



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
   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.  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: tadahiko.ito.public@gmail.com


   Tomofumi Okubo
   DigiCert, Inc.
   Email: tomofumi.okubo+ietf@gmail.com


   Sean Turner
   sn3rd
   Email: sean@sn3rd.com
