LAMPS                                                   A. Melnikov, Ed.
Internet-Draft                                                 Isode Ltd
Intended status: Standards Track                          W. Chuang, Ed.
Expires: September 9, 13, 2017                                 Google, Inc.
                                                          March 8, 12, 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 . . . . . . . . . . . . . . . . . .   8  10
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   8  10
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9  10
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  10  11
   Appendix A.  ASN.1 Module . . . . . . . . . . . . . . . . . . . .  10  12
   Appendix B.  Example of SmtpUTF8Name  . . . . . . . . . . . . . .  11  13
   Appendix C.  Acknowledgements . . . . . . . . . . . . . . . . . .  12  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12  13

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.

   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 ">".

   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 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 (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
   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 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 5.  As with
   rfc822Name name constraints as specified in Section 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.  Name constraint
   comparisons in the context of [RFC5280] that are specified with
   SmtpUTF8Name name are only done on the subjectAltName SmtpUTF8Name
   name and not on other forms.  Similarly rfc822Name name constraints
   do not apply to subjectAltName SmtpUTF8Name name.  This imposes
   requirements on the certificate issuer as described next.

   When name constraints are used with SmtpUTF8Name subjectAltName subject alternative
   names, they the constraints are specified in by the following profile changes to the
   path validator to prevent
   bypassing bypass of the name constraints.  Host name and domain constraints MUST
   use both  The email
   address path validator in Section 6 of [RFC5280] is modified to

   1.  When neither rfc822Name and nor SmtpUTF8Name forms name constraints are
       present in the issuing certificate
   with the constraint.  Complete email address constraint any issuer CA certificate, then path validation does
       not add restrictions on children certificates with UTF-8
   local-part MUST only use rfc822Name or
       SmtpUTF8Name form.  Complete email address
   constraint with ASCII local-part MUST use both subject alternative names.  That is any combination
       of rfc822Name and or SmtpUTF8Name forms. subject alternative names may be

   2.  If issuer CA certificates contain only rfc822Name name
       constraints, then those constraints apply to rfc822Name subject
       alternative name in children certificates.  SmtpUTF8Name subject
       alternative name are prohibited in those same certificates, that
       is those certificates MUST be rejected by the path verifier.

   3.  When both rfc822Name and SmtpUTF8Name name constraints forms are present, they MUST carry the equivalent
   constraints as defined by Section 5 and MUST be found
       present in all issuer CA certificates that have either form, then
       the same
   node and in path verifier applies the same permittedSubtrees or excludedSubtrees.  This
   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 the subject
       alternative name constraint profiles as an additional path
   validation step form in Section 6 of [RFC5280].  SmtpUTF8Name aware
   validators MUST NOT accept children certificates.  This allows any certificate whose path contains an
   issuing certificate whose
       combination of rfc822Name or SmtpUTF8Name subject alternative
       names to be present and implies that the issuer has applied
       appropriate name constraints
   do constraints.  While commonly the alternative
       forms will be equivalent, they need not match be, as the above profile.  That forms can
       represent features not present in its counterpart.  One instance
       of this is when the path validator verifies
   that a rfc822Name issuer wants to name constraint has a corresponding SmtpUTF8Name
   constraint and that constrain domain or
       hostname using the rules of a particular form.

   4.  If some issuer CA certificates contain only SmtpUTF8Name name constraint has a
       constraints, then those are at risk of bypass with rfc822Name
       subject alternative names when processed by legacy verifiers.  To
       prevent this, issuers MUST also publish rfc822Name name
       constraint that prevent those bypasses.  This occurs when the both
       rfc822Name and SmtpUTF8Name constraint contains host
   name, forms can represents the
       same host, domain or email address, and both are needed.  Even
       when the constraints are asymmetric such as when the issuer
       wishes to constrain an email address with an ASCII local-part.  This
   correspondence is required UTF-8 local part, a
       non empty rfc822Name name constraint may be needed if there isn't
       one already so that the path verifier initializes correctly.

   When both name constraints are present, the contents depends on the
   usage.  If the issuer desires to represent the same NR-LDH host or
   domain, then it is the same string in both rfc822Name and
   SmtpUTF8Name.  If the host or domain labels contain UTF-8, then the
   labels may be used directly in SmtpUTF8Name noting the same issuing certificate node restriction in
   Section 5 and transformed to A-label for rfc822Name using the process
   described in [RFC5280].  Email addresses that use ASCII local-part
   use the same nameConstraint permittedSubtrees processing procedures for host or excludedSubtrees. domain.

   If the issuer wishes to represent the name constraint asymmetrically,
   with either rfc822Name or SmtpUTF8Name to respectively represent some
   A-label or U-label in the domain or host, the alternate name
   constraint form must still be present.  If nothing needs be
   represented by the alternate form, then empty name constraint can
   described by the "invalid" TLD that helps initialize the name
   constraint path validation set.  Or alternatively it may be omitted
   if some other name constraint pair, provides a name constraint of
   that form.  In particular this initialization may be needed when
   SmtpUTF8Name is used to represent an email address name constraint
   with an UTF-8 local-part and rfc822Name cannot represent such a email
   address constraint.

