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

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

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

Abstract

   RFC5280 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/.

   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 . . . . . . . . . .   4
     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 . . . . . . . . . . . . . . . . . . . . . . .   5
   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 . . . . . . . . . . . . . . . . . . . .   8
   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 [RFC7299] under the IANA repository
   "SMI Security for PKIX Extended Key Purpose".  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
   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 KeyPurposeIds are used in a
   PKI (or a trust program) governed by the vendor.  However, if the
   KeyPurposeId is used outside of vendor governance, the usage can
   easily become out of control (e.g. - When the end user encounters
   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 KeyPurposeId owner, there is no way for the
   KeyPurposeId owner to know what the impact will be if any change is
   made to the KeyPurposeId in question, and it would restrict vendor's
   choice of OID management. etc.).

   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 a 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.

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3.  Extended Key Purpose for Document Signing

   This specification defines the KeyPurposeId id-kp-documentSigning.

   As described in [RFC5280], If the Extended Key Usage extension is
   present, then the certificate MUST only be used for one of the
   purposes indicated.  [RFC5280] also describes that If 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 Extended Key Usage
   extension be present and that a 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
   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

   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 XX }

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4.  Using the Extended Key Purpose for Document Signing 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 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 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.  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
   during certificate validation:

   The implementation MAY examine the KeyPurposeId(s) included in the
   Extended Key Usage extension as follows: A Restriction on Extended
   Key Usage 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, then process the KeyPurposeId(s) as described
       below.

       Each restriction on "Excluded KeyPurposeId" or "Permitted
       KeyPurposeId" is handled 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.  A
       consideration on prohibiting combinations of KeyPurposeIds is
       described in the Security Considerations section of this
       document.

       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

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          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.

   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, 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 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 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 additionally prohibit use of
   specific combinations of KeyPurposeIds.

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   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 KeyPurposeId or those who do not
   intend to set up an 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

   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, to the "SMI Security for
   PKIX Extended Key Purpose" (1.3.6.1.5.5.7.3) registry.  The other
   assignment is for the addition of the id-mod-docsign-eku ASN.1 module
   [X.680] 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

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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/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 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

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

   Sean Turner
   sn3rd
   Email: sean@sn3rd.com

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