LAMPS                                                   A. Melnikov, Ed.
Internet-Draft                                                 Isode Ltd
Intended status: Standards Track                          W. Chuang, Ed.
Expires: October 17, 2017                                   Google, Inc.
                                                          April 15, 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 Alternate Name
   extension that allows a certificate subject to be associated with an
   Internationalized Email Address.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   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

   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 October 17, 2017.

Copyright Notice

   Copyright (c) 2017 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       Expires October 17, 2017                [Page 1]

Internet-Draft  I18N Mail Addresses in X.509 certificates     April 2017

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 rfc822Name subjectAltName choice for representing
   [RFC5321] email addresses.  This form is restricted to a subset of
   US-ASCII characters and thus can't be used to represent
   Internationalized Email addresses [RFC6531].  To facilitate use of
   these Internationalized Email addresses with X.509 certificates, this
   document specifies a new name form in otherName so that
   subjectAltName and issuerAltName can carry them.  In addition this
   document calls for all email address domain 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 names 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.

Melnikov & Chuang       Expires October 17, 2017                [Page 2]

Internet-Draft  I18N Mail Addresses in X.509 certificates     April 2017

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

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

   When the subjectAltName (or issuerAltName) extension contains an
   Internationalized Email address, 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, sub-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, sub-
   domain that encode non-ASCII characters SHALL use U-label Unicode
   native character labels and MUST NOT use A-label [RFC5890].  This
   restriction prevents having to determine which label encoding A- or
   U-label is present in the Domain.  As per Section of
   [RFC5890], U-label use UTF-8 [RFC3629] with Normalization Form C and
   other properties specified there.  In SmtpUTF8Mailbox, sub-domain
   that encode ASCII character labels SHALL use NR-LDH restrictions as
   specified by section 2.3.1 of [RFC5890] and SHALL be restricted to
   lower case letters.  One suggested approach to apply these sub-
   domains restriction is to restrict sub-domain so that labels not
   start with two letters followed by two hyphen-minus characters.
   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 described shortly and name constraint
   compatibility reasons described in its section, SmtpUTF8Name
   subjectAltName MUST only be used when the local part of the email
   address contains UTF-8.  When the local-part is ASCII, rfc822Name
   subjectAltName MUST be used instead of SmtpUTF8Name.  The use of
   rfc822Name rather than SmtpUTF8Name is currently more likely to be
   supported.  Also use of SmtpUTF8Name incurs higher byte
   representation overhead due to encoding with otherName and the
   additional OID needed.  This may be offset if domain requires non-
   ASCII characters as SmtpUTF8Name supports U-label whereas rfc822Name
   supports A-label.

Melnikov & Chuang       Expires October 17, 2017                [Page 3]

Internet-Draft  I18N Mail Addresses in X.509 certificates     April 2017

   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
   domain in X.509 certificates MUST conform to IDNA2008 [RFC5890].
   Otherwise 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 GeneralName is used.

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

   In equivalence comparison with SmtpUTF8Name, there may be some setup
   work to enable the comparison i.e. processing of the SmtpUTF8Name
   content or the email address that is being compared against.  The
   process for setup for comparing with SmtpUTF8Name is split into
   domain steps and local- part steps.  The comparison form for local-
   part always is UTF-8.  The comparison form for domain depends on
   context.  While some contexts such as certificate path validation in
   [RFC5280] specify transforming domain to A-label, 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 other email address forms such as
   Internationalized email address or rfc822Name requires additional
   setup steps.  Domain setup is particularly important for forms that
   may contain A- or U-label such as International email address, or
   A-label only forms such as rfc822Name.  This document specifies the
   process to transform the domain to U-label.  (To convert the domain
   to A-label, follow the process specified in section 7.5 and 7.2 in
   [RFC5280]) The first step is to detect A-label by using section 5.1
   of [RFC5891].  Next if necessary, transform the A-label to U-label
   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, 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

Melnikov & Chuang       Expires October 17, 2017                [Page 4]

Internet-Draft  I18N Mail Addresses in X.509 certificates     April 2017

   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-label.

