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 athttps://www.rfc-editor.org/info/rfc9598.¶

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Table of Contents

1.Introduction

[RFC5280] defines the rfc822Name subjectAltName name type forrepresenting email addresses as described in[RFC5321]. The syntaxof rfc822Name is restricted to a subset of US-ASCII characters andthus can't be used to represent internationalized email addresses[RFC6531]. This document defines a new otherName variant torepresent internationalized email addresses. In addition, thisdocument requires all email address domains in X.509 certificates toconform to IDNA2008[RFC5890].¶

This document obsoletes[RFC8398]. The primary motivation of this document is to simplify the encoding of domain labelsfound in the domain part of internationalized email addresses. Inparticular,[RFC8398] specifies that domain labels are conditionallyencoded using either A-labels or U-labels. This specification simplifiesencoding and processing of domain labels by mandating that the A-labelrepresentation be used in all cases.¶

2.Conventions Used in This Document

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.Name Definitions

The GeneralName structure[RFC5280] supports manydifferent name forms including otherName for extensibility. Thissection specifies the SmtpUTF8Mailbox name form of otherName so thatinternationalized email addresses can appear in the subjectAltName ofa certificate, the issuerAltName of a certificate, or anywhere elsethat GeneralName is used.¶

id-on-SmtpUTF8Mailbox OBJECT IDENTIFIER ::= { id-on 9 }SmtpUTF8Mailbox ::= UTF8String (SIZE (1..MAX))-- SmtpUTF8Mailbox conforms to Mailbox as specified-- in Section 3.3 of RFC 6531. Additionally, all domain-- labels included in the SmtpUTF8Mailbox value are-- encoded as LDH labels. In particular, domain labels-- are not encoded as U-labels and instead are encoded-- using their A-label representation.

When the subjectAltName (or issuerAltName) extension contains aninternationalized email address with a non-ASCII Local-part, theaddressMUST be stored in the SmtpUTF8Mailbox name form of otherName.The format of SmtpUTF8Mailbox is a modified version of theinternationalized Mailbox that was defined inSection 3.3 of [RFC6531], which was derived from Mailbox as defined inSection 4.1.2 of [RFC5321].[RFC6531] defines the following ABNF rules for Mailboxwhose parts are modified for internationalization:Local-part,Dot-string,Quoted-string,QcontentSMTP,Domain, andAtom.In particular,Local-part was updated to also supportUTF8-non-ascii. UTF8-non-ascii was described bySection 3.1 of [RFC6532]. Also, domain was extended to support U-labels, as definedin[RFC5890].¶

This document further refines internationalized Mailbox ABNF rules asdescribed in[RFC6531] and calls this SmtpUTF8Mailbox. InSmtpUTF8Mailbox, labels that include non-ASCII charactersMUST bestored in A-label (rather than U-label) form[RFC5890]. Thisrestriction reduces complexity for implementations of the certificationpath validation algorithm defined inSection 6 of [RFC5280]. InSmtpUTF8Mailbox, domain labels that solely use ASCII characters (meaningneither A- nor U-labels)SHALL use NR-LDH restrictions as specified bySection 2.3.1 of [RFC5890]. NR-LDH stands for "Non-Reserved LettersDigits Hyphen" and is the set of LDH labels that do not have "--"characters in the third and forth character positions, which excludes"tagged domain names" such as A-labels. To facilitate octet-for-octetcomparisons of SmtpUTF8Mailbox values, all NR-LDH and A-label labelsthat constitute the domain partSHALL only be encoded with lowercaseletters. Consistent with the treatment of rfc822Name in[RFC5280],SmtpUTF8Mailbox is an envelopeMailbox and has no phrase (such as acommon name) before it, has no comment (text surrounded in parentheses)after it, and is not surrounded by "<" and ">" characters.¶

Due to name constraint compatibility reasons described inSection 6,SmtpUTF8Mailbox subjectAltNameMUST NOT be used unless the Local-partof the email address contains non-ASCII characters. When theLocal-part is ASCII, rfc822Name subjectAltNameMUST be used instead ofSmtpUTF8Mailbox. This is compatible with legacy software thatsupports only rfc822Name (and not SmtpUTF8Mailbox). The appropriateusage of rfc822Name and SmtpUTF8Mailbox is summarized in Table 1below.¶

SmtpUTF8Mailbox is encoded as UTF8String. The UTF8String encodingMUST NOT contain a Byte Order Mark (BOM)[RFC3629] to aid consistencyacross implementations, particularly for comparison.¶

