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Internationalized Email Addresses in X.509 Certificates
draft-ietf-lamps-rfc8398bis-05

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 9598.
Authors Alexey Melnikov , Wei Chuang , Corey Bonnell
Last updated 2024-05-23 (Latest revision 2024-02-13)
Replaces draft-bonnell-lamps-rfc8398bis
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
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Stream WG state Submitted to IESG for Publication
Document shepherd Tim Hollebeek
Shepherd write-up Show Last changed 2023-12-13
IESG IESG state Became RFC 9598 (Proposed Standard)
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(None)
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Roman Danyliw
Send notices to tim.hollebeek@digicert.com
IANA IANA review state IANA OK - Actions Needed
IANA action state RFC-Ed-Ack
draft-ietf-lamps-rfc8398bis-05
Network Working Group                                        A. Melnikov
Internet-Draft                                                 Isode Ltd
Obsoletes: 8398 (if approved)                                  W. Chuang
Updates: 5280 (if approved)                                 Google, Inc.
Intended status: Standards Track                              C. Bonnell
Expires: 16 August 2024                                         DigiCert
                                                        13 February 2024

        Internationalized Email Addresses in X.509 Certificates
                     draft-ietf-lamps-rfc8398bis-05

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 and obsoletes RFC 8398.

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/draft-lamps-rfc8398-bis/draft-bonnell-
   lamps-rfc8398bis.html.  Status information for this document may be
   found at https://datatracker.ietf.org/doc/draft-ietf-lamps-
   rfc8398bis/.

   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/draft-lamps-rfc8398-bis.

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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   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 16 August 2024.

Copyright Notice

   Copyright (c) 2024 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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Name Definitions  . . . . . . . . . . . . . . . . . . . . . .   3
   4.  IDNA2008  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Matching of Internationalized Email Addresses in X.509
           Certificates  . . . . . . . . . . . . . . . . . . . . . .   5
   6.  Name Constraints in Path Validation . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  Differences from RFC 8398 . . . . . . . . . . . . . . . . . .   9
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Appendix A.  ASN.1 Module . . . . . . . . . . . . . . . . . . . .  11
   Appendix B.  Example of SmtpUTF8Mailbox . . . . . . . . . . . . .  13
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

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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].

   This document obsoletes [RFC8398].  The primary motivation for
   publication of this document is to simplify the encoding of domain
   labels found in the domain part of internationalized email addresses.
   In particular, [RFC8398] specifies that domain labels are
   conditionally encoded using either A-labels or U-labels.  This
   specification simplifies encoding and processing of domain labels by
   mandating that the A-label representation be used in all cases.

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.  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. 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.

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   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 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 A-label (rather than U-label) form [RFC5890].  This
   restriction reduces complexity for implementations of the
   certification path validation algorithm defined in Section 6 of
   [RFC5280].  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].  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.  To facilitate octet-for-octet comparisons of
   SmtpUTF8Mailbox values, all NR-LDH and A-label labels which
   constitute the domain part SHALL only be encoded with lowercase
   letters.  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.

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                   +=================+=================+
                   | Local-part char | subjectAltName  |
                   +=================+=================+
                   | ASCII-only      | rfc822Name      |
                   +-----------------+-----------------+
                   | non-ASCII       | SmtpUTF8Mailbox |
                   +-----------------+-----------------+

                     Table 1: Email Address Formatting

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

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

   Equivalence comparisons with SmtpUTF8Mailbox consist 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 is always
   performed with the LDH-label ([RFC5890]) encoding of the relevant
   domain labels.  The comparison of LDH-labels in domain parts reduces
   complexity for implementations of the certification path validation
   algorithm as defined in Section 6 of [RFC5280] by obviating the need
   to convert domain labels to their Unicode representation.

   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 of a SmtpUTF8Mailbox and rfc822Name will always fail.
   SmtpUTF8Mailbox values SHALL contain a Local-part which includes one
   or more non-ASCII characters, while rfc822Names only include ASCII
   characters (including the Local-part).  Thus, a SmtpUTF8Mailbox and
   rfc822Name will never match.

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

   For the setup of the domain part, the following conversions SHALL be
   performed:

   1.  Convert all labels which constitute the domain part that include
       non-ASCII characters to A-labels if not already in that form.

       a.  Detect all U-labels present within the domain part using
       Section 5.1 of [RFC5891].

       b.  Transform all detected U-labels (Unicode) to A-labels (ASCII)
       as specified in Section 5.5 of [RFC5891].

