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Internet Engineering Task Force (IETF) A. Melnikov, Ed. Request for Comments: 8398 Isode Ltd Updates: 5280 W. Chuang, Ed. Category: Standards Track Google, Inc. ISSN: 2070-1721 May 2018

      Internationalized Email Addresses in X.509 Certificates


 This document defines a new name form for inclusion in the otherName
 field of an X.509 Subject Alternative Name and Issuer Alternative
 Name extension that allows a certificate subject to be associated
 with an internationalized email address.
 This document updates RFC 5280.

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

Copyright Notice

 Copyright (c) 2018 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
 ( in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Melnikov & Chuang Standards Track [Page 1] RFC 8398 I18N Mail Addresses in X.509 Certificates May 2018

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
 2.  Conventions Used in This Document . . . . . . . . . . . . . .   2
 3.  Name Definitions  . . . . . . . . . . . . . . . . . . . . . .   3
 4.  IDNA2008  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
 5.  Matching of Internationalized Email Addresses in X.509
     Certificates  . . . . . . . . . . . . . . . . . . . . . . . .   4
 6.  Name Constraints in Path Validation . . . . . . . . . . . . .   6
 7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
 8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
 9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
   9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
 Appendix A.  ASN.1 Module . . . . . . . . . . . . . . . . . . . .  10
 Appendix B.  Example of SmtpUTF8Mailbox . . . . . . . . . . . . .  11
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  11
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1. Introduction

 [RFC5280] defines the rfc822Name subjectAltName name type for
 representing email addresses as described in [RFC5321].  The syntax
 of rfc822Name is restricted to a subset of US-ASCII characters and
 thus can't be used to represent internationalized email addresses
 [RFC6531].  This document defines a new otherName variant to
 represent internationalized email addresses.  In addition this
 document requires all email address domains in X.509 certificates to
 conform to IDNA2008 [RFC5890].

2. Conventions Used in This Document

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "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.
 The formal syntax uses the Augmented Backus-Naur Form (ABNF)
 [RFC5234] notation.

Melnikov & Chuang Standards Track [Page 2] RFC 8398 I18N Mail Addresses in X.509 Certificates May 2018

3. Name Definitions

 The GeneralName structure is defined in [RFC5280] and supports many
 different name forms including otherName for extensibility.  This
 section specifies the SmtpUTF8Mailbox name form of otherName so that
 internationalized email addresses can appear in the subjectAltName of
 a certificate, the issuerAltName of a certificate, or anywhere else
 that 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.
 When the subjectAltName (or issuerAltName) extension contains an
 internationalized email address with a non-ASCII local-part, the
 address MUST be stored in the SmtpUTF8Mailbox name form of otherName.
 The format of SmtpUTF8Mailbox is defined as the ABNF rule
 SmtpUTF8Mailbox.  SmtpUTF8Mailbox is a modified version of the
 internationalized Mailbox that was defined in Section 3.3 of
 [RFC6531], which was derived from Mailbox as defined in Section 4.1.2
 of [RFC5321].  [RFC6531] defines the following ABNF rules for Mailbox
 whose parts are modified for internationalization: <Local-part>,
 <Dot-string>, <Quoted-string>, <QcontentSMTP>, <Domain>, and <Atom>.
 In particular, <Local-part> was updated to also support UTF8-non-
 ascii.  UTF8-non-ascii was described by Section 3.1 of [RFC6532].
 Also, domain was extended to support U-labels, as defined in
 This document further refines internationalized Mailbox ABNF rules as
 described in [RFC6531] and calls this SmtpUTF8Mailbox.  In
 SmtpUTF8Mailbox, labels that include non-ASCII characters MUST be
 stored in U-label (rather than A-label) form [RFC5890].  This
 restriction removes the need to determine which label encoding, A- or
 U-label, is present in the domain.  As per Section of
 [RFC5890], U-labels are encoded as UTF-8 [RFC3629] in Normalization
 Form C and other properties specified there.  In SmtpUTF8Mailbox,
 domain labels that solely use ASCII characters (meaning neither A-
 nor U-labels) SHALL use NR-LDH restrictions as specified by
 Section 2.3.1 of [RFC5890] and SHALL be restricted to lowercase
 letters.  NR-LDH stands for "Non-Reserved Letters Digits Hyphen" and
 is the set of LDH labels that do not have "--" characters in the
 third and forth character position, which excludes "tagged domain
 names" such as A-labels.  Consistent with the treatment of rfc822Name
 in [RFC5280], SmtpUTF8Mailbox is an envelope <Mailbox> and has no

