Internet DRAFT - draft-fetch-validation-vmc-wchuang
draft-fetch-validation-vmc-wchuang
Network W. Chuang
Internet-Draft Google
Intended status: Standards Track M. Bradshaw
Expires: 4 September 2024 Fastmail
T. Loder
Skyelogicworks
A. Brotman
Comcast
3 March 2024
Fetch and Validation of Verified Mark Certificates
draft-fetch-validation-vmc-wchuang-06
Abstract
A description of how entities wishing to validate a Verified Mark
Certificate (VMC) should retrieve and validate these documents.
This document is a companion to BIMI core specification, which should
be consulted alongside this document.
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
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This Internet-Draft will expire on 4 September 2024.
Copyright Notice
Copyright (c) 2024 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 (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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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
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Fetch of Verified Mark Certificate . . . . . . . . . . . . . 3
3.1. BIMI Assertion Record . . . . . . . . . . . . . . . . . . 4
3.2. Verified Mark Certificate Fetch . . . . . . . . . . . . . 4
3.3. Fetch Format . . . . . . . . . . . . . . . . . . . . . . 4
4. Verified Mark Certificate Profile . . . . . . . . . . . . . . 5
4.1. Logotype Extension . . . . . . . . . . . . . . . . . . . 5
4.2. SVG Indicators . . . . . . . . . . . . . . . . . . . . . 5
4.3. BIMI Domain . . . . . . . . . . . . . . . . . . . . . . . 6
4.4. BIMI Extended Key Usage . . . . . . . . . . . . . . . . . 6
4.5. Validity . . . . . . . . . . . . . . . . . . . . . . . . 6
4.6. Certificate Transparency . . . . . . . . . . . . . . . . 7
5. Validation of Verified Mark Certificate . . . . . . . . . . . 7
5.1. Issuance and Profile Verification . . . . . . . . . . . . 7
5.2. VMC Domain Verification . . . . . . . . . . . . . . . . . 8
5.3. Validation of VMC Evidence Document . . . . . . . . . . . 8
6. Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. IANA Considerations . . . . . . . . . . . . . . . . . . . 9
7. Normative References . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
Brands Indicators or logos help people visually recognize products
and services, and consequently their providers. In the context of
email, it can help users understand who a sender is. Brand
Indicators for Message Identification is a specification as described
in [I-D.blank-ietf-bimi] for senders to associate and provide brand
Indicators to email. As noted in that document's security
considerations, the potential for abuse with brand indicators is
high, and in particular a risk for look-alike domains and copycat
indicators. One way to mitigate abuse is to validate brand ownership
of some Indicators by some third-party and have that validation
provably demonstrated through an X.509/PKIX [RFC5280] certificate as
an evidence document [I-D.blank-ietf-bimi]. We call such
certificates containing Indicators that meet the profile described
later in this document Verified Mark Certificate (VMC). This
document provides a specification on how email receivers working on
behalf of email users can fetch VMC from the Internet and how they
can validate the content of the VMC. With this, the email receiver
can prove that the VMC was indeed issued by some trusted third-party
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such as a Certification Authority (CA).
2. Terminology
BIMI: Brand Indicators for Message Identification (BIMI)
specification [I-D.blank-ietf-bimi] that combines DMARC-based message
authentication with cryptographic methods to ensure the identity of a
sender.
BIMI Evidence Document: A document published by a Mark Verifying
Authority (MVA) which in the context of a Verified Mark Certificate
(VMC) is a Certification Authority (CA) to assert evidence of
verification.
Email Receivers: The entity or organization that receives and
processes email. Mail Receivers operate one or more Internet facing
Mail Transport Agents (MTAs).
End entity certificate: A non-CA certificate representing a user (or
domain) of the PKI certificates.
Indicator: A brand indicator displayed on a Mail User Agent (MUA
e.g., an email client) when the sender's email meets the BIMI
specification requirements. The brand logo and other associated
identity information is parsed from the Verified Mark Certificate.
Immediate issuing CA certificate: A non-root CA certificate that
issues end entity certificates.
Root certificate: A self-signed CA certificate issued by the root CA
to identify itself and to facilitate verification of certificates
issued by Subordinate CAs.
Verified Mark Certificate (VMC): An end entity certificate that meets
the profile specified in this document and the Verified Mark
Certificate Guideline document.
3. Fetch of Verified Mark Certificate
This section normatively describes the actions needed to receive and
handle BIMI identified messages using Verified Mark Certificates that
is built on the protocol described in the [I-D.blank-ietf-bimi].
