General Purpose Extended Key Usage (EKU) for Document Signing X.509 Certificates

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Table of Contents

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 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.¶

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

    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.¶

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

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