LAMPS                                                   A. Melnikov, Ed.
Internet-Draft                                                 Isode Ltd
Intended status: Standards Track                          W. Chuang, Ed.
Expires: November 19, December 20, 2017                                  Google, Inc.
                                                           June 18, 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 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 choice name type for
   representing [RFC5321] email addresses.  This form  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].  To facilitate use of
   these Internationalized Email addresses with X.509 certificates, this  This
   document specifies calls for a new name form in otherName so that
   subjectAltName and issuerAltName can carry them. variant to represent
   Internationalized Email addresses.  In addition this document calls
   for all email address domain 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 names 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, 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>, <Quoted- string>, <QcontentSMTP>,
   <Domain>, and <Atom>.  In particular, <Local-part> <Local- part> was updated to
   also support UTF8-non-ascii.  UTF8-non-ascii was described by
   Section 3.1 of [RFC6532].  Also, sub-domain 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-
   labels that encode include non-ASCII characters SHALL use U-label Unicode
   native character labels and MUST NOT use A-label [RFC5890]. be stored in U-label
   (rather than A-label) [RFC5890] form.  This restriction prevents having removes the
   need to determine which label encoding A- or U-label is present in
   the Domain.  As per Section of [RFC5890], U-label use are encoded
   as UTF-8 [RFC3629] with in Normalization Form C and other properties
   specified there.  In SmtpUTF8Mailbox, sub-domain domain labels that encode solely use
   ASCII character labels characters (meaning not A- nor U-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  NR-LDH stands for "Non-Reserved
   Letters Digits Hyphen" and is to restrict sub-domain so that the set LDH labels that do not
   start with two letters followed by two hyphen-minus characters. 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 its section, Section 6, SmtpUTF8Name
   subjectAltName MUST only be used when the local part of the email
   address contains UTF-8. contains non-ASCII characters.  When the local-part
   is ASCII, rfc822Name subjectAltName MUST be used instead of
   SmtpUTF8Name.  The use of
   rfc822Name rather than SmtpUTF8Name  This is currently more likely to be
   supported.  Also use of SmtpUTF8Name incurs higher byte
   representation overhead due to encoding compatible with otherName and the
   additional OID needed.  This may be offset if domain requires non-
   ASCII characters as SmtpUTF8Name legacy software that supports U-label whereas
   only rfc822Name
   supports A-label. (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).  Otherwise  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 GeneralName is these are 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 on one or both inputs depending of whether the email address that is being compared against.  The
   process for setup for comparing with SmtpUTF8Name input is split into already
   in comparison form.  Comparing SmtpUTF8Names consists of a domain steps and local-
   part steps. step and a local-part step.  The comparison form for local-
   part 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, A-label (section 7.5 and 7.2
   in [RFC5280]), 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 U- label

   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 addresses such as
   Internationalized email address or rfc822Name requires additional
   setup steps.  Domain setup is particularly important steps for forms that
   may contain A- or U-label such as International domain part and local-part.  The initial preparation
   for the email address, addresses is to remove any phrases or
   A-label only forms such comments, as rfc822Name. well
   as "<" and ">" present.  This document specifies the
   process to transform the domain to U-label.  (To convert the calls for comparison of domain
   labels that include non-ASCII characters be tranformed to A-label, follow the process specified in section 7.5 and 7.2 U-label if
   not already in
   [RFC5280]) that form.  The first step is to detect use of the
   A-label by using section 5.1 of [RFC5891].  Next if necessary,
   transform the A-label any A-labels to U-label U-labels 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 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. U-labels.

   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 wildcard characters
   and SmtpUTF8Name comparison implementations MUST NOT interpret any
   character as wildcards.  Instead, to specify multiple email addresses
   through SmtpUTF8Name, the certificate SHOULD MUST use multiple
   subjectAltNames or issuerAltNames to explicitly carry those any additional
   email addresses.

6.  Name constraints in path validation

   This section updates section of [RFC5280] to extend
   rfc822Name name constraints defined in section to work with SmtpUTF8Name subjectAltName.  The following
   specifies that a SmtpUTF8Name aware CA use a compatible name
   constraint representation.  Similarly a subjectAltNames.  A
   SmtpUTF8Name aware path validators MUST be able to will apply name constraint
   comparison to the subject distinguished name and both forms of
   subject alternative name rfc822Name and SmtpUTF8Name.


   Both rfc822Name and SmtpUTF8Name aware 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 modifies and
   extends rfc822Name name constraint form SmtpUTF8Name does not violate existing name
   constraints.  Since it is
   specified not valid to be include non-ASCII UTF-8
   characters in the local-part of rfc822Name motivated by compatibility considerations
   with legacy systems that already understand name constraints, and
   since name constraints that form.  This include a local-part are rarely, if at
   all, used in practice, this specification modifies [RFC5280] name constraint
   constraints to only require with
   MAY that it represents admit the forms represent all addresses at a host
   or all mailboxes in a domain, and require with MAY NOT deprecates rfc822Name name
   constraints that it represent a particular mailbox.  For context, [RFC5280] Section specifies with MAY
   that name constraint 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.  The change is due to  That is, rfc822Name
   constraints inability to represent a specific mailbox with a
   UTF-8 email local part email address.  CA certificate issuers should local-part SHOULD NOT be aware of this lessened support. used.

   Constraint comparison with SmtpUTF8Name subjectAltName starts with
   the setup steps defined by Section 5.  The setup applies to  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.  Non-normatively the setup constraint, to constraint comparison form.  For rfc822Name name
   constraint, this will convert any domain A-label A-labels to U-label in U-labels.  For
   both the rfc822Name name constraint, constraint and
   to the subject, this will lower case any doman
   domain NR-LDH label in both labels.  Strip the name constraint local-part and "@" separator from
   each rfc822Name and SmtpUTF8Name, leaving just the subject. domain-part.
   After setup, this follows the comparison steps defined in of
   [RFC5280] with some modifications as follows.  The
   comparison process  If the resulting name constraint domain starts by determining
   with a "." character, then for the name constraint
   representation i.e. email host to match, a suffix
   of the resulting subject alternative name or domain part, then comparing MUST match the name
   constraint against (including the corresponding part in leading ".") octet for octet.  If the email
   address using
   resulting name constraint domain does not start with a byte "." character,
   then for byte comparison.  This document suggests
   that the name constraint comparison with to match, the entire resulting subject distinguished
   alternative name or domain MUST match the name constraint octet for

   Certificate Authorities that wish to issue CA certificates with email
   address name constraint MUST use rfc822Name subjectAltName also follow these setup subject alternative names
   only.  These MUST be IDNA2008 conformant names with no mappings, and comparisons
   steps as well.
   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 for of SmtpUTF8Name for certificate subjectAltName (and
   issuerAltName) will incur many of the same security considerations of as
   in Section 8 in [RFC5280] , but is further complicated introduces a new issue by permitting non-
   non-ASCII characters in the email address local-part.  This complication, 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 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,

   [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

   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