Melnikov & Chuang Standards Track [Page 3] RFC 8398 I18N Mail Addresses in X.509 Certificates May 2018

 phrase (such as a common 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 in Section 6,
 SmtpUTF8Mailbox subjectAltName MUST NOT be used unless the local-part
 of the email address contains non-ASCII characters.  When the local-
 part is ASCII, rfc822Name subjectAltName MUST be used instead of
 SmtpUTF8Mailbox.  This is compatible with legacy software that
 supports only rfc822Name (and not SmtpUTF8Mailbox).  The appropriate
 usage of rfc822Name and SmtpUTF8Mailbox is summarized in Table 1
 SmtpUTF8Mailbox is encoded as UTF8String.  The UTF8String encoding
 MUST NOT contain a Byte-Order-Mark (BOM) [RFC3629] to aid consistency
 across implementations, particularly for comparison.
  | local-part char | domain char | domain label |  subjectAltName |
  |    ASCII-only   |  ASCII-only | NR-LDH label |    rfc822Name   |
  |    non-ASCII    |  ASCII-only | NR-LDH label | SmtpUTF8Mailbox |
  |    ASCII-only   |  non-ASCII  |   A-label    |    rfc822Name   |
  |    non-ASCII    |  non-ASCII  |   U-label    | SmtpUTF8Mailbox |
         Non-ASCII may additionally include ASCII characters.
                   Table 1: Email Address Formatting

4. IDNA2008

 To facilitate comparison between email addresses, all email address
 domains in X.509 certificates MUST conform to IDNA2008 [RFC5890] (and
 avoid any "mappings" mentioned in that document).  Use of
 non-conforming email address domains introduces the possibility of
 conversion errors between alternate forms.  This applies to
 SmtpUTF8Mailbox and rfc822Name in subjectAltName, issuerAltName, and
 anywhere else that these are used.

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

 In equivalence comparison with SmtpUTF8Mailbox, there may be some
 setup work on one or both inputs depending on whether the input is
 already in comparison form.  Comparing SmtpUTF8Mailboxes consists of
 a domain part step and a local-part step.  The comparison form for
 local-parts is always UTF-8.  The comparison form for domain parts
 depends on context.  While some contexts such as certificate path

Melnikov & Chuang Standards Track [Page 4] RFC 8398 I18N Mail Addresses in X.509 Certificates May 2018

 validation in [RFC5280] specify transforming domain to A-label
 (Sections 7.2 and 7.5 in [RFC5280] as updated by [RFC8399]), this
 document recommends transforming to UTF-8 U-label instead.  This
 reduces the likelihood of errors by reducing conversions as more
 implementations natively support U-label domains.
 Comparison of two SmtpUTF8Mailboxes is straightforward with no setup
 work needed.  They are considered equivalent if there is an exact
 octet-for-octet match.  Comparison with email addresses such as
 internationalized email address or rfc822Name requires additional
 setup steps for domain part and local-part.  The initial preparation
 for the email addresses is to remove any phrases, comments, and "<"
 or ">" characters.  This document calls for comparison of domain
 labels that include non-ASCII characters to be transformed to
 U-labels if not already in that form.  The first step is to detect
 use of the A-label by using Section 5.1 of [RFC5891].  Next, if
 necessary, transform any A-labels (US-ASCII) to U-labels (Unicode) as
 specified in Section 5.2 of [RFC5891].  Finally, if necessary,
 convert the Unicode to UTF-8 as specified in Section 3 of [RFC3629].
 For ASCII NR-LDH labels, uppercase letters are converted to lowercase
 letters.  In setup for SmtpUTF8Mailbox, the email address local-part
 MUST conform to the requirements of [RFC6530] and [RFC6531],
 including being a string in UTF-8 form.  In particular, the local-
 part MUST NOT be transformed in any way, such as by doing case
 folding or normalization of any kind.  The <Local-part> part of an
 internationalized email address is already in UTF-8.  For rfc822Name,
 the local-part, which is IA5String (ASCII), trivially maps to UTF-8
 without change.  Once setup is complete, they are again compared
 octet for octet.
 To summarize non-normatively, the comparison steps, including setup,
 1.  If the domain contains A-labels, transform them to U-labels.
 2.  If the domain contains ASCII NR-LDH labels, lowercase them.
 3.  Compare strings octet for octet for equivalence.
 This specification expressly does not define any wildcard characters,
 and SmtpUTF8Mailbox comparison implementations MUST NOT interpret any
 characters as wildcards.  Instead, to specify multiple email
 addresses through SmtpUTF8Mailbox, the certificate MUST use multiple
 subjectAltNames or issuerAltNames to explicitly carry any additional
 email addresses.