   The name constraint requirement with SmtpUTF8Name subjectAltName subject alternative
   name is illustrated in the following non-normative diagram Figure 1.  This
   show 1 with
   several examples.  (3a) shows an issuer constraining a NR-LDH
   hostname with rfc822Name and SmtpUTF8Name aware so that they can issue
   ASCII and UTF-8 local-name email addresses certificates.  (3b) shows
   an issuer constraining a hostname containing a non-ASCII label for
   u+5C0Fu+5B66 (elementary school).  (3c) demonstrates that constrained the intermediate CA a hostname
   constraint with host name an rfc822Name is distinguishable from its
   SmtpUTF8Name constraint, and that only the rfc822Name form is
   permitted.  No 'invalid' SmtpUTF8Name constraint is needed since
   other SmtpUTF8Name constraints are present.  (3d) similarly
   demonstrates this capability to restrict a name constraint to
   SmtpUTF8Name only.  (3e) shows that a non-ASCII local- part email
   address name constraints.  In particular can also be constrained to be permitted using SmtpUTF8Name.
   It too does not need an 'invalid' rfc822Name as other rfc822Name
   constrains are present.  Diagram Figure 2 illustrates (non-
   normatively) a different certificate chain that does need the
   'invalid' name constraint. (3f) constrains a non-ASCII local-part
   email address constraint with UTF8 local-part only used using a single SmtpUTF8Name name constraint, while the email address constraint with
   ASCII local-part used both but requires a
   rfc822Name and SmtpUTF8Name 'invalid' constraint because it lacks any other rfc822Name
   constraints needed to initialize the name
   constraints. constraint path
   verification.  The next non-normative diagram Figure 2 3 illustrates
   legacy name constraints to that contrasts the changes this document
   specifies.  The legacy approach (2) has only a single rfc822Name name
   email address name constraint.


   |  Root CA Cert                                                     |
   |  Intermediate CA Cert                                             |
   |    Name Constraint Extension                                 |
       |      Permitted                                                    |
   |        rfc822Name: (3a)                   |
   |        SmtpUTF8Name: (3a)                 |
   |                                                                   |
   |        rfc822Name: (3b)             |
   |        SmtpUTF8Name: (3b)              |
   |                                                                   |
   |        rfc822Name: (3c)            |
   |                                                                   |
   |        SmtpUTF8Name: (3d)        |
       +--------------------------------------------------------------+                                                                   |
   |        SmtpUTF8Name: (3e)     |
   |  Entity Cert (w/explicitly permitted subjects)                    |
   |    SubjectAltName Extension                                       |
   |      rfc822Name: (3a)             |
   |      SmtpUTF8Name: (3a)      |
   |                                                                   |
   |      rfc822Name: (3b)       |
   |      SmtpUTF8Name: (3b)   |
   |                                                                   |
   |      rfc822Name: (3c)      |
   |                                                                   |
   |      SmtpUTF8Name: (3d)  |
   |                                                                   |
       +--------------------------------------------------------------+      SmtpUTF8Name: (3e)       |

   Name constraints with SmtpUTF8Name and rfc822Name

                                 Figure 1


   |  Root CA Cert                                                     |
   |  Intermediate CA Cert                                             |
   |    Name Constraint Extension                                      |
   |      Permitted                                                    |
   |        rfc822Name: invalid (3f)                                   |
   |        SmtpUTF8Name: (3f)     |
   |  Entity Cert (w/explicitly permitted subjects)                    |
   |    SubjectAltName Extension                                       |
   |      SmtpUTF8Name: (3f)       |

   Name constraints with SmtpUTF8Name email address and empty rfc822Name

                                 Figure 2

   |  Root CA Cert                                                     |
   |  Intermediate CA Cert                                             |
   |    Name Constraint Extension                                      |
   |      Permitted                                                    |
   |        rfc822Name: (2)                  |
   |  Entity Cert (w/explicitly permitted subjects)                    |
   |    SubjectAltName Extension                                       |
   |      rfc822Name: (2)                    |

   Legacy name constraints with rfc822Name

                                 Figure 2 3

7.  Deployment Considerations

   For email addresses whose local-part is ASCII it may be more
   reasonable to continue using rfc822Name 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 smptUtf8Name SmtpUTF8Name supports U-label whereas rfc822Name
   supports A-label.  This document RECOMMENDS using SmtpUTF8Name when
   local-part contains non-ASCII characters, and otherwise rfc822Name.

8.  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.

9.  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

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
   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 3 4

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 4 5

   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. 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 Amphitheatre Parkway
   Mountain View, CA  94043