X.509 Certificate General-Purpose Extended Key Usage (EKU) for Document SigningSECOM CO., LTD.tadahiko.ito.public@gmail.comDigiCert, Inc.tomofumi.okubo+ietf@gmail.comsn3rdsean@sn3rd.com
sec
lampsRFC 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.Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by
the Internet Engineering Steering Group (IESG). Further
information on Internet Standards is available in Section 2 of
RFC 7841.
Information about the current status of this document, any
errata, and how to provide feedback on it may be obtained at
.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
() in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with
respect to this document. Code Components extracted from this
document must include Revised BSD License text as described in
Section 4.e of the Trust Legal Provisions and are provided without
warranty as described in the Revised BSD License.
Table of Contents
. Introduction
. Conventions and Definitions
. Extended Key Purpose for Document Signing
. Including the Extended Key Purpose for Document Signing in Certificates
. Using the Extended Key Purpose for Document Signing in a Certificate
. Implications for a Certification Authority
. Security Considerations
. IANA Considerations
. References
. Normative References
. Informative References
. ASN.1 Module
Acknowledgments
Authors' Addresses
Introduction specifies several extended key purpose
identifiers (KeyPurposeIds) for X.509 certificates. In addition, the
IANA repository "SMI Security for PKIX Extended Key Purpose" includes a number of KeyPurposeIds. 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 pose 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.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
when, and only when, they appear in all capitals, as shown here.
Extended Key Purpose for Document SigningThis specification defines the KeyPurposeId id-kp-documentSigning.As described in , "[i]f the [Extended Key Usage] extension is present,
then the certificate MUST only be used for one of the purposes indicated."
also notes that "[i]f 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 EKU 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 in a printable or displayable form by
means of services or software, rather than processed by machines.
Including the Extended Key Purpose for Document Signing in Certificates 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 , the 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 }
Using the Extended Key Purpose for Document Signing in a CertificateOur 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 signature that 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
EKU extension.
Restrictions on EKU 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 the presence of a
certain KeyPurposeId or the 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 exist,
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 that the relying party or the relying party software
prohibits. Examples of Excluded KeyPurposeId include the presence of the
anyExtendedKeyUsage KeyPurposeId or the 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 that the relying party or
the relying party software accepts. Examples of Permitted
KeyPurposeId include the presence of this general-purpose Document-Signing
KeyPurposeId and/or the protocol-specific
KeyPurposeIds that are relevant to Document Signing. If a KeyPurposeId of the certificate meets the
condition set by a Permitted KeyPurposeId restriction, the
certificate is acceptable. Otherwise, the 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 and PDF).Implications for a Certification AuthorityThe 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, 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.Security ConsiderationsThe usage of the 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, which include
TLS (id-kp-clientAuth) and S/MIME (id-kp-emailProtection),
in order to minimize the risk of cross-protocol attacks.To reduce the risk of specific cross-protocol attacks, the relying party
or the 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-purpose
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.IANA ConsiderationsIANA has registered the following OID in the "SMI Security for PKIX
Extended Key Purpose" registry (1.3.6.1.5.5.7.3). This OID is defined in
.
Decimal
Description
References
36
id-kp-documentSigning
RFC 9336
IANA has also registered the following ASN.1 module OID in the "SMI
Security for PKIX Module Identifier" registry (1.3.6.1.5.5.7.0). This OID is defined in .
Decimal
Description
References
104
id-mod-docsign-eku
RFC 9336
ReferencesNormative ReferencesKey words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) ProfileThis memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK]Ambiguity of Uppercase vs Lowercase in RFC 2119 Key WordsRFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.Information technology - Abstract Syntax Notation One (ASN.1): Specification of basic notationITU-TInformative ReferencesObject Identifier Registry for the PKIX Working GroupWhen the Public-Key Infrastructure using X.509 (PKIX) Working Group was chartered, an object identifier arc was allocated by IANA for use by that working group. This document describes the object identifiers that were assigned in that arc, returns control of that arc to IANA, and establishes IANA allocation policies for any future assignments within that arc.ASN.1 ModuleThe 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(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
AcknowledgmentsWe would like to thank for verifying the ASN.1 module.
Additionally, we would like to thank , , , , and for their comments.Authors' AddressesSECOM CO., LTD.tadahiko.ito.public@gmail.comDigiCert, Inc.tomofumi.okubo+ietf@gmail.comsn3rdsean@sn3rd.com