Non-ASCII Local-part values may additionally include ASCII characters.¶

4.IDNA2008

To facilitate comparison between email addresses, all email addressdomains in X.509 certificatesMUST conform to IDNA2008[RFC5890] (andavoid any "mappings" mentioned in that document). Use ofnon-conforming email address domains introduces the possibility ofconversion errors between alternate forms. This applies toSmtpUTF8Mailbox and rfc822Name in subjectAltName, issuerAltName, andanywhere else that these are used.¶

5.Matching of Internationalized Email Addresses in X.509 Certificates

Equivalence comparisons with SmtpUTF8Mailbox consist ofa domain part step and a Local-part step. The comparison form forLocal-parts is always UTF-8. The comparison form for domain partsis always performed with the LDH label ([RFC5890]) encoding of therelevant domain labels. The comparison of LDH labels in domain partsreduces complexity for implementations of the certification pathvalidation algorithm as defined inSection 6 of [RFC5280] by obviatingthe need to convert domain labels to their Unicode representation.¶

Comparison of two SmtpUTF8Mailboxes is straightforward with no setupwork needed. They are considered equivalent if there is an exactoctet-for-octet match.¶

Comparison of an SmtpUTF8Mailbox and rfc822Name will always fail.SmtpUTF8Mailbox valuesSHALL contain a Local-part that includesone or more non-ASCII characters, while rfc822Names onlyincludes ASCII characters (including the Local-part). Thus, anSmtpUTF8Mailbox and rfc822Name will never match.¶

Comparison of SmtpUTF8Mailbox values with internationalized emailaddresses from other sources (such as received email messages, userinput, etc.) requires additional setup steps for domain part andLocal-part. The initial preparation for the email address to comparewith the SmtpUTF8Mailbox value is to remove any phrases, comments, and"<" or ">" characters.¶

For the setup of the domain part, the following conversionsSHALL beperformed:¶

1. Convert all labels that constitute the domain part that includenon-ASCII characters to A-labels, if not already in that form.¶

   1. Detect all U-labels present within the domain part usingSection 5.1 of [RFC5891].¶

   2. Transform all detected U-labels (Unicode) to A-labels (ASCII) as specified inSection 5.5 of [RFC5891].¶

2. Convert all uppercase letters found within the NR-LDH and A-labellabels that constitute the domain part to lowercase letters.¶

For the setup of the Local-part, the Local-partMUST be verified toconform to the requirements of[RFC6530] and[RFC6531], includingbeing a string in UTF-8 form. In particular, the Local-partMUST NOT be transformed in any way, such as by doing casefolding or normalization of any kind. TheLocal-part of aninternationalized email address is already in UTF-8. Once setup iscomplete, they are again compared octet for octet.¶

To summarize non-normatively, the comparison steps, including setup,are:¶

1. If the domain contains U-labels, transform them to A-labels.¶

2. If any NR-LDH or A-label domain label in the domain partcontains uppercase letters, lowercase them.¶

3. Compare strings octet for octet for equivalence.¶

This specification expressly does not define any wildcard characters,and SmtpUTF8Mailbox comparison implementationsMUST NOT interpret anycharacters as wildcards. Instead, to specify multiple emailaddresses through SmtpUTF8Mailbox, the certificateMUST use multiplesubjectAltNames or issuerAltNames to explicitly carry any additionalemail addresses.¶

6.Name Constraints in Path Validation

This section updatesSection 4.2.1.10 of [RFC5280] to extendrfc822Name name constraints to SmtpUTF8Mailbox subjectAltNames.SmtpUTF8Mailbox-aware path validators will apply name constraintcomparison to the subject distinguished name and both forms ofsubject alternative names, rfc822Name and SmtpUTF8Mailbox.¶

Both rfc822Name and SmtpUTF8Mailbox subject alternative namesrepresent the same underlying email address namespace. Since legacyCertification Authorities (CAs) constrained to issue certificates for a specific set of domainswould lack corresponding UTF-8 constraints,[RFC9549] updates,modifies, and extends rfc822Name name constraints defined in[RFC5280] to cover SmtpUTF8Mailbox subject alternative names. Thisensures that the introduction of SmtpUTF8Mailbox does not violateexisting name constraints. Since it is not valid to includenon-ASCII UTF-8 characters in the Local-part of rfc822Name nameconstraints, and since name constraints that include a Local-part arerarely, if at all, used in practice, name constraints updated in[RFC9549] allow the forms that represent all addresses at a host, orall mailboxes in a domain and deprecates rfc822Name name constraintsthat represent a particular mailbox. That is, rfc822Name constraintswith a Local-partSHOULD NOT be used.¶