Receivers use these specified processes to fetch Verified Mark
Certificates securely, and then to validate the certificates and
their embedded Indicators. If all requirements are met, receivers
may then display the Indicators. Details of these steps are
described below, and the indicator display procedure is described
here.
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3.1. BIMI Assertion Record
The sender declares associating BIMI to a domain via an Assertion
record as normatively described by [I-D.blank-ietf-bimi]. That
record describes the BIMI policy and that policy may include a means
to find the BIMI Indicator via the publication of evidence documents.
The email receiver uses [RFC1035] to look for the publication of the
Assertion record as well as the lack of publication when [RFC1035]
returns NXDOMAIN. Assertion records containing the "a=" tag and
associated populated value indicates that a sender has published a
BIMI evidence document for use with that domain such a Verified Mark
Certificate. The evidence document may be found at a hosting service
specified by the URI in the value. The rest of this document
describes the process for fetching and validating the Verified Mark
Certificate with the clarification that other types of evidence
document will have their own specification for fetch and validation.
3.2. Verified Mark Certificate Fetch
Verified Mark Certificates, in this specification, are published by
the sender's web hosting service. The service MUST use HTTPS
protocol and SHOULD use TLS version v1.2 [RFC5246] or newer to avoid
protocol security bugs with earlier versions of TLS. That secure TLS
connection MUST be established by using the protocol specified in
[RFC8446]. The TLS certificate MUST have a valid issuance path to
some client trusted server root CA certificate using the procedures
in [RFC5280].
3.3. Fetch Format
Certificates fetched from the hosting service MUST be in PEM encoding
[RFC7468]. To facilitate X.509/PKIX certificate issuance validation,
the full issuance chain up to and optionally including the root CA
certificate, MUST be present. The downloaded file SHOULD be ordered
by starting with the Verified Mark Certificate, followed by its
issuer CA certificate and potential successive issuers all the way to
the optional root CA certificate. If the certificates appear out of
issuance order, contain duplicates or more than one VMC, the receiver
may choose to reject the validation. The filename specified of the
BIMI Assertion record "a=" tag URI SHOULD have a ".pem" file name
extension. Email receivers MAY cache the certificates and other
evidence documents, and if so the receivers SHOULD set a Time-To-Live
(TTL) on the cache entries as well as index by URI. This document
intentionally does not specify what that TTL value is. If the sender
wants force a certificate update, the sender MAY change the URI to a
new unique location that will "bust the cache".
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4. Verified Mark Certificate Profile
The profile describes the metadata contained within the Verified Mark
Certificate. This section normatively describes a subset of the
Verified Mark Certificate profile that pertains to the certificate
issuance and validity verification. The remaining content of the VMC
profile is defined via guidance documents from the Authindicators
(aka BIMI) Working Group, and in particular the process for obtaining
that content (with some overlap). For example, while this section
defines a requirement for the VMC to contain the Logotype extension,
the Authindicators' VMC Guidelines document defines the content of
the logotype extension. The following section describes the VMC
profile checks using the profile described here as part of the VMC
validation process.
4.1. Logotype Extension
Verified Mark Certificates MUST use logotype extension to carry the
Indicators in the certificate as specified in [RFC3709]. That RFC
uses secure URIs to specify the Indicators, and this specification
calls for the Indicators to be embedded directly in the certificate
via a DATA URI as defined by [RFC6170] and [RFC2397].
4.2. SVG Indicators
The Indicators image format MUST be SVG (an open W3C specification)
as this helps with supporting different display resolutions that
likely change in the future as SVG is a vectorized (meaning
dimensionless) format. We believe that constraining the Indicators
to a single image type will help with interoperability. This SVG
MUST use the secure profile as defined by [RFC6170]. Non-
normatively, to reiterate the secure profile defined there, it is
summarized as:
* Use SVG Tiny profile
* No script
* No external resource references
Additionally this document normatively specifies additional security
restrictions on the SVG formatting as defined in
[I-D.svg-tiny-ps-abrotman]. The SVG MUST be compressed to be space
efficient, and base64 encoded for the DATA URI encoding as defined by
[RFC6170] and [RFC2397].
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4.3. BIMI Domain
A Verified Mark Certificate MUST define one or more Subject
Alternative Name (SAN) extension dNSName domain as defined by
[RFC5280] that identifies the location of the BIMI Assertion record
that was used to fetch the VMC. There may be stronger properties
that can be said about the relationship between the VMC and the
Assertion record depending on the validation done on dNSName, but
that is outside the scope of this document. The domain name may
either be the author or organizational name as defined in
[I-D.blank-ietf-bimi] i.e. the Assertion record domain without the
BIMI header selector or "default" selector and without the "_bimi"
well-known label, meaning that it matches against any BIMI header
selector including "default". Alternatively the domain name may
specify also the BIMI header selector or "default" selector along
with the "_bimi" well-known label, and will only match against that
specific selector. If the domain is internationalized, it MUST
follow canonicalization procedure specified in section 7.2 of
[RFC5280].
