]> DigiCert, Inc.

corey.bonnell@digicert.com

Security caa certification authority authorization email address 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 The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Limited Additional Mechanisms for PKIX and SMIME (lamps) Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

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. contains the core CAA specification, where Property Tags that restrict the issuance of certificates which certify domain names are defined. 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 .

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.

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 () 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 ): The production rules for "WSP", "ALPHA", and "DIGIT" are defined in Appendix B.1 of . 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 LDH domain 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

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 . The input domain to the discovery algorithm SHALL be the domain "part" () of the email address that is being certified. If the domain "part" of the email address being certified is an Internationalized Domain Name () that contains one or more U-Labels, then all U-Labels MUST be converted to their A-Label representation () for the purpose of discovering the Relevant RRSet for that email address. 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 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 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.

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".

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:

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:

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:

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.

Malformed issuemail Property The following RRSet contains a single "issuemail" Property whose sub-syntax does not conform to the ABNF as specified in . 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.

Security Considerations The security considerations that are expressed in 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: 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".

IANA Considerations The author requests the registration of the following "Certification Authority Restriction Properties":

Tag	Meaning	Reference
issuemail	Authorization Entry by Email Address	[This document]

References Normative References This document specifies the Internet Message Format (IMF), a syntax for text messages that are sent between computer users, within the framework of "electronic mail" messages. This specification is a revision of Request For Comments (RFC) 2822, which itself superseded Request For Comments (RFC) 822, "Standard for the Format of ARPA Internet Text Messages", updating it to reflect current practice and incorporating incremental changes that were specified in other RFCs. [STANDARDS-TRACK] Internet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK] This document is the revised protocol definition for Internationalized Domain Names (IDNs). The rationale for changes, the relationship to the older specification, and important terminology are provided in other documents. This document specifies the protocol mechanism, called Internationalized Domain Names in Applications (IDNA), for registering and looking up IDNs in a way that does not require changes to the DNS itself. IDNA is only meant for processing domain names, not free text. [STANDARDS-TRACK] The Certification Authority Authorization (CAA) DNS Resource Record allows a DNS domain name holder to specify one or more Certification Authorities (CAs) authorized to issue certificates for that domain name. CAA Resource Records allow a public CA to implement additional controls to reduce the risk of unintended certificate mis-issue. This document defines the syntax of the CAA record and rules for processing CAA records by CAs. This document obsoletes RFC 6844. In 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. RFC 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. Informative References This document is one of a collection that, together, describe the protocol and usage context for a revision of Internationalized Domain Names for Applications (IDNA), superseding the earlier version. It describes the document collection and provides definitions and other material that are common to the set. [STANDARDS-TRACK]

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 Russ Housley, Seo Suchan, Michael Richardson, Alexey Melnikov, Roman Danyliw, and John Levine for their suggestions which greatly improved the quality of this document.