Constraint comparison with SmtpUTF8Mailbox subjectAltName starts withthe setup steps defined inSection 5. Setup converts the inputs ofthe comparison (which is one of a subject distinguished name, anrfc822Name, or an SmtpUTF8Mailbox subjectAltName, and one of anrfc822Name name constraint) to constraint comparison form. For both thename constraint and the subject, this will convert all A-labels andNR-LDH labels to lowercase. Strip the Local-part and "@"separator from each rfc822Name and SmtpUTF8Mailbox, which leaves just thedomain part. After setup, follow the comparison steps definedinSection 4.2.1.10 of [RFC5280] as follows. If the resulting nameconstraint domain starts with a "." character, then for the nameconstraint to match, a suffix of the resulting subject alternativename domainMUST match the name constraint (including the leading".") octet for octet. If the resulting name constraint domain doesnot start with a "." character, then for the name constraint tomatch, the entire resulting subject alternative name domainMUSTmatch the name constraint octet for octet.¶

Certificate Authorities that wish to issue CA certificates with emailaddress name constraintsMUST use rfc822Name subject alternativenames only. TheseMUST be IDNA2008-conformant names with no mappingsand with non-ASCII domains encoded in A-labels only.¶

The name constraint requirement with an SmtpUTF8Mailbox subjectalternative name is illustrated in the non-normative diagram inFigure 1. The first example (1) illustrates a permitted rfc822NameASCII-only host name name constraint and the corresponding validrfc822Name subjectAltName and SmtpUTF8Mailbox subjectAltName emailaddresses. The second example (2) illustrates a permitted rfc822Namehost name name constraint with an A-label, and the corresponding validrfc822Name subjectAltName and SmtpUTF8Mailbox subjectAltName emailaddresses. Note that an email address with an ASCII-only Local-part isencoded as rfc822Name despite also having Unicode present in thedomain.¶

+-------------------------------------------------------------------+|  Root CA Cert                                                     |+-------------------------------------------------------------------+                                  |                                  v+-------------------------------------------------------------------+|  Intermediate CA Cert                                             ||      Permitted                                                    ||        rfc822Name: elementary.school.example.com (1)              ||                                                                   ||        rfc822Name: xn--pss25c.example.com (2)                     ||                                                                   |+-------------------------------------------------------------------+                                  |                                  v+-------------------------------------------------------------------+|  Entity Cert (w/explicitly permitted subjects)                    ||    SubjectAltName Extension                                       ||      rfc822Name: student@elementary.school.example.com (1)        ||      SmtpUTF8Mailbox: u+5B66u+751F@elementary.school.example.com  ||        (1)                                                        ||                                                                   ||      rfc822Name: student@xn--pss25c.example.com (2)               ||      SmtpUTF8Mailbox: u+533Bu+751F@xn--pss25c.example.com (2)     ||                                                                   |+-------------------------------------------------------------------+

7.Security Considerations

Use of SmtpUTF8Mailbox for certificate subjectAltName (andissuerAltName) will incur many of the same security considerations described inSection 8 of [RFC5280], but it introduces a new issue bypermitting non-ASCII characters in the email address Local-part.This issue, as mentioned inSection 4.4 of [RFC5890] and inSection 4 of [RFC6532], is that use of Unicode introduces the risk of visuallysimilar and identical characters that can be exploited to deceive therecipient. The former document references some means to mitigateagainst these attacks. See[WEBER] for more background on securityissues with Unicode.¶

Additionally, it is possible to encode a string of Unicodeuser-perceived characters in multiple ways. While various Unicodenormalization forms exist,[RFC6531] does not mandate the use of anysuch forms for the encoding of the Local-part. Thus, it may be possibleto encode a Local-part value in multiple ways. To mitigate againstattacks where different encodings are used by the mail system and theCertification Authority issues certificates containingSmtpUTF8Mailbox values, this specification requires an octet-for-octetcomparison of the Local-part. However, requiring the use of binarycomparison may raise interoperability concerns where the mail systememploys one encoding and the Certification Authority employs another.¶

8.Differences from RFC 8398

This document obsoletes[RFC8398]. There are three major changesdefined in this specification:¶

1. In all cases, domain labels in mail addressesSHALL be encoded asLDH labels. In particular, domain namesSHALL NOT be encoded usingU-Labels; instead, use A-Labels.¶

