Internet DRAFT - draft-ietf-lamps-caa-issuemail
draft-ietf-lamps-caa-issuemail
Network Working Group C. Bonnell
Internet-Draft DigiCert, Inc.
Intended status: Standards Track 10 August 2023
Expires: 11 February 2024
Certification Authority Authorization (CAA) Processing for Email
Addresses
draft-ietf-lamps-caa-issuemail-07
Abstract
The Certification Authority Authorization (CAA) DNS resource record
(RR) provides a mechanism for domains to express the allowed set of
Certification Authorities (CAs) 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 which certify domain names are defined. This
specification defines a Property Tag that grants authorization to CAs
to issue certificates which contain the id-kp-emailProtection key
purpose in the extendedKeyUsage extension and one or more rfc822Name
or otherName of type id-on-SmtpUTF8Mailbox that include the domain
name in the subjectAltName extension.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://CBonnell.github.io/caa-issuemail/draft-ietf-lamps-caa-
issuemail.html. Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-lamps-caa-issuemail/.
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/CBonnell/caa-issuemail.
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. Syntax of the "issuemail" Property Tag . . . . . . . . . . . 3
4. Processing of the "issuemail" Property Tag . . . . . . . . . 4
5. Examples of the "issuemail" Property Tag . . . . . . . . . . 6
5.1. No issuemail Property . . . . . . . . . . . . . . . . . . 6
5.2. Single issuemail Property . . . . . . . . . . . . . . . . 6
5.3. Single issuemail Property with Parameters . . . . . . . . 6
5.4. Multiple issuemail Properties . . . . . . . . . . . . . . 6
5.5. Malformed issuemail Property . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 8
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
The Certification Authority Authorization (CAA) DNS resource record
(RR) provides a mechanism for domains to express the allowed set of
Certification Authorities (CAs) that are authorized to issue
certificates for the domain. [RFC8659] contains the core CAA
specification, where Property Tags that restrict the issuance of
certificates which certify domain names are defined. [RFC8659] does
not define a mechanism to restrict the issuance of certificates which
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 the email address is included as a rfc822Name or
otherName of type id-on-SmtpUTF8Mailbox in the subjectAltName
extension.
This document defines a CAA Property Tag which restricts the allowed
set of issuers of certificates which 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 ([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]):
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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 CA comprised of one or
more labels
* "label": A single domain label which 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 which 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
Resource Record Set (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.
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If the Relevant RRSet is empty, or the Relevant RRSet 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 which 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 which
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 are specific to
each Certification Authority and are 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.
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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 which
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, one of
which matches the issuer-domain-name of the example Certification
Authority ("authority.example") and one Property which does not
match. Although this example is contrived, this example 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"
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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 the 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".
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7. IANA Considerations
The author requests the registration of the following "Certification
Authority Restriction Properties" in the registry group "Public Key
Infrastructure using X.509 (PKIX) Parameters":
+===========+======================================+===========+
| Tag | Meaning | Reference |
+===========+======================================+===========+
| issuemail | Authorization Entry by Email Address | [This |
| | | document] |
+-----------+--------------------------------------+-----------+
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/rfc/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/rfc/rfc5234>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/rfc/rfc5322>.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891,
DOI 10.17487/RFC5891, August 2010,
<https://www.rfc-editor.org/rfc/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/rfc/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/rfc/rfc8659>.
8.2. Informative References
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[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/rfc/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
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