LAMPS                                                   A. Melnikov, Ed.
Internet-Draft                                                 Isode Ltd
Intended status: Standards Track                          W. Chuang, Ed.
Expires: May 4, 2017                                        Google, Inc.
                                                        October 31, 2016

        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 extension that allows a
   certificate subject to be associated with an Internationalized Email

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   This Internet-Draft will expire on May 4, 2017.

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Table of Contents

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

1.  Introduction

   [RFC5280] defines rfc822Name subjectAltName choice for representing
   [RFC5322] 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 fascilitate 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.

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.

   id-on-smtputf8Name OBJECT IDENTIFIER ::= { id-on TBD }

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   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 is defined in
   Section 3.3 of [RFC6531] which is 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 is described
   by Section 3.1 of [RFC6532].  Also sub-domain is 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 solely ASCII character labels SHALL use NR-LDH
   restrictions as specified by section 2.3.1 of [RFC5890].  Note that a
   smtputf8Mailbox 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 ">".

   In the context of building name constraint as needed by [RFC5280],
   the smtputf8Mailbox rules are modified to allow partial productions
   to allow for additional forms required by Section 5.  Name
   constraints may specify a complete email address, host name, or
   domain.  This means that the local-part may be missing, and domain
   partially specified.

   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.  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-

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   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 to A-label, this document RECOMMENDS
   transforming to UTF-8 U-label even in place of those other
   specifications.  As more implementations natively support U-label
   domain, requiring U-label reduces conversions required, which then
   reduces likelihood of errors caused by bugs in implementation.

   Comparison of two smtputf8Name can be straightforward.  No setup work
   is needed and it can be an octet for octet comparison.  For other
   email address forms such as Internationalized email address or
   rfc822Name, the comparison requires additional setup to convert the
   format for comparison.  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 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].  In setup for smtputf8Mailbox,
   the email address local-part MUST be converted to UTF-8 if it is not
   already.  The <Local-part> part of an Internationalized email address
   is already in UTF-8.  For the rfc822Name local-part is IA5String
   (ASCII), and conversion to UTF-8 is trivial since ASCII octets maps
   to UTF-8 without change.  Once the setup is completed, comparison is
   an octet for octet comparison.

   This specification expressly does not define any wildcards characters
   and smtputf8Name comparison implementations MUST NOT interpret any
   character as wildcards.  Instead, to specify multiple specifying
   multiple email addresses through smtputf8Name, the certificate should
   use multiple subjectAltNames or issuerAltNames to explicitly carry
   those email addresses.

5.  Name constraints in path validation

   This section defines use of smtputf8Name name for name constraints.
   The format for smtputf8Name in name constraints is identical to the
   use in subjectAltName as specified in Section 3 with the extension as
   noted there for partial productions.

   Constraint comparison on complete email address with smtputf8Name
   name uses the matching procedure defined by Section 4.  As with
   rfc822Name name constraints as specified in Section of
   [RFC5280], smtputf8Name name can specify a particular mailbox, all

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   addresses at a host, or all mailboxes in a domain by specifying the
   complete email address, a host name, or a domain.

   Name constraint comparisons in the context [RFC5280] is specified
   with smtputf8Name name are only done on the subjectAltName (and
   issuerAltName) smtputf8Name name, and says nothing more about
   constaints on other email address forms such as rfc822Name.
   Consequently it may be necessary to include other name constraints
   such as rfc822Name in addition to smtputf8Name to constrain all
   potential email addresses.  For example a domain with both ascii and
   non-ascii local-part email addresses may require both rfc822Name and
   smtputf8Name name constraints.  This can be illustrated in the
   following non-normative diagram Figure 1 which shows a name
   constraint set in the intermediate CA certificate, which then applies
   to the children entity certificates.  Note that a constraint on
   rfc822Name does not apply to smtputf8Name and vice versa.

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       |  Root CA Cert                                        |
       |  Intermediate CA Cert                                |
       |    Name Constraint Extension                         |
       |      Permitted                                       |
       |        rfc822Name:               |
       |        smtputf8Name:             |
       |      Excluded                                        |
       |        rfc822Name:               |
                             |   |
                             v   |
       |  Entity Cert (w/explicitly permitted subjects)       |
       |    SubjectAltName Extension                          |
       |      rfc822Name:         |
       |      smtputf8Name: \u8001\  |
       |  Entity Cert (w/permitted subject- excluded          |
       |  rfc822Name does not exclude smtputf8Name)           |
       |    SubjectAltName Extension                          |
       |      smtputf8Name: \u4E0D\  |

                                 Figure 1

6.  Resource Considerations

   For email addresses whose local-part is ASCII it may be more
   reasonable to continue using rfc822Name instead of smtputf8Name.  Use
   of smtputf8Name incurs higher byte representation overhead due to
   encoding with otherName and the additional OID needed.  This document
   RECOMMENDS using smtputf8Name when local-part contains non-ASCII
   characters, and otherwise rfc822Name.

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 further complicated by permitting non-
   ASCII characters in the email address local-part.  As mentioned in

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   Section 4.4 of [RFC5890] and in Section 4 of [RFC6532]  Unicode
   introduces the risk for visually similar characters which can be
   exploited to deceive the recipient.  The former document references
   some means to mitigate against these attacks.

8.  IANA Considerations

   This document makes use of object identifiers for the smtputf8Name
   defined 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:

      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,

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   [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,

   [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,

   [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

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,

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.

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    { 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) }



    id-pkix  OBJECT IDENTIFIER  ::=
        {iso(1) identified-organization(3) dod(6) internet(1) security(5)
        mechanisms(5) pkix(7)}

  -- 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 TBD }

    SmtpUtf8Name ::= UTF8String (SIZE (1..MAX))


                                 Figure 2

Appendix B.  Acknowledgements

   Thank you to Magnus Nystrom for motivating this document.  Thanks to
   Russ Housley, Nicolas Lidzborski, Laetitia Baudoin, Ryan Sleevi, Sean
   Leonard, Sean Turner, and Jim Schaad for their feedback.  Also thanks
   to John Klensin for his valuable input on internationalization,
   Unicode and ABNF formatting.

Authors' Addresses

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


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


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