Internet Engineering Task Force (IETF) C. Bonnell
Request for Comments: 9495 DigiCert, Inc.
Category: Standards Track October 2023
ISSN: 2070-1721
Certification Authority Authorization (CAA) Processing for Email
Addresses
Abstract
The Certification Authority Authorization (CAA) DNS resource record
(RR) provides a mechanism for domains to express the allowed set of
Certification Authorities that are authorized to issue certificates
for the domain. RFC 8659 contains the core CAA specification, where
Property Tags that restrict the issuance of certificates that certify
domain names are defined. This specification defines a Property Tag
that grants authorization to Certification Authorities to issue
certificates that contain the id-kp-emailProtection key purpose in
the extendedKeyUsage extension and at least one rfc822Name value or
otherName value of type id-on-SmtpUTF8Mailbox that includes the
domain name in the subjectAltName extension.
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
https://www.rfc-editor.org/info/rfc9495.
Copyright Notice
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Table of Contents
1. Introduction
2. Conventions and Definitions
3. Syntax of the "issuemail" Property Tag
4. Processing of the "issuemail" Property Tag
5. Examples of the "issuemail" Property Tag
5.1. No "issuemail" Property
5.2. Single "issuemail" Property
5.3. Single "issuemail" Property with Parameters
5.4. Multiple "issuemail" Properties
5.5. Malformed "issuemail" Property
6. Security Considerations
7. IANA Considerations
8. References
8.1. Normative References
8.2. Informative References
Acknowledgments
Author's Address
1. Introduction
The Certification Authority Authorization (CAA) DNS resource record
(RR) provides a mechanism for domains to express the allowed set of
Certification Authorities that are authorized to issue certificates
for the domain. [RFC8659] contains the core CAA specification, where
Property Tags that restrict the issuance of certificates that certify
domain names are defined. [RFC8659] does not define a mechanism to
restrict the issuance of certificates that certify email addresses.
For the purposes of this document, a certificate "certifies" an email
address if the certificate contains the id-kp-emailProtection key
purpose in the extendedKeyUsage extension and at least one rfc822Name
value or otherName value of type id-on-SmtpUTF8Mailbox that includes
the domain name in the subjectAltName extension.
This document defines a CAA Property Tag that restricts the allowed
set of issuers of certificates that certify email addresses. Its
syntax and processing are similar to the "issue" Property Tag as
defined in Section 4.2 of [RFC8659].
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. Syntax of the "issuemail" Property Tag
This document defines the "issuemail" Property Tag. The presence of
one or more "issuemail" Properties in the Relevant Resource Record
Set (RRSet) [RFC8659] indicates that the domain is requesting that
Certification Authorities restrict the issuance of certificates that
certify email addresses.
The CAA "issuemail" Property Value has the following sub-syntax
(specified in ABNF as per [RFC5234]):
issuemail-value = *WSP [issuer-domain-name *WSP]
[";" *WSP [parameters *WSP]]
issuer-domain-name = label *("." label)
label = (ALPHA / DIGIT) *( *("-") (ALPHA / DIGIT))
parameters = (parameter *WSP ";" *WSP parameters) / parameter
parameter = tag *WSP "=" *WSP value
tag = (ALPHA / DIGIT) *( *("-") (ALPHA / DIGIT))
value = *(%x21-3A / %x3C-7E)
The production rules for "WSP", "ALPHA", and "DIGIT" are defined in
Appendix B.1 of [RFC5234]. Readers who are familiar with the sub-
syntax of the "issue" and "issuewild" Property Tags will recognize
that this sub-syntax is identical.
The meanings of each production rule within "issuemail-value" are as
follows:
"issuer-domain-name":
A domain name of the Certification Authority comprised of one or
more labels
"label":
A single domain label that consists solely of ASCII letters,
digits, and the hyphen (known as an "LDH label")
"parameters":
A semicolon-separated list of parameters
"parameter":
A tag and a value, separated by an equals sign ("=")
"tag":
A keyword that identifies the type of parameter
"value":
The string value for a parameter
4. Processing of the "issuemail" Property Tag
Prior to issuing a certificate that certifies an email address, the
Certification Authority MUST check for publication of a Relevant
RRSet. The discovery of such a Relevant RRSet MUST be performed
using the algorithm specified in Section 3 of [RFC8659]. The input
domain to the discovery algorithm SHALL be the domain "part"
[RFC5322] of the email address that is being certified. If the
domain "part" of the email address being certified is an
Internationalized Domain Name [RFC5890] that contains one or more
U-Labels, then all U-Labels MUST be converted to their A-Label
representation [RFC5891] for the purpose of discovering the Relevant
RRSet for that email address.
If the Relevant RRSet is empty or if it does not contain any
"issuemail" Properties, then the domain has not requested any
restrictions on the issuance of certificates for email addresses.
The presence of other Property Tags, such as "issue" or "issuewild",
does not restrict the issuance of certificates that certify email
addresses.
For each "issuemail" Property in the Relevant RRSet, the
Certification Authority SHALL compare its issuer-domain-name with the
issuer-domain-name as expressed in the Property Value. If there is
not any "issuemail" record whose issuer-domain-name (as expressed in
the Property Value) matches the Certification Authority's issuer-
domain-name, then the Certification Authority MUST NOT issue the
certificate. If the Relevant RRSet contains any "issuemail" Property
whose issuemail-value does not conform to the ABNF syntax as defined
in Section 3 of this document, then those records SHALL be treated as
if the issuer-domain-name in the issuemail-value is the empty string.
If the certificate certifies more than one email address, then the
Certification Authority MUST perform the above procedure for each
email address being certified.
The assignment of issuer-domain-names to Certification Authorities is
beyond the scope of this document.
