Individual T. Ito
Internet-Draft SECOM CO., LTD.
Intended status: Informational T. Okubo
Expires: October 3, 2021 DigiCert, Inc.
April 01, 2021
General Purpose Extended Key Usage (EKU) for Document Signing X.509
Certificates
draft-ito-documentsigning-eku-00
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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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
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
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.
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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).
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. 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
The id-kp-documentSigning purpose requires an object identifier
(OID). The objects are defined in an arc delegated by IANA to
Limited Additional Mechanisms for PKIX and SMIME (lamps). 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>.
[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>.
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Authors' Addresses
Tadahiko Ito
SECOM CO., LTD.
Email: tadahiko.ito.public@gmail.com
Tomofumi Okubo
DigiCert, Inc.
Email: tomofumi.okubo+ietf@gmail.com
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