   2.  If the domain contains ASCII NR-LDH labels, lowercase them.

   3.  Ensure local-part is UTF-8.

   4.  Compare strings octet-for-octet for equivalence.

   This specification expressly does not define any wildcards characters
   and SmtpUTF8Name comparison implementations MUST NOT interpret any
   character as wildcards.  Instead, to specify multiple email addresses
   through SmtpUTF8Name, the certificate SHOULD use multiple
   subjectAltNames or issuerAltNames to explicitly carry those email

6.  Name constraints in path validation

   This section updates [RFC5280] name constraints to work with
   SmtpUTF8Name subjectAltName.  In the following, a CA or path verifier
   implementation that follows this specification is called SmtpUTF8Name

   SmtpUTF8Name aware path validators MUST be able to apply name
   constraint to the subject distinguished name and both forms of
   subject alternative name.  That is rfc822Name name constraint applies
   to emailAddress subject distinguished name, and to SmtpUTF8Name and
   rfc822Name subject alternative name, as mentioned in Section
   of [RFC5280].  Constraint comparison with SmtpUTF8Name subjectAltName
   uses the matching procedure defined by Section 5 including any setup
   steps.  The lack of a SmtpUTF8Name name constraint form is
   intentional and motivated as described next.

   This specification requires that SmtpUTF8Name aware CAs continue to
   issue certificates with rfc822Name name constraints form due to
   compatibility concerns with legacy systems.  Using rfc822Name name
   constraints allows backwards compatibility with legacy path verifiers
   that only understand rfc822Name form, yet is forward compatible by
   being able to describe the intent of the CA to constrain both
   rfc822Name and SmtpUTF8Name subjectAltName to SmtpUTF8Name aware path
   verifiers.  Oblivious legacy path verifier will not see the
   SmtpUTF8Name subjectAltName (nor the unknown otherNames), and thereby
   prevent the use of an unconstrained SmtpUTF8Name subjectAltName.

Melnikov & Chuang       Expires October 17, 2017                [Page 5]

Internet-Draft  I18N Mail Addresses in X.509 certificates     April 2017

   Other implementations may detect an unknown otherNames, along with
   the critical bit set on the name constraints extension and then fail
   path verification.  This too prevents use of an unconstrained
   SmtpUTF8Name subjectAltName.  A legacy CA will use rfc822Name name
   constraints.  As the CA's intent is to constrain all email addresses
   matching the constraint, this will be forward compatible with a
   SmtpUTF8Name aware path verifiers that applies the name constraint to
   either forms rfc822Name and SmtpUTF8Name subjectAltName.

   The representation of name constraints are specified in
   Section of [RFC5280] and there MAY represent a particular
   mailbox, all addresses at a host, or all mailboxes in a domain by
   specifying the complete email address, a host name, or a domain.
   This specification modifies [RFC5280] name constraint to only require
   with a MAY that it represents all addresses at a host or all
   mailboxes in a domain, and require with a MAY NOT that it represent a
   particular mailbox.  This is motivated by rfc822Name name constraints
   inability to represent a specific mailbox with a UTF-8 email local
   part email address.  Certificate issuers should be aware of this
   lessened support.

   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.

Melnikov & Chuang       Expires October 17, 2017                [Page 6]

Internet-Draft  I18N Mail Addresses in X.509 certificates     April 2017

   |  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 for SmtpUTF8Name for certificate subjectAltName (and
   issuerAltName) will incur many of the same security considerations of
   Section 8 in [RFC5280] but is further complicated by permitting non-
   ASCII characters in the email address local-part.  This complication,
   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 attacks.

8.  IANA Considerations

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

Melnikov & Chuang       Expires October 17, 2017                [Page 7]

Internet-Draft  I18N Mail Addresses in X.509 certificates     April 2017

      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

   [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, <>.

Melnikov & Chuang       Expires October 17, 2017                [Page 8]

Internet-Draft  I18N Mail Addresses in X.509 certificates     April 2017

   [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.

Melnikov & Chuang       Expires October 17, 2017                [Page 9]

Internet-Draft  I18N Mail Addresses in X.509 certificates     April 2017

    { 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

Melnikov & Chuang       Expires October 17, 2017               [Page 10]

Internet-Draft  I18N Mail Addresses in X.509 certificates     April 2017

      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

   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


Melnikov & Chuang       Expires October 17, 2017               [Page 11]