General Purpose Extended Key Usage (EKU) for Document Signing X.509 Certificates
draft-ietf-lamps-documentsigning-eku-01

LAMPS Working Group                                               T. Ito
Internet-Draft                                           SECOM CO., LTD.
Intended status: Standards Track                                T. Okubo
Expires: 3 September 2022                                 DigiCert, Inc.
                                                               S. Turner
                                                                   sn3rd
                                                            2 March 2022

  General Purpose Extended Key Usage (EKU) for Document Signing X.509
                              Certificates
                draft-ietf-lamps-documentsigning-eku-01

Abstract

   RFC 5280 specifies several extended key usages for X.509
   certificates.  This document defines a general purpose document
   signing extended key usage for X.509 public key certificates which
   restricts the usage of the certificates for document signing.

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/.

   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.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 3 September 2022.

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Table of Contents

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

1.  Introduction

   [RFC5280] specifies several extended key usages for X.509
   certificates.  In addition, several extended key usage had been added
   [RFC7299] as public Object Identifier (OID) under the IANA
   repository.  While usage of any extended key usage is bad practice
   for publicly trusted certificates, there are no public and general
   extended key usage explicitly assigned for Document Signing
   certificates.  The current practice is to use id-kp-emailProtection,
   id-kp-codeSigning or vendor defined OID for general document signing

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   purposes.

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

   There is no issue if the vendor defined OIDs are used in a PKI (or a
   trust program) governed by the vendor.  However, if the OID is used
   outside of the vendor governance, the usage can easily become out of
   control (e.g. - When the end user encounters vendor defined OIDs,
   they might want to ask that vendor about use of the certificate,
   however, the vendor may not know about the particular use. - If the
   issuance of the cert is not under the control of the OID owner, there
   is no way for the OID owner to know what the impact will be if any
   change is made to the OID in question, and it would restrict vendor's
   choice of OID management. etc.).

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

   This document defines a general Document Signing extended key purpose
   identifier.

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 usage for DocumentSigning

   This specification defines the KeyPurposeId id-kp-documentSigning.
   Inclusion of this KeyPurposeId in a certificate indicates that the
   use of any Subject names in the certificate is restricted to use by a
   document signing.

   Term of "Document Sign" in this document is digitally sign contents
   that are consumed by humans.  To be more precise, contents are
   intended to be shown to human with printable or displayable form by
   means of services or software, rather than processed by machines.

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3.1.  Extended Key Usage Values for Document Signing

   [RFC5280] specifies the EKU X.509 certificate extension for use in
   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 how the
   public key in the certificate is used, in a more basic cryptographic
   way.

   The EKU extension syntax is repeated here for convenience:

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

     KeyPurposeId  ::=  OBJECT IDENTIFIER

   This specification defines the KeyPurposeId id-kp-documentSigning.
   Inclusion of this KeyPurposeId in a certificate indicates that the
   use of any Subject names in the certificate is restricted to use by a
   document signing service or a software (along with any usages allowed
   by other EKU values).

     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 XX }

4.  Using the Document Signing EKU in a Certificate

   [RFC8358] specifies the conventions for digital signatures on
   Internet-Drafts.  This is one of the intended use cases for the
   general document signing EKU described in this document.  [RFC8358]
   uses CMS to digitally sign a wide array of files such as ASCII, PDF,
   EPUB, HTML etc.  Currently, there are no specification regarding EKU
   for certificates signing those files except those which are defined
   by the software vendor.

   The signed contents of Internet-Drafts are primarily intended to be
   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.  To validate
   the digital signature on the contents that is intended to be consumed
   by people, implementations MAY perform the steps below as a
   certificate validation.

   The implementation MAY examine the Extended Key Usage value(s):

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

   2.  If there are restrictions set for the replying party and relying
       party software, proceed as following.

       Each Restriction on the EKUs can be "Excluded EKU" or "Permitted
       EKU" and handled.

       The procedure is intended to permit or prohibit presence of a
       certain EKU or complete absence of EKUs.  It is outside the scope
       of this document, but the relying party can permit or exclude
       combinations of EKU.  A consideration on prohibiting combination
       of EKUs is described in the security consideration section of
       this document.

       Excluded EKUs procedure:  "Excluded EKU" is an EKU which the
          relying party or the relying party software prohibits.
          Examples of "Excluded EKU" are, presence of
          anyExtendedKeyUsage or complete absence of EKU extension on a
          certificate.  If an EKU of the certificate meets the
          conditions set by the "Excluded EKU" restriction, the relying
          party or the relying party software rejects the certificate.

       Permitted EKU procedure:  "Permitted EKU" is an EKU which the
          relying party or the relying party software accepts.  Examples
          of "Permitted EKU" are, presence of this general document
          signing EKU and/or protocol specific document signing-type
          EKUs.  If an EKU of the certificate meets the condition set by
          a "Permitted EKU" restriction, the certificate is acceptable.
          Otherwise, relying party or the relying party software rejects
          the certificate.

   When a single software has 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, etc).

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 (or trust program), certificates
   that indicate usage for document signing MAY include the id-kp-
   documentSigning EKU extension.  This does not encompass the mandatory
   usage of the id-kp-documentSigning EKU in conjunction with the vendor
   specific EKU.  However, this does not restrict the CA from including

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   multiple EKUs related to document signing.

6.  Security Considerations

   The usage of id-kp-documentSigning EKU intends to prevent id-kp-
   emailProtection from being used for purposes other than email and id-
   kp-codeSigning used to sign objects other than binary codes.  This
   EKU does not introduce new security risks but instead reduces
   existing security risks by providing means to separate other EKUs
   used for communication protocols namely, TLS or S/MIME 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 additionaly prohibit use of
   specific combination of EKUs.

   While a specific protocol or signing scheme may choose to come up
   with their own EKU, some may not have significant motive or resource
   to set up and manage thier own EKU.  This general document signing
   EKU may be used as a stop gap for those that intend to set up their
   own EKU or those who do not intend to set up an EKU but still would
   like to distinguish from other usage.

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

7.  IANA Considerations

   This document requests that IANA make two assignments.  One for the
   id-kp-documentSigning object identifier (OID), as defined in
   Section 3.1, for the EKU from the "SMI Security for PKIX Extended Key
   Purpose" (1.3.6.1.5.5.7.3) registry.  Another for the id-mod-docsign-
   eku, as defined in Appendix A, for the ASN.1 module [X.680] from the
   in the "SMI Security for PKIX Module Identifier" (1.3.6.1.5.5.7.0)
   registry.  No further action is necessary by IANA.

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>.

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   [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>.

   [RFC8358]  Housley, R., "Update to Digital Signatures on Internet-
              Draft Documents", RFC 8358, DOI 10.17487/RFC8358, March
              2018, <https://www.rfc-editor.org/rfc/rfc8358>.

Appendix A.  ASN.1 Module

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

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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(TBD1) }

     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 TBD2 }

     END

Acknowledgments

   We would like to thank Russ Housley for verifying the ASN.1 module.

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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