4.4. BIMI Extended Key Usage
This document describes a new [RFC5280] Extended Key Usage OID that
identifies Verified Mark Certificate as id-kp-
BrandIndicatorforMessageIdentification. Certificates conforming to
the Verified Mark Certificate profile is distinguished by using this
extended key usage.
id-kp-BrandIndicatorforMessageIdentification OBJECT IDENTIFIER ::= {id-kp 31 } OID.
Verified Mark Certificates MUST contain an Extended Key Usage
extension with the id-kp-BrandIndicatorforMessageIdentification OID.
Also the CA certificate representing the immediate issuer of Verified
Mark Certificates MUST also contain an Extended Key Usage extension
with the id-kp-BrandIndicatorforMessageIdentification OID designating
its usage.
4.5. Validity
A Verified Mark Certificate MUST specify a certificate validity
period using the notBefore and notAfter fields. It MUST also define
a location to check for certificate revocation using a Certificate
Revocation List (CRL) Distribution Point and that is encoded in the
VMC as a [RFC5280] cRLDistributionPoints extension.
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4.6. Certificate Transparency
CT as specified provides transparency for the issued certificates.
The Verified Mark Certificate MUST be logged to a set of Certificate
Transparency (CT) logs, and the proof of that logging must be present
in the certificate as a [RFC6962] SCT extension. The SCT extension
MUST contain one or more SCTs.
5. Validation of Verified Mark Certificate
Verified Mark Certificates provide the means to securely authenticate
as well as identify the third-party Mark Verifying Authority which in
this case is a Certification Authority by verifying the issuance
chain. It also provides the means to associate the Verified Mark
Certificate to the BIMI Assertion record. This section concludes
with a BIMI validation procedure for determining whether the VMC is
valid or not. This should be used as part of a procedure in
determining whether to display a BIMI Indicator based on a VMC.
5.1. Issuance and Profile Verification
To ensure the correctness of that certificate information, the
receiver verifies the authenticity of the certificate, its validity
and that it is a Verified Mark Certificate. After the certificate is
downloaded, the receiver MUST validate the certificate with the
following procedure:
1. Validate the authenticity of the certificate by checking the
certificate signatures, that the end-entity certificate issuance
chain leads back to some BIMI root CA certificate, and confirm
membership of the root CA certificate in the receiver's trusted
BIMI root set following the path validation procedures specified
in section 6.1 of [RFC5280]. The downloaded certificates MUST
contain all intermediate CA certificates up to but not
necessarily including the root certificates.
2. Check the validity of the certificates in the certificate chain
using the procedures in section 4.1.2.5 of [RFC5280].
3. Check that the certificates in the certificate chain are not
revoked using the procedures in section 6.3 of [RFC5280]. The
end-entity certificate MUST identify the CRL by a
cRLDistributionPoints extension.
4. Validate the proof of CT logging. The receiver MUST find one or
more SCTs, and validate that they are signed by a CT log
recognized by the receiver using the procedures in [RFC6962].
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5. Verify that the end-entity certificate is a Verified Mark
Certificate. The certificate MUST contain an Extended-Key-Usage
extension, and that it contains extended-key-usage is id-kp-
BrandIndicatorforMessageIdentification. The entity certificate
must contain a logotype extension, and that it contains a SVG as
described in [#svg_indicators].
5.2. VMC Domain Verification
Next the receiver checks VMC SAN dNSName domain name relationship to
the Assertion Record domain name, to demonstrate that they mutually
identify each other. The following procedure allows the dNSName to
either specify and thus match against the Assertion Record's domain
name only, or selector + domain name. To do this, the receiver
creates two domain name sets: 1) selector-set and 2) domain-set. The
receiver iterates over the VMC SAN dNSName domain names and adds them
to the domain name sets as follows.
* If the domain name contains "_bimi" i.e. prefixed with the labels
<selector>._bimi, add to selector-set.
* Otherwise add the domain name to domain-set.
Then check if the Assertion record domain matches by checking the
following:
* Check if the Assertion record author or organizational domain
names as defined in [draft-blank-ietf-bimi-01], which includes the
<selector>._bimi prefix, is present in the selector-set. If
found, the VMC Domain Verification is considered to match.