   2.  Convert all uppercase letters found within the NR-LDH and A-label
       labels which constitute the domain part to lowercase letters.

   For the setup of the Local-part, the Local-part MUST be verified to
   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.  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 U-labels, transform them to A-labels.

   2.  If any NR-LDH or A-label domain label in the domain part contains
       uppercase letters, 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.

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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, [RFC8399BIS] 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
   [RFC8399BIS] 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 both
   the name constraint and the subject, this will convert all A-labels
   and NR-LDH labels to lowercase.  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.

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   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@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)     |
  |                                                                   |
  +-------------------------------------------------------------------+

       Figure 1: Name Constraints with SmtpUTF8Name and rfc822Name

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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.

   Additionally, it is possible to encode a string of Unicode user-
   perceived characters in multiple ways.  While various Unicode
   normalization forms exist, [RFC6531] does not mandate the use of any
   such forms for the encoding of the Local-part.  Thus, it may be
   possible to encode a Local-part value in multiple ways.  To mitigate
   against attacks where different encodings are used by the mail system
   and the Certification Authority issuing certificates containing
   SmtpUTF8Mailbox values, this specification requires an octet-for-
   octet comparison of the Local-part.  However, requiring the use of
   binary comparison may raise interoperability concerns where the mail
   system employs one encoding and the Certification Authority employs
   another.

8.  Differences from RFC 8398

   This document obsoletes [RFC8398].  There are three major changes
   defined in this specification which deviate from [RFC8398]:

   1.  In all cases, domain labels in mail addresses SHALL be encoded as
       LDH-labels.  In particular, domain names SHALL NOT be encoded
       using U-Labels and instead use A-Labels.

   2.  To accommodate the first change listed above, the mail address
       matching algorithm defined in Section 5 of [RFC8398] has been
       modified to only accept domain labels that are encoded using
       their A-label representation.

   3.  Additionally, the name constraints processing algorithm defined
       in Section 6 of [RFC8398] has been modified to only accept domain
       labels that are encoded using their A-label representation.

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9.  IANA Considerations

   Update the document 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 from RFC 8398 to this document.

   Update the document reference for the SmtpUTF8Mailbox otherName in
   the "SMI Security for PKIX Other Name Forms" (1.3.6.1.5.5.7.8)
   registry from RFC 8398 to this document.

10.  References

10.1.  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>.

   [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/rfc/rfc3629>.

   [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/rfc/rfc5280>.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,
              <https://www.rfc-editor.org/rfc/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/rfc/rfc5890>.

   [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>.

   [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/rfc/rfc6530>.

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   [RFC6531]  Yao, J. and W. Mao, "SMTP Extension for Internationalized
              Email", RFC 6531, DOI 10.17487/RFC6531, February 2012,
              <https://www.rfc-editor.org/rfc/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/rfc/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/rfc/rfc8174>.

   [RFC8398]  Melnikov, A., Ed. and W. Chuang, Ed., "Internationalized
              Email Addresses in X.509 Certificates", RFC 8398,
              DOI 10.17487/RFC8398, May 2018,
              <https://www.rfc-editor.org/rfc/rfc8398>.

   [RFC8399BIS]
              Housley, R., "Internationalization Updates to RFC 5280",
              n.d., <https://datatracker.ietf.org/doc/draft-housley-
              lamps-rfc8399bis/>.

10.2.  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/rfc/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.

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   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. 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

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Appendix B.  Example of SmtpUTF8Mailbox

   This non-normative example demonstrates using SmtpUTF8Mailbox as an
   otherName 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 6e2d2d70
   73733235 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 the
   above text decoding is an output of Peter Gutmann's "dumpasn1"
   program.

Acknowledgments

   The authors thank David Benjamin for providing the motivation for
   this document.  Additionally, the authors thank √Čric Vyncke, John
   Levine, Peter van Dijk, Rich Salz, Russ Housley, and Tim Hollebeek
   for their reviews and feedback which meaningfully improved the
   document.

   The authors also recognize and appreciate the following individuals
   for their contributions to the previous version of this document:

   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

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   Alexey Melnikov
   Isode Ltd
   14 Castle Mews
   Hampton
   TW12 2NP
   United Kingdom
   Email: Alexey.Melnikov@isode.com

   Wei Chuang
   Google, Inc.
   1600 Amphitheater Parkway
   Mountain View, CA
   United States of America
   Email: weihaw@google.com

   Corey Bonnell
   DigiCert
   Pittsburgh, PA
   United States of America
   Email: corey.bonnell@digicert.com

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