2. To accommodate the first change listed above, the mail addressmatching algorithm defined inSection 5 of [RFC8398] has been modifiedto only accept domain labels that are encoded using their A-labelrepresentation.¶

3. Additionally, the procedure to process rfc822Name name constraints as defined inSection 6 of [RFC8398] has been modified to only accept domain labelsthat are encoded using their A-label representation.¶

9.IANA Considerations

IANA has updated the reference for the id-mod-lamps-eai-addresses-2016 module in the "SMI Security for PKIX Module Identifier"(1.3.6.1.5.5.7.0) registry to refer to this document instead of[RFC8398].¶

IANA has updated the reference for the SmtpUTF8Mailbox otherName in the"SMI Security for PKIX Other Name Forms" (1.3.6.1.5.5.7.8) registry to refer to this document instead of[RFC8398].¶

Appendix A.ASN.1 Module

The following ASN.1 module normatively specifies the SmtpUTF8Mailboxstructure. This specification uses the ASN.1 definitions from[RFC5912] with the 2002 ASN.1 notation used in that document.[RFC5912] updates normative documents using older ASN.1 notation.¶

LAMPS-EaiAddresses-2016{ iso(1) identified-organization(3) dod(6)  internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)  id-mod-lamps-eai-addresses-2016(92) }DEFINITIONS IMPLICIT TAGS ::=BEGINIMPORTSOTHER-NAMEFROM PKIX1Implicit-2009  { iso(1) identified-organization(3) dod(6) internet(1) security(5)  mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-implicit-02(59) }id-pkixFROM PKIX1Explicit-2009  { iso(1) identified-organization(3) dod(6) internet(1) security(5)  mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-explicit-02(51) } ;---- otherName carries additional name types for subjectAltName,-- issuerAltName, and other uses of GeneralNames.--id-on OBJECT IDENTIFIER ::= { id-pkix 8 }SmtpUtf8OtherNames OTHER-NAME ::= { on-SmtpUTF8Mailbox, ... }on-SmtpUTF8Mailbox OTHER-NAME ::= {    SmtpUTF8Mailbox IDENTIFIED BY id-on-SmtpUTF8Mailbox}id-on-SmtpUTF8Mailbox OBJECT IDENTIFIER ::= { id-on 9 }SmtpUTF8Mailbox ::= UTF8String (SIZE (1..MAX))-- SmtpUTF8Mailbox conforms to Mailbox as specified-- in Section 3.3 of RFC 6531. Additionally, all domain-- labels included in the SmtpUTF8Mailbox value are-- encoded as LDH Labels. In particular, domain labels-- are not encoded as U-Labels and instead are encoded-- using their A-label representation.END

Appendix B.Example of SmtpUTF8Mailbox

This non-normative example demonstrates using SmtpUTF8Mailbox as anotherName in GeneralName to encode the email address"u+533Bu+751F@xn--pss25c.example.com".¶

The hexadecimal DER encoding of the block is:¶

a02b0608 2b060105 05070809 a01f0c1d e58cbbe7 949f4078 6e2d2d7073733235 632e6578 616d706c 652e636f 6d

The text decoding is:¶

0  43: [0] {2   8:   OBJECT IDENTIFIER '1 3 6 1 5 5 7 8 9'12  31:   [0] {14  29:     UTF8String 'u+533Bu+751F@xn--pss25c.example.com'      :     }      :   }

The example was encoded using Google's "der-ascii" program and theabove text decoding is an output of Peter Gutmann's "dumpasn1"program.¶

Acknowledgments

The authors thank David Benjamin for providing the motivation for thisdocument. Additionally, the authors thank Éric Vyncke, John Levine,Peter van Dijk, Rich Salz, Russ Housley, and Tim Hollebeek for theirreviews and feedback, which meaningfully improved the document.¶

The authors also recognize and appreciate the following individuals fortheir contributions to[RFC8398]:¶

Thank you to Magnus Nystrom for motivating this document. Thanks toRuss Housley, Nicolas Lidzborski, Laetitia Baudoin, Ryan Sleevi, SeanLeonard, Sean Turner, John Levine, and Patrik Falstrom for theirfeedback. Also special thanks to John Klensin for his valuable inputon internationalization, Unicode, and ABNF formatting; to Jim Schaadfor his help with the ASN.1 example and his helpful feedback; andespecially to Viktor Dukhovni for helping us with name constraintsand his many detailed document reviews.¶

Authors' Addresses