* Check if the remaining Assertion record author or organizational
domain name is present in the domain-set. If found, the VMC
Domain Verification is considered to match.
If internationalization is present, the receiver MUST canonicalize
the domain names using the internationalization procedures specified
in section 7.2 of [RFC5280].
5.3. Validation of VMC Evidence Document
The following procedure combines the above steps to determine whether
a VMC contains a valid BIMI Evidence Document. This should be a part
of a larger receiver defined procedure to determine whether to
display a BIMI Indicator that may take into account other receiver
specific signals such as reputation.
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As a preamble, consider if the receiver supports VMC validation and
an Assertion Record is found which has BIMI VMC location in the "a="
tag value. If so then validate the VMC using the following
algorithm:
1. Use the mechanism in the "a=" tag location to retrieve the VMC,
this MUST be a URI with HTTPS transport.
2. If the TLS connection setup as described in
[#verified_mark_certificate_fetch] fails, then validation returns
with an error.
3. If the evidence document does not contain a single valid VMC
certificate chain then validation returns with an error.
4. Validate the VMC path validation procedure described in
[#issuance_and_profile_verification]. If path validation fails
then validation returns with an error.
5. Validate the VMC Domain relationship to the Assertion record as
described in [#VMC_domain_verification] i.e. matches. If the VMC
Domain is not related to the Assertion record, then validation
returns with an error.
6. Retrieve the SVG Indicator from the [Logotype] Extension (oid
1.3.6.1.5.5.7.1.12) of the validated VMC.
7. Optionally, the receiver MAY choose to retrieve the SVG Indicator
from the URI specified in the l= tag location of the Assertion
Record and compare this to the SVG Indicator embedded within the
VMC. The receiver MAY fail validation if these Indicators
differ.
8. Validate the certificate meets the remaining profile
specification of the VMC as described in
[#verified_mark_certificate_profile], otherwise validation
returns with an error.
9. Proceed to the Indicator Validation as described in section 8.6
in [I-D.blank-ietf-bimi].
6. Appendix
6.1. IANA Considerations
IANA is kindly requested to reserve the following assignments for:
* The LAMPS-Bimi-Certificate-2018 ASN.1 module in the "SMI Security
for PKIX Extended Key Purpose" registry (1.3.6.1.5.5.7.3).
* The BIMI certificate extended key usage (1.3.6.1.5.5.7.3.31).
7. Normative References
[I-D.blank-ietf-bimi]
Blank, S., Goldstein, P., Loder, T., Zink, T., and M.
Bradshaw, "Brand Indicators for Message Identification
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(BIMI)", Work in Progress, Internet-Draft, draft-blank-
ietf-bimi-02, 9 March 2021,
<https://datatracker.ietf.org/doc/html/draft-blank-ietf-
bimi-02>.
[I-D.svg-tiny-ps-abrotman]
Brotman, A. and J. T. Adams, "SVG Tiny Portable/Secure",
Work in Progress, Internet-Draft, draft-svg-tiny-ps-
abrotman-06, 3 September 2023,
<https://datatracker.ietf.org/doc/html/draft-svg-tiny-ps-
abrotman-06>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC2397] Masinter, L., "The "data" URL scheme", RFC 2397,
DOI 10.17487/RFC2397, August 1998,
<https://www.rfc-editor.org/info/rfc2397>.
[RFC3709] Santesson, S., Housley, R., and T. Freeman, "Internet
X.509 Public Key Infrastructure: Logotypes in X.509
Certificates", RFC 3709, DOI 10.17487/RFC3709, February
2004, <https://www.rfc-editor.org/info/rfc3709>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.
[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>.
[RFC6170] Santesson, S., Housley, R., Bajaj, S., and L. Rosenthol,
"Internet X.509 Public Key Infrastructure -- Certificate
Image", RFC 6170, DOI 10.17487/RFC6170, May 2011,
<https://www.rfc-editor.org/info/rfc6170>.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<https://www.rfc-editor.org/info/rfc6962>.
[RFC7468] Josefsson, S. and S. Leonard, "Textual Encodings of PKIX,
PKCS, and CMS Structures", RFC 7468, DOI 10.17487/RFC7468,
April 2015, <https://www.rfc-editor.org/info/rfc7468>.
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[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
Authors' Addresses
Weihaw Chuang
Google
Email: weihaw@google.com
Marc Bradshaw
Fastmail
Email: marc@fastmailteam.com
Thede Loder
Skyelogicworks
Email: thede@skyelogicworks.com
Alex Brotman (ed)
Comcast
Email: alex_brotman@comcast.com
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