LAMPS                                                   A. Melnikov, Ed.
Internet-Draft                                                 Isode Ltd
Intended status: Standards Track                          W. Chuang, Ed.
Expires: August 5, September 9, 2017                                  Google, Inc.
                                                        February 1,
                                                           March 8, 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.

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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.  Deployment Considerations . . . . . . . . . . . . . . . . . .   7   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7   8
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8   9
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   8   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .   9  10
   Appendix A.  ASN.1 Module . . . . . . . . . . . . . . . . . . . .   9  10
   Appendix B.  Example of SmtpUtf8Name SmtpUTF8Name  . . . . . . . . . . . . . .  10  11
   Appendix C.  Acknowledgements . . . . . . . . . . . . . . . . . .  11  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11  12

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 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, the address MUST be stored in the
   SmtpUTF8Name name form of otherName.  The format of SmtpUtf8Name SmtpUTF8Name is
   defined as the ABNF rule SmtpUtf8Mailbox.  SmtpUtf8Mailbox SmtpUTF8Mailbox.  SmtpUTF8Mailbox is a
   modified version of the Internationalized Mailbox which is was defined
   in Section 3.3 of [RFC6531] which is 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 is was
   described by Section 3.1 of [RFC6532].  Also, sub-domain is was extended
   to support U-label, as defined in [RFC5890].

   This document further refines Internationalized [RFC6531] Mailbox
   ABNF rules and calls this SmtpUtf8Mailbox. SmtpUTF8Mailbox.  In SmtpUtf8Mailbox, 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, SmtpUTF8Mailbox, sub-domain
   that encode solely 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.  Note  One suggested approach to apply these sub-
   domains restriction is to restrict sub-domain so that a SmtpUtf8Mailbox 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 ">".

   In the context of building name constraint as needed by [RFC5280],
   the SmtpUtf8Mailbox SmtpUTF8Mailbox rules are modified to allow partial productions
   to allow for additional forms required by Section 6.  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 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 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, SmtpUTF8Name, there may be some setup
   work to enable the comparison i.e. processing of the SmtpUtf8Name SmtpUTF8Name
   content or the email address that is being compared against.  The
   process for setup for comparing with SmtpUtf8Name SmtpUTF8Name is split into
   domain steps and local-part 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 can be straightforward.  No SmtpUTF8Name is straightforward with no setup work
   needed.  They are considered equivalent if there is needed and it can be an octet for octet comparison.  For exact octet-
   for-octet match.  Comparison with other email address forms such as
   Internationalized email address or
   rfc822Name, the comparison rfc822Name requires additional
   setup to convert the
   format for comparison. 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, SmtpUTF8Mailbox, the email
   address local-part MUST conform to the requirements of [RFC6530] and
   [RFC6531], including already 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 completed,
   comparison is complete, they are again checking for octet for octet equivalence.
   compared octet-for-octet.

   To summarize non-normatively non-normatively, the domain setup comparison steps including setup

   1.  if  If the domain contains A-labels, transform them to U-label U-label.

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

   This enables an octet them.

   3.  Ensure local-part is UTF-8.

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

   This specification expressly does not define any wildcards characters
   and SmtpUtf8Name SmtpUTF8Name comparison implementations MUST NOT interpret any
   character as wildcards.  Instead, to specify multiple specifying
   multiple email addresses
   through SmtpUtf8Name, SmtpUTF8Name, the certificate should SHOULD use multiple
   subjectAltNames or issuerAltNames to explicitly carry those email

6.  Name constraints in path validation

   This section defines use of SmtpUtf8Name SmtpUTF8Name name for name constraints.
   The format for SmtpUtf8Name 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 SmtpUTF8Name
   name uses the matching procedure defined by Section 5.  As with
   rfc822Name name constraints as specified in Section of
   [RFC5280], SmtpUtf8Name 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.  Name constraint
   comparisons in the context of [RFC5280] is that are specified with SmtpUtf8Name
   SmtpUTF8Name name are only done on the subjectAltName (and
   issuerAltName) SmtpUtf8Name name, SmtpUTF8Name
   name and says nothing more about
   constraints not on other email address forms such as rfc822Name.
   Consequently it may be necessary to include other forms.  Similarly rfc822Name name constraints
   do not apply to subjectAltName SmtpUTF8Name name.  This imposes
   requirements on the certificate issuer as rfc822Name described next.

   When name constraints are used with SmtpUTF8Name subjectAltName
   names, they are specified in addition to SmtpUtf8Name the following profile to constrain all
   potential email addresses.  For example a prevent
   bypassing of name constraints.  Host name and domain with constraints MUST
   use both ASCII rfc822Name and
   non-ASCII SmtpUTF8Name forms in the issuing certificate
   with the constraint.  Complete email address constraint with UTF-8
   local-part MUST only use SmtpUTF8Name form.  Complete email addresses may require address
   constraint with ASCII local-part MUST use both rfc822Name and
   SmtpUTF8Name forms.  When both rfc822Name and SmtpUTF8Name name constraints.
   constraints forms are present, they MUST carry the equivalent
   constraints as defined by Section 5 and MUST be found in the same
   node and in the same permittedSubtrees or excludedSubtrees.  This can
   specification intentionally leaves unchanged rfc822Name name
   constraint processing as described in Section of [RFC5280].

