X.509 Certificate General-Purpose Extended Key Usage (EKU) for Document Signing
draft-ietf-lamps-documentsigning-eku-06

LAMPS Working Group                                               T. Ito
Internet-Draft                                           SECOM CO., LTD.
Intended status: Standards Track                                T. Okubo
Expires: 2 April 2023                                     DigiCert, Inc.
                                                               S. Turner
                                                                   sn3rd
                                                       29 September 2022

  General Purpose Extended Key Usage (EKU) for Document Signing X.509
                              Certificates
                draft-ietf-lamps-documentsigning-eku-06

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.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-lamps-documentsigning-
   eku/.

   Discussion of this document takes place on the Limited Additional
   Mechanisms for PKIX and SMIME (LAMPS) Working Group mailing list
   (mailto:spasm@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/spasm/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/spasm/.

   Source for this draft and an issue tracker can be found at
   https://github.com/lamps-wg/documentsigning-eku.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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Table of Contents

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

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1.  Introduction

   [RFC5280] specifies several extended key purpose identifiers
   (KeyPurposeIds) for X.509 certificates.  In addition, several
   KeyPurposeIds have been added under the IANA repository "SMI Security
   for PKIX Extended Key Purpose" [RFC7299].  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 poses 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 describes that "[i]f multiple
   [key] purposes are indicated the application need not recognize all
   purposes indicated, as long as the intended purpose is present."

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   Document Signing applications MAY require that the Extended Key Usage
   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 with printable or
   displayable form by means of services or software, rather than
   primarily 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], 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

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   signature which 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 Extended Key Usage extension.  Restrictions on
   Extended Key Usage 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 presence of a
      certain KeyPurposeId or 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" exists, 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 which the relying party or the relying party
         software prohibits.  Examples of "Excluded KeyPurposeId" are,
         presence of the anyExtendedKeyUsage KeyPurposeId or 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 which the relying party or the relying party
         software accepts.  Examples of "Permitted KeyPurposeId" are,
         presence of this general document signing KeyPurposeId and/or
         protocol specific document signing-type KeyPurposeIds.  If a
         KeyPurposeId of the certificate meets the condition set by a
         "Permitted KeyPurposeId" restriction, the certificate is
         acceptable.  Otherwise, relying party or the relying party
         software rejects the certificate.

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   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, 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(s) 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 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 namely, TLS (id-kp-clientAuth) or S/MIME (id-
   kp-emailProtection) etc. in order to minimize the risk of cross-
   protocol attacks.

   To reduce the risk of specific cross-protocol attacks, the relying
   party or 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
   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.

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7.  IANA Considerations

   IANA made one assignment for the id-kp- documentSigning object
   identifier (OID), as defined in Section 3.1, in the "SMI Security for
   PKIX Extended Key Purpose" (1.3.6.1.5.5.7.3) registry.  The other
   assignment was made for the id-mod-docsign-eku ASN.1 module [X.680]
   object identifier (OID), as defined in Appendix A, in the "SMI
   Security for PKIX Module Identifier" (1.3.6.1.5.5.7.0) registry.

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/rfc/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/rfc/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/rfc/rfc8174>.

   [X.680]    ITU-T, "Information technology - Abstract Syntax Notation
              One (ASN.1): Specification of basic notation", ISO/
              IEC 8824-1:2015, November 2015.

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/rfc/rfc7299>.

Appendix A.  ASN.1 Module

   The following ASN.1 module provides the complete definition of the
   Document Signing KeyPurposeId.

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     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

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