LAMPS Working Group T. Ito
Internet-Draft SECOM CO., LTD.
Intended status: Standards Track T. Okubo
Expires: 3 October 2022 DigiCert, Inc.
S. Turner
sn3rd
1 April 2022
General Purpose Extended Key Usage (EKU) for Document Signing X.509
Certificates
draft-ietf-lamps-documentsigning-eku-03
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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This Internet-Draft will expire on 3 October 2022.
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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 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
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 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 relying 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 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.
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.
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
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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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