 3 }

id-kp-documentSigning OBJECT IDENTIFIER ::= { id-kp 36 }

4. Using the Extended Key Purpose for Document Signing in a Certificate

Our intended use case is people consuming the contents of signed
documents. To be more precise, contents are intended to be shown to
a person in a printable or displayable form by means of services or
software, rather than processed by machines. The digital signature
on the contents is to indicate to the recipient of the contents that
the content has not changed since it was signed by the identity
indicated as the subject of the certificate. To validate the digital
signature that is signed on contents intended to be consumed by
people, implementations MAY perform the steps below during
certificate validation.

The following procedure is used to examine the KeyPurposeId(s)
included in the EKU extension. Restrictions on EKU is derived and
implemented from (or configured with) the policy to which the
implementation conforms.

* If there are no restrictions set for the relying party and the
relying party software, the certificate is acceptable.

* If there are restrictions set for the relying party and relying
party software, then process the KeyPurposeId(s) as described
below.

This procedure is intended to permit or prohibit the presence of a
certain KeyPurposeId or the complete absence of KeyPurposeIds. It
is outside the scope of this document, but the relying party can
permit or prohibit combinations of KeyPurposeIds, instead of a
single KeyPurposeId. A consideration on prohibiting combinations
of KeyPurposeIds is described in the Security Considerations
section of this document. If both Excluded KeyPurposeId and
Permitted KeyPurposeId exist, the relying party or the relying
party software processes each restriction on Excluded KeyPurposeId
first and then processes each restriction on Permitted
KeyPurposeId.

Excluded KeyPurposeId procedure:
Excluded KeyPurposeId is a KeyPurposeId that the relying party
or the relying party software prohibits. Examples of Excluded
KeyPurposeId include the presence of the anyExtendedKeyUsage
KeyPurposeId or the complete absence of the EKU extension in a
certificate. If a KeyPurposeId of the certificate meets the
conditions set by the Excluded KeyPurposeId restriction, the
relying party or the relying party software rejects the
certificate.

Permitted KeyPurposeId procedure:
Permitted KeyPurposeId is a KeyPurposeId that the relying party
or the relying party software accepts. Examples of Permitted
KeyPurposeId include the presence of this general-purpose
Document-Signing KeyPurposeId and/or the protocol-specific
KeyPurposeIds that are relevant to Document Signing. If a
KeyPurposeId of the certificate meets the condition set by a
Permitted KeyPurposeId restriction, the certificate is
acceptable. Otherwise, the relying party or the relying party
software rejects the certificate.

When a single application has the capability to process various data
formats, the software may choose to make the excluded and permitted
decisions separately in accordance with the format it is handling
(e.g., TEXT and PDF).

5. Implications for a Certification Authority

The procedures and practices employed by a certification authority
MUST ensure that the correct values for the EKU extension are
inserted in each certificate that is issued. Unless certificates are
governed by a vendor-specific PKI, certificates that indicate usage
for Document Signing MAY include the id-kp-documentSigning
KeyPurposeId. The inclusion of the id-kp-documentSigning
KeyPurposeId does not preclude the inclusion of other KeyPurposeIds.

6. Security Considerations

The usage of the id-kp-documentSigning KeyPurposeId is to provide an
alternative to id-kp-emailProtection being used for non-email
purposes and id-kp-codeSigning being used to sign objects other than
binary code. This extended key purpose does not introduce new
security risks but instead reduces existing security risks by
providing means to separate other extended key purposes used for
communication protocols, which include TLS (id-kp-clientAuth) and
S/MIME (id-kp-emailProtection), in order to minimize the risk of
cross-protocol attacks.

To reduce the risk of specific cross-protocol attacks, the relying
party or the relying party software may additionally prohibit use of
specific combinations of KeyPurposeIds.

While a specific protocol or signing scheme may choose to come up
with their own KeyPurposeIds, some may not have significant motive or
resources to set up and manage their own KeyPurposeIds. This
general-purpose Document-Signing KeyPurposeId may be used as a stop-
gap for those that intend to define their own Document-Signing
KeyPurposeId or those who do not intend to set up a KeyPurposeId but
still would like to distinguish Document Signing from other usages.

Introduction of this id-kp-documentSigning KeyPurposeId does not
introduce any new security or privacy concerns.

7. IANA Considerations

IANA has registered the following OID in the "SMI Security for PKIX
Extended Key Purpose" registry (1.3.6.1.5.5.7.3). This OID is
defined in Section 3.1.

+=========+=======================+============+
| Decimal | Description | References |
+=========+=======================+============+
| 36 | id-kp-documentSigning | RFC 9336 |
+---------+-----------------------+------------+

Table 1

IANA has also registered the following ASN.1 [X.680] module OID in
the "SMI Security for PKIX Module Identifier" registry
(1.3.6.1.5.5.7.0). This OID is defined in Appendix A.

+=========+====================+============+
| Decimal | Description | References |
+=========+====================+============+
| 104 | id-mod-docsign-eku | RFC 9336 |
+---------+--------------------+------------+

Table 2

8. References

8.1. 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>.

[X.680] ITU-T, "Information technology - Abstract Syntax Notation
One (ASN.1): Specification of basic notation", ITU-T
Recommendation X.680, February 2021.

8.2. Informative References

[RFC7299] Housley, R., "Object Identifier Registry for the PKIX
Working Group", RFC 7299, DOI 10.17487/RFC7299, July 2014,
<https://www.rfc-editor.org/info/rfc7299>.

Appendix A. ASN.1 Module

The following ASN.1 [X.680] module provides the complete definition
of the Document-Signing KeyPurposeId.

DocSignEKU { iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-docsign-eku(104) }

DEFINITIONS EXPLICIT TAGS ::=

BEGIN

-- EXPORTS ALL --

-- IMPORTS NOTHING --

-- OID Arc --

id-kp OBJECT IDENTIFIER ::= {
iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) kp(3) }

-- Document-Signing Extended Key Usage --

id-kp-documentSigning OBJECT IDENTIFIER ::= { id-kp 36 }

END

Acknowledgments

We would like to thank Russ Housley for verifying the ASN.1 module.
Additionally, we would like to thank Corey Bonnell, Wendy Brown, Russ
Housley, Prachi Jain, and Stefan Santesson for their comments.

Authors' Addresses

Tadahiko Ito
SECOM CO., LTD.
Email: [email protected]

Tomofumi Okubo
DigiCert, Inc.
Email: [email protected]

Sean Turner
sn3rd
Email: [email protected]