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

Abstract

   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
   https://www.rfc-editor.org/info/rfc8398.

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
   (https://trustee.ietf.org/license-info) 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.

   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",
   "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.

   The formal syntax uses the Augmented Backus-Naur Form (ABNF)
   [RFC5234] notation.

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
   [RFC5890].

   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 2.3.2.1 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

   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
   below.

   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

   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,
   are:

   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.

6. Name Constraints in Path Validation

   This section updates Section 4.2.1.10 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 4.2.1.10 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.

   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
   domain.

   +-------------------------------------------------------------------+
   |  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@elemenary.school.example.com (1)         |
   |      SmtpUTF8Mailbox: u+5B66u+751F@elementary.school.example.com  |
   |        (1)                                                        |
   |                                                                   |
   |      rfc822Name: student@xn--pss25c.example.com (2)               |
   |      SmtpUTF8Mailbox: u+533Bu+751F@u+5927u+5B66.example.com (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

   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" (1.3.6.1.5.5.7.0) registry.

   o  For the SmtpUTF8Mailbox otherName, IANA has registered value 9 for
      id-on-SmtpUTF8Mailbox in the "SMI Security for PKIX Other Name
      Forms" (1.3.6.1.5.5.7.8) 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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [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>.

   [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,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,
              <https://www.rfc-editor.org/info/rfc5321>.

   [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>.

   [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>.

   [RFC6530]  Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", RFC 6530, DOI 10.17487/RFC6530,
              February 2012, <https://www.rfc-editor.org/info/rfc6530>.

   [RFC6531]  Yao, J. and W. Mao, "SMTP Extension for Internationalized
              Email", RFC 6531, DOI 10.17487/RFC6531, February 2012,
              <https://www.rfc-editor.org/info/rfc6531>.

   [RFC6532]  Yang, A., Steele, S., and N. Freed, "Internationalized
              Email Headers", RFC 6532, DOI 10.17487/RFC6532, February
              2012, <https://www.rfc-editor.org/info/rfc6532>.

   [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>.

   [RFC8399]  Housley, R., "Internationalization Updates to RFC 5280",
              RFC 8399, DOI 10.17487/RFC8399, May 2018,
              <https://www.rfc-editor.org/info/rfc8399>.

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,
              <https://www.rfc-editor.org/info/rfc5912>.

   [WEBER]    Weber, C., "Attacking Software Globalization", March 2010,
              <https://www.lookout.net/files/
              Chris_Weber_Character%20Transformations%20v1.7_IUC33.pdf>.

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.

  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 ::=
  BEGIN

  IMPORTS
    OTHER-NAME
    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) }

    id-pkix
    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) } ;

  --
  -- 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.

  END

Appendix B. Example of SmtpUTF8Mailbox

   This non-normative example demonstrates using SmtpUTF8Mailbox as an
   otherName in GeneralName to encode the email address
   "u+8001u+5E2B@example.com".

      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 '..@example.com'
             :     }
             :   }

                                 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"
   program.

Acknowledgements

   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.

Authors' Addresses

   Alexey Melnikov (editor)
   Isode Ltd
   14 Castle Mews
   Hampton, Middlesex  TW12 2NP
   United Kingdom

   Email: Alexey.Melnikov@isode.com

   Weihaw Chuang (editor)
   Google, Inc.
   1600 Amphitheater Parkway
   Mountain View, CA  94043
   United States of America

   Email: weihaw@google.com