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

Individual                                                        T. Ito
Internet-Draft                                           SECOM CO., LTD.
Intended status: Informational                                  T. Okubo
Expires: 13 January 2022                                  DigiCert, Inc.
                                                               S. Turner
                                                                   sn3rd
                                                            12 July 2021

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

Abstract

   [RFC5280] 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.

Status of This Memo

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Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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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  . . . . .   3
   4.  Implications for a Certification Authority  . . . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   5
   Appendix A.  ASN.1 Module . . . . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   [RFC5280] specifies several extended key usages for X.509
   certificates.  In addition, several extended key usage had been
   added[RFC7299] as public 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 Object ID for general document signing 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

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   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 usage.

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 paper is digitaly signing human
   readable data or data that is intended to be human readable by means
   of services and software.

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).

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       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.  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
   multiple EKUs related to document signing.

5.  Security Considerations

   The Use of id-kp-documentSigning EKU can prevents the usage of id-kp-
   emailProtection for none-email purposes and id-kp-codeSigning for
   signing objects other than binary codes.  An id-kp-documentSigning
   EKU value does not introduce any new security or privacy concerns.

6.  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.

7.  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>.

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   [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>.

   [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", ISO/
              IEC 8824-1:2015, November 2015.

Acknowledgments

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

Appendix A.  ASN.1 Module

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

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

Authors' Addresses

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   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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