Melnikov & Chuang Standards Track [Page 5] RFC 8398 I18N Mail Addresses in X.509 Certificates May 2018

6. Name Constraints in Path Validation

 This section updates Section of [RFC5280] to extend
 rfc822Name name constraints to SmtpUTF8Mailbox subjectAltNames.
 SmtpUTF8Mailbox-aware path validators will apply name constraint
 comparison to the subject distinguished name and both forms of
 subject alternative names rfc822Name and SmtpUTF8Mailbox.
 Both rfc822Name and SmtpUTF8Mailbox subject alternative names
 represent the same underlying email address namespace.  Since legacy
 CAs constrained to issue certificates for a specific set of domains
 would lack corresponding UTF-8 constraints, [RFC8399] updates,
 modifies, and extends rfc822Name name constraints defined in
 [RFC5280] to cover SmtpUTF8Mailbox subject alternative names.  This
 ensures that the introduction of SmtpUTF8Mailbox does not violate
 existing name constraints.  Since it is not valid to include
 non-ASCII UTF-8 characters in the local-part of rfc822Name name
 constraints, and since name constraints that include a local-part are
 rarely, if at all, used in practice, name constraints updated in
 [RFC8399] allow the forms that represent all addresses at a host or
 all mailboxes in a domain and deprecates rfc822Name name constraints
 that represent a particular mailbox.  That is, rfc822Name constraints
 with a local-part SHOULD NOT be used.
 Constraint comparison with SmtpUTF8Mailbox subjectAltName starts with
 the setup steps defined by Section 5.  Setup converts the inputs of
 the comparison (which is one of a subject distinguished name, an
 rfc822Name, or an SmtpUTF8Mailbox subjectAltName, and one of an
 rfc822Name name constraint) to constraint comparison form.  For an
 rfc822Name name constraint, this will convert any domain A-labels to
 U-labels.  For both the name constraint and the subject, this will
 lowercase any domain NR-LDH labels.  Strip the local-part and "@"
 separator from each rfc822Name and SmtpUTF8Mailbox, leaving just the
 domain part.  After setup, this follows the comparison steps defined
 in Section of [RFC5280] as follows.  If the resulting name
 constraint domain starts with a "." character, then for the name
 constraint to match, a suffix of the resulting subject alternative
 name domain MUST match the name constraint (including the leading
 ".") octet for octet.  If the resulting name constraint domain does
 not start with a "." character, then for the name constraint to
 match, the entire resulting subject alternative name domain MUST
 match the name constraint octet for octet.
 Certificate Authorities that wish to issue CA certificates with email
 address name constraints MUST use rfc822Name subject alternative
 names only.  These MUST be IDNA2008-conformant names with no mappings
 and with non-ASCII domains encoded in A-labels only.

Melnikov & Chuang Standards Track [Page 6] RFC 8398 I18N Mail Addresses in X.509 Certificates May 2018

 The name constraint requirement with SmtpUTF8Mailbox subject
 alternative name is illustrated in the non-normative diagram in
 Figure 1.  The first example (1) illustrates a permitted rfc822Name
 ASCII-only host name name constraint and the corresponding valid
 rfc822Name subjectAltName and SmtpUTF8Mailbox subjectAltName email
 addresses.  The second example (2) illustrates a permitted rfc822Name
 host name name constraint with A-label, and the corresponding valid
 rfc822Name subjectAltName and SmtpUTF8Mailbox subjectAltName email
 addresses.  Note that an email address with ASCII-only local-part is
 encoded as rfc822Name despite also having Unicode present in the
 |  Root CA Cert                                                     |
 |  Intermediate CA Cert                                             |
 |      Permitted                                                    |
 |        rfc822Name: (1)              |
 |                                                                   |
 |        rfc822Name: (2)                     |
 |                                                                   |
 |  Entity Cert (w/explicitly permitted subjects)                    |
 |    SubjectAltName Extension                                       |
 |      rfc822Name: (1)         |
 |      SmtpUTF8Mailbox:  |
 |        (1)                                                        |
 |                                                                   |
 |      rfc822Name: (2)               |
 |      SmtpUTF8Mailbox: (2)   |
 |                                                                   |
      Figure 1: Name Constraints with SmtpUTF8Name and rfc822Name

7. Security Considerations

 Use of SmtpUTF8Mailbox for certificate subjectAltName (and
 issuerAltName) will incur many of the same security considerations as
 in Section 8 in [RFC5280], but it introduces a new issue by
 permitting non-ASCII characters in the email address local-part.
 This issue, as mentioned in Section 4.4 of [RFC5890] and in Section 4

Melnikov & Chuang Standards Track [Page 7] RFC 8398 I18N Mail Addresses in X.509 Certificates May 2018

 of [RFC6532], is that use of Unicode introduces the risk of visually
 similar and identical characters that can be exploited to deceive the
 recipient.  The former document references some means to mitigate
 against these attacks.  See [WEBER] for more background on security
 issues with Unicode.

