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 athttps://www.rfc-editor.org/info/rfc9336.¶

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

[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 alsoused for Document Signing, technical or policy changes made to thecode signing and S/MIME ecosystem may cause unexpected behaviors orhave an adverse impact such as decreased cryptographicagility 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 forsigning a document that is not governed by the vendor.¶

This document defines an extended key purpose identifier for DocumentSigning.¶

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 certificateMUST only be used for one of the purposes indicated."[RFC5280] also notes that "[i]f multiple [key] purposes are indicatedthe application need not recognize all purposes indicated,as long as the intended purpose is present."¶

Document-Signing applicationsMAY require that the EKU extension be presentand that the id-kp-documentSigning be indicated in order for the certificate to be acceptableto that Document-Signing application.¶

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

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

  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, implementationsMAY perform the steps below during certificate validation.¶

The following procedure is used to examine the KeyPurposeId(s) included in theEKU 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 therelying party software, the certificate is acceptable.¶

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

  This procedure is intended to permit or prohibit the presence of acertain KeyPurposeId or the complete absence of KeyPurposeIds. It isoutside the scope of this document, but the relying party can permitor prohibit combinations of KeyPurposeIds, instead of a singleKeyPurposeId.A consideration onprohibiting combinations of KeyPurposeIds is described in theSecurity Considerations section of this document.If both Excluded KeyPurposeId and Permitted KeyPurposeId exist,the relying party or the relying party software processes each restrictionon Excluded KeyPurposeId first and then processes each restriction onPermitted KeyPurposeId.¶

  Excluded KeyPurposeId procedure:

  Excluded KeyPurposeId is aKeyPurposeId that the relying party or the relying party softwareprohibits. Examples of Excluded KeyPurposeId include the presence of theanyExtendedKeyUsage KeyPurposeId or the complete absence of the EKUextension in a certificate. If a KeyPurposeId of the certificatemeets the conditions set by the Excluded KeyPurposeId restriction,the relying party or the relying party software rejects thecertificate.¶

  Permitted KeyPurposeId procedure:

  Permitted KeyPurposeId is a KeyPurposeId that the relying party orthe relying party software accepts. Examples of PermittedKeyPurposeId include the presence of this general-purpose Document-SigningKeyPurposeId and/or the protocol-specificKeyPurposeIds that are relevant to Document Signing. If a KeyPurposeId of the certificate meets thecondition set by a Permitted KeyPurposeId restriction, thecertificate is acceptable. Otherwise, the relying party or the relyingparty software rejects the certificate.¶

When a single application has the capability to process various dataformats, the software may choose to make the excluded and permitteddecisions 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 authorityMUSTensure that the correct values for the EKU extension are inserted ineach certificate that is issued. Unless certificates are governed by avendor-specific PKI, certificates that indicate usagefor Document SigningMAY include the id-kp-documentSigning KeyPurposeId.The inclusion of the id-kp-documentSigning KeyPurposeId does notpreclude the inclusion of other KeyPurposeIds.¶

6.Security Considerations

The usage of the id-kp-documentSigning KeyPurposeId is to provide analternative to id-kp-emailProtection being used for non-email purposesand id-kp-codeSigning being used to sign objects other than binary code.This extended key purpose does not introduce new security risks butinstead reduces existing security risks by providing means to separateother extended key purposes used for communication protocols, which includeTLS (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 partyor the relying party software may additionally prohibit use of specificcombinations of KeyPurposeIds.¶

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

Introduction of this id-kp-documentSigning KeyPurposeId does notintroduce 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 inSection 3.1.¶

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 inAppendix A.¶

Appendix A.ASN.1 Module

The following ASN.1[X.680] module provides the complete definition of theDocument-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 thankRuss Housley for verifying the ASN.1 module.Additionally, we would like to thankCorey Bonnell,Wendy Brown,Russ Housley,Prachi Jain, andStefan Santesson for their comments.¶

Authors' Addresses