Network Working Group                                   A. Melnikov, Ed.
Internet-Draft                                                 Isode Ltd
Intended status: Standards Track                          W. Chuang, Ed.
Expires: January 2, 2017                                    Google, Inc.
                                                            July 1, 2016


        Internationalized Email Addresses in X.509 certificates
                 draft-melnikov-spasm-eai-addresses-02

Abstract

   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
   Address.

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   This Internet-Draft will expire on January 2, 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 . . . . . . . . . . . . . . . . . . .   4
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

.  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].

.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

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

.  Name Definitions

   This section defines the SMTPUTF8Name name as a form of otherName
   from the GeneralName structure in SubjectAltName defined in
   [RFC5280].

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

   SMTPUTF8Name ::= UTF8String (SIZE (1..MAX))

   When the subjectAltName 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



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   [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 SHALL use U-label Unicode native character labels and MUST NOT
   use A-label [RFC5890] to encode non-ascii characters.  This
   restriction prevents having to determine which label encoding A- or
   U-label is present in the Domain.  Further U-label SHALL use UTF-8
   Unicode [RFC3629] for encoding efficiency.  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 ">".

.  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 normalization and local-part normalization.  The normalized
   form for local-part always is UTF-8.  The normalized form for domain
   depends on context.  While some contexts such as certificate path
   validation in [RFC5280] specify normalizing to A-label, this document
   RECOMMENDS normalizing to UTF-8 U-label even in place of those other
   specifications.  As more implementations natively support UTF-8
   U-label domain, normalizing to UTF-8 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 normalize the
   format for comparison.  Domain normalization is particularly
   important for forms that may contain A- or U-label such as
   International email address, or A-label only such as rfc822Name.
   This document specifies the process to normalize domain to UTF-8
   U-label.  (To normalize to 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



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   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.

.  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.

   Name constraint comparisons in the context [RFC5280] is specified
   with smtputf8Name name are only done on the subjectAltName
   smtputf8Name name, and says nothing more about constaints on other
   email address forms such as rfc822Name.  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 4.2.1.10 of [RFC5280], smtputf8Name name can
   specify 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.

.  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 the
   use of otherName and the additiona OID needed.  This document
   RECOMMENDS using smtputf8Name when local-part contains non-ascii
   characters, and otherwise rfc822Name.

.  IANA Considerations

   [[CREF1: Just need a new OID.]]

.  Security Considerations

   Use for smtputf8Name for certificate subjectAltName will incur many
   of the same security considerations of Section 8 [RFC5280] but
   further complicated by permitting non-ASCII characters to the email
   address local-part.  As mentioned in 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.




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   The former document references some means to mitigate against these
   attacks.

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

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

   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891,
              DOI 10.17487/RFC5891, August 2010,
              <http://www.rfc-editor.org/info/rfc5891>.

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







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9.2.  Informative References

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <http://www.rfc-editor.org/info/rfc5322>.

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

Appendix A.  Acknowledgements

   Thank you to Magnus Nystrom for motivating this document.  Thanks to
   Nicolas Lidzborski, Laetitia Baudoin, Ryan Sleevi and the reviewers
   for their early feedback.

Authors' Addresses

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

   Email: Alexey.Melnikov@isode.com


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

   Email: weihaw@google.com

















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