Internet DRAFT - draft-ietf-lamps-documentsigning-eku
draft-ietf-lamps-documentsigning-eku
LAMPS Working Group T. Ito
Internet-Draft SECOM CO., LTD.
Intended status: Standards Track T. Okubo
Expires: 2 April 2023 DigiCert, Inc.
S. Turner
sn3rd
29 September 2022
General Purpose Extended Key Usage (EKU) for Document Signing X.509
Certificates
draft-ietf-lamps-documentsigning-eku-06
Abstract
RFC 5280 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/. Subscribe at
https://www.ietf.org/mailman/listinfo/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 . . . . . . . . . . 3
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 . . . . . . . . . . . . . . . . . . . . . . . 4
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 . . . . . . . . . . . . . . . . . . . . 7
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 under the IANA repository "SMI Security
for PKIX Extended Key Purpose" [RFC7299]. 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
used for document signing, technical or policy changes made to the
code signing and S/MIME ecosystem may cause unexpected behaviors or
have an adverse impact such as decreased cryptographic agility on the
document signing ecosystem and vice versa.
Vendor-defined KeyPurposeIds that are used in a PKI governed by the
vendor or a group of vendors poses no interoperability concern.
Appropriating, or misappropriating as the case may be, KeyPurposeIDs
for use outside of their originally intended vendor or group of
vendors controlled environment can introduce problems, the impact of
which is difficult to determine.
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 an 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.
3. Extended Key Purpose for Document Signing
This specification defines the KeyPurposeId id-kp-documentSigning.
As described in [RFC5280], "[i]f the [Extended Key Usage] extension
is present, then the certificate MUST only be used for one of the
purposes indicated." [RFC5280] also describes that "[i]f multiple
[key] purposes are indicated the application need not recognize all
purposes indicated, as long as the intended purpose is present."
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Document Signing applications MAY require that the Extended Key Usage
extension be present and that the 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
primarily 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
As described in [RFC5280], EKU extension may, at the option of the
certificate issuer, be either critical or non-critical.
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 36 }
4. Using the Extended Key Purpose for Document Signing in a Certificate
Our intended use case is people consuming the contents of signed
documents. 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
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signature which is signed on contents intended to be consumed by
people, implementations MAY perform the steps below during
certificate validation:
The following procedure is used to examine the KeyPurposeId(s)
included in the Extended Key Usage extension. Restrictions on
Extended Key Usage is derived and implemented from (or configured
with) the policy to which the implementation conforms.
* If there are no restrictions set for the relying party and the
relying party software, the certificate is acceptable.
* If there are restrictions set for the relying party and relying
party software, then process the KeyPurposeId(s) 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, instead of a
single KeyPurposeId. A consideration on prohibiting combinations
of KeyPurposeIds is described in the Security Considerations
section of this document. If both "Excluded KeyPurposeId" and
"Permitted KeyPurposeId" exists, the relying party or the relying
party software processes each restriction on "Excluded
KeyPurposeId" first, and then processes each restriction on
"Permitted KeyPurposeId".
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
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.
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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).
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(s) specific PKI, 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 (id-kp-clientAuth) or S/MIME (id-
kp-emailProtection) 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.
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 document signing KeyPurposeId or
those who do not intend to set up a 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.
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7. IANA Considerations
IANA made one assignment for the id-kp- documentSigning object
identifier (OID), as defined in Section 3.1, in the "SMI Security for
PKIX Extended Key Purpose" (1.3.6.1.5.5.7.3) registry. The other
assignment was made for the id-mod-docsign-eku ASN.1 module [X.680]
object identifier (OID), as defined in Appendix A, in the "SMI
Security for PKIX Module Identifier" (1.3.6.1.5.5.7.0) registry.
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>.
Appendix A. ASN.1 Module
The following ASN.1 module provides the complete definition of the
Document Signing KeyPurposeId.
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DocSignEKU { iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-docsign-eku(104) }
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 36 }
END
Acknowledgments
We would like to thank Russ Housley for verifying the ASN.1 module.
Additionally, we would like to thank Corey Bonnell, Wendy Brown, Russ
Housley, Prachi Jain, and Stefan Santesson for their comments.
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
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