LAMPS                                                   A. Melnikov, Ed.
Internet-Draft                                                 Isode Ltd
Intended status: Standards Track                          W. Chuang, Ed.
Expires: December 20, 31, 2017                                  Google, Inc.
                                                           June 18, 29, 2017

        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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   2
   3.  Name Definitions  . . . . . . . . . . . . . . . . . . . . . .   2
   4.  IDNA2008  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Matching of Internationalized Email Addresses in X.509
       certificates  . . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Name constraints in path validation . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  ASN.1 Module . . . . . . . . . . . . . . . . . . . .   9
   Appendix B.  Example of SmtpUTF8Name  . . . . . . . . . . . . . .  10
   Appendix C.  Acknowledgements . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   [RFC5280] defines the rfc822Name subjectAltName name type for
   representing [RFC5321] email addresses.  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 calls for a new otherName variant to represent
   Internationalized Email addresses.  In addition this document calls
   for 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",
   document are to be interpreted as described in [RFC2119].

   The formal syntax use the Augmented Backus-Naur Form (ABNF) [RFC5234]

3.  Name Definitions

   The GeneralName structure is defined in [RFC5280], and supports many
   different name forms including otherName for extensibility.  This
   section specifies the SmtpUTF8Name 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-SmtpUTF8Name OBJECT IDENTIFIER ::= { id-on 9 }

   SmtpUTF8Name ::= UTF8String (SIZE (1..MAX))

   When the subjectAltName (or issuerAltName) extension contains an
   Internationalized Email address with a non-ASCII local-part, the
   address MUST be stored in the SmtpUTF8Name name form of otherName.
   The format of SmtpUTF8Name is defined as the ABNF rule
   SmtpUTF8Mailbox.  SmtpUTF8Mailbox is a modified version of the
   Internationalized Mailbox which was defined in Section 3.3 of
   [RFC6531] which was itself derived from SMTP Mailbox from
   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-label, as defined in [RFC5890].

   This document further refines Internationalized [RFC6531] Mailbox
   ABNF rules and calls this SmtpUTF8Mailbox.  In SmtpUTF8Mailbox,
   labels that include non-ASCII characters MUST be stored in U-label
   (rather than A-label) [RFC5890] form.  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-label 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 not A- nor U-labels) SHALL use NR-LDH
   restrictions as specified by section Section 2.3.1 of [RFC5890] and SHALL be
   restricted to lower case letters.  NR-LDH stands for "Non-Reserved
   Letters Digits Hyphen" and is the set 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], SmtpUTF8Name 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 ">".

   Due to operational reasons to be described shortly and name
   constraint compatibility reasons described in Section 6, SmtpUTF8Name
   subjectAltName MUST only be used when the local part of the email
   address contains contains non-ASCII characters.  When the local-part
   is ASCII, rfc822Name subjectAltName MUST be used instead of
   SmtpUTF8Name.  This is compatible with legacy software that supports
   only rfc822Name (and not SmtpUTF8Name).

   SmtpUTF8Name is encoded as UTF8String.  The UTF8String encoding MUST
   NOT contain a Byte-Order- Mark (BOM) [RFC3629] to aid consistency
   across implementations particularly for comparison.

4.  IDNA2008

   To facilitate comparison between email addresses, all email address
   domains in X.509 certificates MUST conform to IDNA2008 [RFC5890] (and
   avoids 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
   SmtpUTF8Name 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 SmtpUTF8Name, there may be some setup
   work on one or both inputs depending of whether the input is already
   in comparison form.  Comparing SmtpUTF8Names consists of a domain
   part step and a local-part step.  The comparison form for local-parts
   always is 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 (section (Section 7.5 and 7.2
   in [RFC5280]), [RFC5280] as updated by [ID-lamps-rfc5280-i18n-update]), 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 SmtpUTF8Name 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 or comments, as well
   as "<" and ">" present.  This document calls for comparison of domain
   labels that include non-ASCII characters be tranformed to U-label if
   not already in that form.  The first step is to detect use of the
   A-label by using section Section 5.1 of [RFC5891].  Next if necessary,
   transform any A-labels to U-labels Unicode as specified in section
   Section 5.2 of [RFC5891].  Finally if necessary convert the Unicode
   to UTF-8 as specified in section Section 3 of [RFC3629].  For ASCII NR-LDH
   labels, upper case letters are converted to lower case 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

   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 SmtpUTF8Name comparison implementations MUST NOT interpret any
   character as wildcards.  Instead, to specify multiple email addresses
   through SmtpUTF8Name, 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 Section of [RFC5280] to extend
   rfc822Name name constraints to SmtpUTF8Name subjectAltNames.  A
   SmtpUTF8Name aware path validators will apply name constraint
   comparison to the subject distinguished name and both forms of
   subject alternative name rfc822Name and SmtpUTF8Name.