8. IANA Considerations

 As described in Section 3 and the ASN.1 module identifier defined in
 Appendix A, IANA has assigned the values described here.
 o  For the LAMPS-EaiAddresses-2016 ASN.1 module, IANA has registered
    value 92 for "id-mod-lamps-eai-addresses-2016" in the "SMI
    Security for PKIX Module Identifier" ( registry.
 o  For the SmtpUTF8Mailbox otherName, IANA has registered value 9 for
    id-on-SmtpUTF8Mailbox in the "SMI Security for PKIX Other Name
    Forms" ( registry.

9. References

9.1. 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,
 [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
            10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
            2003, <>.
 [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234,
            DOI 10.17487/RFC5234, January 2008,
 [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,
 [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
            DOI 10.17487/RFC5321, October 2008,

Melnikov & Chuang Standards Track [Page 8] RFC 8398 I18N Mail Addresses in X.509 Certificates May 2018

 [RFC5890]  Klensin, J., "Internationalized Domain Names for
            Applications (IDNA): Definitions and Document Framework",
            RFC 5890, DOI 10.17487/RFC5890, August 2010,
 [RFC5891]  Klensin, J., "Internationalized Domain Names in
            Applications (IDNA): Protocol", RFC 5891,
            DOI 10.17487/RFC5891, August 2010,
 [RFC6530]  Klensin, J. and Y. Ko, "Overview and Framework for
            Internationalized Email", RFC 6530, DOI 10.17487/RFC6530,
            February 2012, <>.
 [RFC6531]  Yao, J. and W. Mao, "SMTP Extension for Internationalized
            Email", RFC 6531, DOI 10.17487/RFC6531, February 2012,
 [RFC6532]  Yang, A., Steele, S., and N. Freed, "Internationalized
            Email Headers", RFC 6532, DOI 10.17487/RFC6532, February
            2012, <>.
 [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
            2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
            May 2017, <>.
 [RFC8399]  Housley, R., "Internationalization Updates to RFC 5280",
            RFC 8399, DOI 10.17487/RFC8399, May 2018,

9.2. Informative References

 [RFC5912]  Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
            Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
            DOI 10.17487/RFC5912, June 2010,
 [WEBER]    Weber, C., "Attacking Software Globalization", March 2010,

Melnikov & Chuang Standards Track [Page 9] RFC 8398 I18N Mail Addresses in X.509 Certificates May 2018

Appendix A. ASN.1 Module

 The following ASN.1 module normatively specifies the SmtpUTF8Mailbox
 structure.  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.
  { 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) }
  FROM 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) }
  FROM 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) } ;
  1. -
  2. - otherName carries additional name types for subjectAltName,
  3. - issuerAltName, and other uses of GeneralNames.
  4. -
  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.

Melnikov & Chuang Standards Track [Page 10] RFC 8398 I18N Mail Addresses in X.509 Certificates May 2018

Appendix B. Example of SmtpUTF8Mailbox

 This non-normative example demonstrates using SmtpUTF8Mailbox as an
 otherName in GeneralName to encode the email address
    The hexadecimal DER encoding of the email address is:
    A022060A 2B060105 05070012 0809A014 0C12E880 81E5B8AB 40657861
    6D706C65 2E636F6D
    The text decoding is:
      0  34: [0] {
      2  10:   OBJECT IDENTIFIER '1 3 6 1 5 5 7 0 18 8 9'
     14  20:   [0] {
     16  18:     UTF8String ''
           :     }
           :   }
                               Figure 2
 The example was encoded on the OSS Nokalva ASN.1 Playground and the
 above text decoding is an output of Peter Gutmann's "dumpasn1"


 Thank you to Magnus Nystrom for motivating this document.  Thanks to
 Russ Housley, Nicolas Lidzborski, Laetitia Baudoin, Ryan Sleevi, Sean
 Leonard, Sean Turner, John Levine, and Patrik Falstrom for their
 feedback.  Also special thanks to John Klensin for his valuable input
 on internationalization, Unicode, and ABNF formatting; to Jim Schaad
 for his help with the ASN.1 example and his helpful feedback; and
 especially to Viktor Dukhovni for helping us with name constraints
 and his many detailed document reviews.

Melnikov & Chuang Standards Track [Page 11] RFC 8398 I18N Mail Addresses in X.509 Certificates May 2018

Authors' Addresses

 Alexey Melnikov (editor)
 Isode Ltd
 14 Castle Mews
 Hampton, Middlesex  TW12 2NP
 United Kingdom
 Weihaw Chuang (editor)
 Google, Inc.
 1600 Amphitheater Parkway
 Mountain View, CA  94043
 United States of America

Melnikov & Chuang Standards Track [Page 12]

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