Parameters may be defined by a Certification Authority as a means for
domains to further restrict the issuance of certificates. For
example, a Certification Authority may define a parameter that
contains an account identifier. If the domain elects to add this
parameter in an "issuemail" Property, the Certification Authority
will verify that the account that is requesting the certificate
matches the account specified in the Property and will refuse to
issue the certificate if they do not match.
The processing of parameters in the issuemail-value is specific to
each Certification Authority and is beyond the scope of this
document. In particular, this document does not define any
parameters and does not specify any processing rules for when
parameters must be acknowledged by a Certification Authority.
However, parameters that do not conform to the ABNF syntax as defined
in Section 3 will result in the issuemail-value being not conformant
with the ABNF syntax. As stated above, a Property whose issuemail-
value is malformed SHALL be treated as if the issuer-domain-name in
the issuemail-value is the empty string.
5. Examples of the "issuemail" Property Tag
Several illustrative examples of Relevant RRSets and their expected
processing semantics follow. All examples assume that the issuer-
domain-name for the Certification Authority is "authority.example".
5.1. No "issuemail" Property
The following RRSet does not contain any "issuemail" Properties, so
there are no restrictions on the issuance of certificates that
certify email addresses for that domain:
mail.client.example CAA 0 issue "authority.example"
mail.client.example CAA 0 issue "other-authority.example"
5.2. Single "issuemail" Property
The following RRSet contains a single "issuemail" Property where the
issuer-domain-name is the empty string, so the issuance of
certificates certifying email addresses for the domain is prohibited:
mail.client.example CAA 0 issuemail ";"
5.3. Single "issuemail" Property with Parameters
The following RRSet contains a single "issuemail" Property where the
issuer-domain-name is "authority.example" and contains a single
"account" parameter of "123456". In this case, the Certification
Authority MAY issue the certificate, or it MAY refuse to issue the
certificate, depending on its practices for processing the "account"
parameter:
mail.client.example
CAA 0 issuemail "authority.example; account=123456"
5.4. Multiple "issuemail" Properties
The following RRSet contains multiple "issuemail" Properties, where
one Property matches the issuer-domain-name of the example
Certification Authority ("authority.example") and one Property does
not match. Although this example is contrived, it demonstrates that
since there is at least one record whose issuer-domain-name matches
the Certification Authority's issuer-domain-name, issuance is
permitted.
mail.client.example CAA 0 issuemail ";"
mail.client.example CAA 0 issuemail "authority.example"
5.5. Malformed "issuemail" Property
The following RRSet contains a single "issuemail" Property whose sub-
syntax does not conform to the ABNF as specified in Section 3. Given
that "issuemail" Properties with malformed syntax are treated the
same as "issuemail" Properties whose issuer-domain-name is the empty
string, issuance is prohibited.
malformed.client.example CAA 0 issuemail "%%%%%"
6. Security Considerations
The security considerations that are expressed in [RFC8659] are
relevant to this specification.
The processing of "issuemail" Properties as specified in this
document is a supplement to the Certification Authority's validation
process. The Certification Authority MUST NOT treat solely the
presence of an "issuemail" Property with its issuer-domain-name
specified within the Relevant CAA RRSet as sufficient validation of
the email address. The Certification Authority MUST validate the
email address according to the relevant policy documents and practice
statements.
CAA Properties may have the "critical" flag asserted, which specifies
that a given Property is critical and must be processed by conforming
Certification Authorities. If a Certification Authority does not
understand the Property, then it MUST NOT issue the certificate in
question.
If a single CAA RRSet is processed by multiple Certification
Authorities for the issuance of multiple certificate types, then a
Certification Authority's lack of support for a critical CAA Property
in the RRSet will prevent the Certification Authority from issuing
any certificates for that domain.
For example, assume that an RRSet contains the following Properties:
client.example CAA 128 issue "other-authority.example"
client.example CAA 0 issuemail "authority.example"
In this case, if the Certification Authority whose issuer-domain-name
matches "authority.example" does not recognize the "issue" Property
Tag, then that Certification Authority will not be able to issue
S/MIME certificates that certify email addresses for
"client.example".
7. IANA Considerations
IANA has registered the following entry in the "Certification
Authority Restriction Properties" subregistry of the "Public Key
Infrastructure using X.509 (PKIX) Parameters" registry group:
+===========+======================================+===========+
| Tag | Meaning | Reference |
+===========+======================================+===========+
| issuemail | Authorization Entry by Email Address | RFC 9495 |
+-----------+--------------------------------------+-----------+
Table 1
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/info/rfc2119>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/info/rfc5322>.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891,
DOI 10.17487/RFC5891, August 2010,
<https://www.rfc-editor.org/info/rfc5891>.
[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/info/rfc8174>.
[RFC8659] Hallam-Baker, P., Stradling, R., and J. Hoffman-Andrews,
"DNS Certification Authority Authorization (CAA) Resource
Record", RFC 8659, DOI 10.17487/RFC8659, November 2019,
<https://www.rfc-editor.org/info/rfc8659>.
8.2. Informative References
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<https://www.rfc-editor.org/info/rfc5890>.
Acknowledgments
The author would like to thank the participants on the LAMPS Working
Group mailing list for their insightful feedback and comments. In
particular, the author extends sincere appreciation to Alexey
Melnikov, Christer Holmberg, Éric Vyncke, John Levine, Lars Eggert,
Michael Richardson, Murray Kucherawy, Paul Wouters, Phillip Hallam-
Baker, Roman Danyliw, Russ Housley, Sean Turner, Seo Suchan, Tim
Chown, and Tim Wicinski for their official reviews and suggestions,
which greatly improved the quality of this document.
Author's Address
Corey Bonnell
DigiCert, Inc.
Email: corey.bonnell@digicert.com