LAMPS Working Group                                               T. Ito
Internet-Draft                                           SECOM CO., LTD.
Intended status: Standards Track                                T. Okubo
Expires: 3 8 September 2022                                 DigiCert, Inc.
                                                               S. Turner
                                                                   sn3rd
                                                            2
                                                            7 March 2022

  General Purpose Extended Key Usage (EKU) for Document Signing X.509
                              Certificates
                draft-ietf-lamps-documentsigning-eku-01
                draft-ietf-lamps-documentsigning-eku-02

Abstract

   RFC 5280

   RFC5280 specifies several extended key usages purpose identifiers
   (KeyPurposeIds) for X.509 certificates.  This document defines a
   general purpose document signing extended key usage KeyPurposeId for inclusion in the
   Extended Key Usage (EKU) extension of X.509 public key certificates which
   restricts the usage of certificates.
   Document Signing applications may require that the certificates for EKU extension be
   present and that a document signing. 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/.

   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/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 3 8 September 2022.

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Table of Contents

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

1.  Introduction

   [RFC5280] specifies several extended key usages purpose identifiers
   (KeyPurposeIds) for X.509 certificates.  In addition, several extended key usage had
   KeyPurposeIds have been added [RFC7299] as public Object Identifier (OID) under the IANA
   repository. repository
   "SMI Security for PKIX Extended Key Purpose".  While usage of any extended key usage the
   "anyExtendedKeyUsage" KeyPurposeId is bad practice for publicly
   trusted certificates, there are is no public and general
   extended key usage KeyPurposeId
   explicitly assigned for Document Signing
   certificates. Signing.  The current practice is to
   use id-kp-emailProtection, id-kp-codeSigning or vendor defined OID a vendor-defined
   KeyPurposeId 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 vendor-defined KeyPurposeIds are used in a
   PKI (or a trust program) governed by the vendor.  However, if the OID
   KeyPurposeId 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,
   vendor-defined KeyPurposeIds, 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 KeyPurposeId owner, there is no way for the OID
   KeyPurposeId owner to know what the impact will be if any change is
   made to the OID KeyPurposeId in question, and it would restrict vendor's
   choice of OID management. etc.).

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

   This document defines a general Document Signing extended key purpose
   identifier. 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 usage Purpose for DocumentSigning Document Signing

   This specification defines the KeyPurposeId id-kp-documentSigning.
   Inclusion of this KeyPurposeId

   As described in a [RFC5280], If the Extended Key Usage extension is
   present, then the certificate indicates MUST only be used for one of the
   purposes indicated.  [RFC5280] also describes that If multiple key
   purposes are indicated the
   use of any Subject names application need not recognize all
   purposes indicated, as long as the intended purpose is present.

   Document Signing applications MAY require that the Extended Key Usage
   extension be present and that a id-kp-documentSigning be indicated in
   order for the certificate is restricted to use by a
   document signing.

   Term of be acceptable to that Document Signing
   application.

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

3.1.  Including the Extended Key Usage Values Purpose for Document Signing in
      Certificates

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

   This specification defines the KeyPurposeId id-kp-documentSigning.
   Inclusion of this KeyPurposeId in a certificate indicates that the
   use of any Subject names
   public key encoded in the certificate is restricted has been certified to use by a
   document signing service or a software (along with any usages allowed be used
   for cryptographic operations on contents that are consumed by other EKU values). 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 XX }

4.  Using the Extended Key Purpose for 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 key purpose 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 key purposes 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  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 which is signed on contents intended to be
   consumed by people, implementations MAY perform the steps below as a
   during certificate validation. validation:

   The implementation MAY examine the KeyPurposeId(s) included in the
   Extended Key Usage extension as follows: A Restriction on Extended
   Key Usage value(s): is derived and implemented from (or configured with) the
   policy to which the implementation conforms.

   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 then process the KeyPurposeId(s) as following. described
       below.

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

       The
       KeyPurposeId" is handled as described below.

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

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

       Permitted EKU KeyPurposeId procedure:

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

   When a single software 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, 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 KeyPurposeId.  The inclusion of the id-kp-documentSigning EKU in conjunction with the vendor
   specific EKU.  However, this id-kp-
   documentSigning KeyPurposeId does not restrict preclude the CA from including
   multiple EKUs related to document signing. inclusion of other
   KeyPurposeIds.

6.  Security Considerations

   The usage of id-kp-documentSigning EKU intends KeyPurposeId is to prevent id-kp-
   emailProtection from provide an
   alternative to id-kp-emailProtection being used for non-email
   purposes other than email and id-
   kp-codeSigning id-kp-codeSigning being used to sign objects other than
   binary codes. code.  This
   EKU extended key purpose does not introduce new
   security risks but instead reduces existing security risks by
   providing means to separate other EKUs extended key purposes used for
   communication protocols namely, TLS or S/MIME etc. in order to
   minimize the risk of cross protocol cross-protocol attacks.

   To reduce the risk of specific cross protocol cross-protocol attacks, the relying
   party or relying party software may additionaly additionally prohibit use of
   specific combination combinations of EKUs. KeyPurposeIds.

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

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

7.  IANA Considerations

   This document requests that IANA make two assignments.  One
   assignment is for the addition of the id-kp-documentSigning object
   identifier (OID), as defined in Section 3.1, for the EKU from to the "SMI Security for
   PKIX Extended Key Purpose" (1.3.6.1.5.5.7.3) registry.  Another  The other
   assignment is for the id-mod-docsign-
   eku, as defined in Appendix A, for addition of the id-mod-docsign-eku ASN.1 module
   [X.680] from the object identifier (OID), as defined in Appendix A, to 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>.

   [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. KeyPurposeId.

     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