wdiff draft-ietf-lamps-documentsigning-eku-01.txt draft-ietf-lamps-documentsigning-eku-02.txt

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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Please review these documents carefully, as they describe your rights
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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