   Both rfc822Name and SmtpUTF8Name 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, this specification [ID-lamps-rfc5280-i18n-update]
   updates modifies and extends rfc822Name name constraints defined in
   [RFC5280] to cover SmtpUTF8Name subject alternative names.  This
   ensures that the introduction of SmtpUTF8Name does not violate
   existing name constraints.  Since it is not valid to include non-ASCII 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, this specification modifies [RFC5280] name constraints to only updated in
   [ID-lamps-rfc5280-i18n-update] admit 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 SmtpUTF8Name 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 or a
   rfc822Name or SmtpUTF8Name subjectAltName, and one of a rfc822Name
   name constraint, to constraint comparison form.  For rfc822Name name
   constraint, this will convert any domain A-labels to U-labels.  For
   both the name constraint and the subject, this will lower case any
   domain NR-LDH labels.  Strip the local-part and "@" separator from
   each rfc822Name and SmtpUTF8Name, leaving just the domain-part.
   After setup, this follows the comparison steps defined in 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

   Certificate Authorities that wish to issue CA certificates with email
   address name constraint 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 SmtpUTF8Name subject alternative
   name is illustrated in the non-normative diagram Figure 1.  The first
   example (1) illustrates a permitted rfc822Name ASCII only hostname
   name constraint, and the corresponding valid rfc822Name
   subjectAltName and SmtpUTF8Name subjectAltName email addresses.  The
   second example (2) illustrates a permitted rfc822Name hostname name
   constraint with A-label, and the corresponding valid rfc822Name
   subjectAltName and SmtpUTF8Name 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                                                     |
   |  Intermediate CA Cert                                             |
   |      Permitted                                                    |
   |        rfc822Name: (1)              |
   |                                                                   |
   |        rfc822Name: (2)                     |
   |                                                                   |
   |  Entity Cert (w/explicitly permitted subjects)                    |
   |    SubjectAltName Extension                                       |
   |      rfc822Name: (1)         |
   |      SmtpUTF8Name: (1) |
   |                                                                   |
   |      rfc822Name: (2)               |
   |      SmtpUTF8Name: (2)      |
   |                                                                   |

   Name constraints with SmtpUTF8Name and rfc822Name

                                 Figure 1

7.  Security Considerations

   Use of SmtpUTF8Name for certificate subjectAltName (and
   issuerAltName) will incur many of the same security considerations as
   in Section 8 in [RFC5280] , but 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 which can be exploited to deceive the recipient.
   The former document references some means to mitigate against these

8.  IANA Considerations

   in Section

   In Section 3 and the ASN.1 module identifier defined in
   Section Appendix A.
   IANA is kindly requested to make the following assignments for:

      The LAMPS-EaiAddresses-2016 ASN.1 module in the "SMI Security for
      PKIX Module Identifier" registry (

      The SmtpUTF8Name otherName in the "PKIX Other Name Forms" registry

9.  References

9.1.  Normative References

              Housley, R., "Internationalization Updates to RFC 5280",
              June 2017, <

   [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,

   [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, <>.

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,

Appendix A.  ASN.1 Module

   The following ASN.1 module normatively specifies the SmtpUTF8Name
   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(TBD) }


    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) } ;

  -- otherName carries additional name types for subjectAltName,
  -- issuerAltName, and other uses of GeneralNames.

    id-on OBJECT IDENTIFIER ::= { id-pkix 8 }

    SmtpUtf8OtherNames OTHER-NAME ::= { on-SmtpUTF8Name, ... }

    on-SmtpUTF8Name OTHER-NAME ::= {
        SmtpUTF8Name IDENTIFIED BY id-on-SmtpUTF8Name

    id-on-SmtpUTF8Name OBJECT IDENTIFIER ::= { id-on 9 }

    SmtpUTF8Name ::= UTF8String (SIZE (1..MAX))


                                 Figure 2

Appendix B.  Example of SmtpUTF8Name

   This non-normative example demonstrates using SmtpUTF8Name 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 3 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"

Appendix C.  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 to
   Viktor Dukhovni for his help with name constraints.

Authors' Addresses

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


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