   This document specifies that SmtpUTF8Name aware path validators check
   for SmtpUTF8Name name constraint profiles as an additional path
   validation step in Section 6 of [RFC5280].  SmtpUTF8Name aware
   validators MUST NOT accept any certificate whose path contains an
   issuing certificate whose rfc822Name or SmtpUTF8Name name constraints
   do not match the above profile.  That is the path validator verifies
   that a rfc822Name name constraint has a corresponding SmtpUTF8Name
   constraint and that a SmtpUTF8Name name constraint has a
   corresponding rfc822Name constraint when the constraint contains host
   name, domain or email address with an ASCII local-part.  This
   correspondence is required to be in the same issuing certificate node
   and in the same nameConstraint permittedSubtrees or excludedSubtrees.

   The name constraint requirement with SmtpUTF8Name subjectAltName is
   illustrated in the following non-normative diagram Figure 1 which shows 1.  This
   show a name
   constraint set in SmtpUTF8Name aware issuer that constrained the intermediate CA certificate, which then applies
   with host name and email address name constraints.  In particular the children entity certificates.  Note that
   email address constraint with UTF8 local-part only used a single
   SmtpUTF8Name name constraint, while the email address constraint on with
   ASCII local-part used both rfc822Name does not apply to SmtpUtf8Name and vice versa as is shown
   in SmtpUTF8Name name
   constraints.  The next non-normative diagram Figure 2. 2 illustrates
   legacy name constraints to contrasts the changes this document
   specifies.  The legacy approach has only a single rfc822Name name
   email address name constraint.

       |  Root CA Cert                                                |
       |  Intermediate CA Cert                                        |
       |    Name Constraint Extension                                 |
       |      Permitted                                               |
       |        rfc822Name:                  |
       |        SmtpUTF8Name:                |
       |                                                              |
       |        SmtpUtf8Name:        SmtpUTF8Name:  |
       |      Excluded                                                              |
       |        rfc822Name:         |
       |        SmtpUTF8Name:       |
       |  Entity Cert (w/explicitly permitted subjects)               |
       |    SubjectAltName Extension                                  |
       |      rfc822Name:      SmtpUTF8Name:     |
       |                                                              |
       |      SmtpUTF8Name:    |
       |      SmtpUtf8Name:                                                              |
       |      rfc822Name:           |

   Name constraints with SmtpUTF8Name

                                 Figure 1

       |  Root CA Cert                                                |
       |  Intermediate CA Cert                                        |
       |    Name Constraint Extension                                 |
       |      Permitted                                               |
       |        rfc822Name:                       |
       |        SmtpUtf8Name:                     |
       |      Excluded                                                |
       |        rfc822Name:                  |
       |  Entity Cert (w/permitted subject i.e. excluded rfc822Name   |
       |  does not exclude SmtpUtf8Name) (w/explicitly permitted subjects)               |
       |    SubjectAltName Extension                                  |
       |      SmtpUtf8Name:      rfc822Name:            |

   Legacy name constraints with rfc822Name

                                 Figure 2

7.  Deployment Considerations

   For email addresses whose local-part is ASCII it may be more
   reasonable to continue using rfc822Name instead of SmtpUtf8Name. SmtpUTF8Name.  The
   use of rfc822Name rather than SmtpUtf8Name SmtpUTF8Name is currently more likely
   to be supported.  Also use of SmtpUtf8Name 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 smptUtf8Name supports U-label whereas rfc822Name
   supports A-label.  This document RECOMMENDS using SmtpUtf8Name SmtpUTF8Name when
   local-part contains non-ASCII characters, and otherwise rfc822Name.

8.  Security Considerations

   Use for SmtpUtf8Name 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.

9.  IANA Considerations

   This document makes use of object identifiers for the SmtpUtf8Name

   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 SmtpUTF8Name otherName in the "PKIX Other Name Forms" registry

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,

   [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, <>.

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,

Appendix A.  ASN.1 Module

   The following ASN.1 module normatively specifies the SmtpUtf8Name 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, ... }


    on-SmtpUTF8Name OTHER-NAME ::= {
        SmtpUTF8Name IDENTIFIED BY id-on-smtpUtf8Name id-on-SmtpUTF8Name


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


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


                                 Figure 3

Appendix B.  Example of SmtpUtf8Name SmtpUTF8Name

   This non-normative example demonstrates using SmtpUtf8Name SmtpUTF8Name as an
   otherName in GeneralName to encode the email address

      The hexidecimal 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 4

   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, Viktor Dukhovni and Patrik
   Falstrom for their feedback.  Also special thanks to John Klensin for
   his valuable input on internationalization, Unicode and ABNF
   formatting, and to Jim Schaad for his help with the ASN.1 example and
   his helpful feedback.

Authors' Addresses

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


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