



Internet Engineering Task Force (IETF)                       A. Melnikov
Request for Comments: 9598                                     Isode Ltd
Obsoletes: 8398                                                W. Chuang
Updates: 5280                                               Google, Inc.
Category: Standards Track                                     C. Bonnell
ISSN: 2070-1721                                                 DigiCert
                                                                May 2024


        Internationalized Email Addresses in X.509 Certificates

Abstract

   This document defines a new name form for inclusion in the otherName
   field of an X.509 Subject Alternative Name and Issuer Alternative
   Name extension that allows a certificate subject to be associated
   with an internationalized email address.

   This document updates RFC 5280 and obsoletes RFC 8398.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9598.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Conventions Used in This Document
   3.  Name Definitions
   4.  IDNA2008
   5.  Matching of Internationalized Email Addresses in X.509
           Certificates
   6.  Name Constraints in Path Validation
   7.  Security Considerations
   8.  Differences from RFC 8398
   9.  IANA Considerations
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Appendix A.  ASN.1 Module
   Appendix B.  Example of SmtpUTF8Mailbox
   Acknowledgments
   Authors' Addresses

1.  Introduction

   [RFC5280] defines the rfc822Name subjectAltName name type for
   representing email addresses as described in [RFC5321].  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].  This document defines a new otherName variant to
   represent internationalized email addresses.  In addition, this
   document requires all email address domains in X.509 certificates to
   conform to IDNA2008 [RFC5890].

   This document obsoletes [RFC8398].  The primary motivation of this
   document is to simplify the encoding of domain labels found in the
   domain part of internationalized email addresses.  In particular,
   [RFC8398] specifies that domain labels are conditionally encoded
   using either A-labels or U-labels.  This specification simplifies
   encoding and processing of domain labels by mandating that the
   A-label representation be used in all cases.

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Name Definitions

   The GeneralName structure [RFC5280] supports many different name
   forms including otherName for extensibility.  This section specifies
   the SmtpUTF8Mailbox 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-SmtpUTF8Mailbox OBJECT IDENTIFIER ::= { id-on 9 }

   SmtpUTF8Mailbox ::= UTF8String (SIZE (1..MAX))
   -- SmtpUTF8Mailbox conforms to Mailbox as specified
   -- in Section 3.3 of RFC 6531. Additionally, all domain
   -- labels included in the SmtpUTF8Mailbox value are
   -- encoded as LDH labels. In particular, domain labels
   -- are not encoded as U-labels and instead are encoded
   -- using their A-label representation.

   When the subjectAltName (or issuerAltName) extension contains an
   internationalized email address with a non-ASCII Local-part, the
   address MUST be stored in the SmtpUTF8Mailbox name form of otherName.
   The format of SmtpUTF8Mailbox is a modified version of the
   internationalized Mailbox that was defined in Section 3.3 of
   [RFC6531], which was derived from Mailbox as defined in 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,
   domain was extended to support U-labels, as defined in [RFC5890].

   This document further refines internationalized Mailbox ABNF rules as
   described in [RFC6531] and calls this SmtpUTF8Mailbox.  In
   SmtpUTF8Mailbox, labels that include non-ASCII characters MUST be
   stored in A-label (rather than U-label) form [RFC5890].  This
   restriction reduces complexity for implementations of the
   certification path validation algorithm defined in Section 6 of
   [RFC5280].  In SmtpUTF8Mailbox, domain labels that solely use ASCII
   characters (meaning neither A- nor U-labels) SHALL use NR-LDH
   restrictions as specified by Section 2.3.1 of [RFC5890].  NR-LDH
   stands for "Non-Reserved Letters Digits Hyphen" and is the set of LDH
   labels that do not have "--" characters in the third and forth
   character positions, which excludes "tagged domain names" such as
   A-labels.  To facilitate octet-for-octet comparisons of
   SmtpUTF8Mailbox values, all NR-LDH and A-label labels that constitute
   the domain part SHALL only be encoded with lowercase letters.
   Consistent with the treatment of rfc822Name in [RFC5280],
   SmtpUTF8Mailbox 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 ">"
   characters.

   Due to name constraint compatibility reasons described in Section 6,
   SmtpUTF8Mailbox subjectAltName MUST NOT be used unless the Local-part
   of the email address contains non-ASCII characters.  When the Local-
   part is ASCII, rfc822Name subjectAltName MUST be used instead of
   SmtpUTF8Mailbox.  This is compatible with legacy software that
   supports only rfc822Name (and not SmtpUTF8Mailbox).  The appropriate
   usage of rfc822Name and SmtpUTF8Mailbox is summarized in Table 1
   below.

   SmtpUTF8Mailbox is encoded as UTF8String.  The UTF8String encoding
   MUST NOT contain a Byte Order Mark (BOM) [RFC3629] to aid consistency
   across implementations, particularly for comparison.

                   +=================+=================+
                   | Local-part char | subjectAltName  |
                   +=================+=================+
                   | ASCII-only      | rfc822Name      |
                   +-----------------+-----------------+
                   | non-ASCII       | SmtpUTF8Mailbox |
                   +-----------------+-----------------+

                     Table 1: Email Address Formatting

   Non-ASCII Local-part values may additionally include ASCII
   characters.

4.  IDNA2008

   To facilitate comparison between email addresses, all email address
   domains in X.509 certificates MUST conform to IDNA2008 [RFC5890] (and
   avoid any "mappings" mentioned in that document).  Use of 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 these are used.

5.  Matching of Internationalized Email Addresses in X.509 Certificates

   Equivalence comparisons with SmtpUTF8Mailbox consist of a domain part
   step and a Local-part step.  The comparison form for Local-parts is
   always UTF-8.  The comparison form for domain parts is always
   performed with the LDH label ([RFC5890]) encoding of the relevant
   domain labels.  The comparison of LDH labels in domain parts reduces
   complexity for implementations of the certification path validation
   algorithm as defined in Section 6 of [RFC5280] by obviating the need
   to convert domain labels to their Unicode representation.

   Comparison of two SmtpUTF8Mailboxes is straightforward with no setup
   work needed.  They are considered equivalent if there is an exact
   octet-for-octet match.

   Comparison of an SmtpUTF8Mailbox and rfc822Name will always fail.
   SmtpUTF8Mailbox values SHALL contain a Local-part that includes one
   or more non-ASCII characters, while rfc822Names only includes ASCII
   characters (including the Local-part).  Thus, an SmtpUTF8Mailbox and
   rfc822Name will never match.

   Comparison of SmtpUTF8Mailbox values with internationalized email
   addresses from other sources (such as received email messages, user
   input, etc.) requires additional setup steps for domain part and
   Local-part.  The initial preparation for the email address to compare
   with the SmtpUTF8Mailbox value is to remove any phrases, comments,
   and "<" or ">" characters.

   For the setup of the domain part, the following conversions SHALL be
   performed:

   1.  Convert all labels that constitute the domain part that include
       non-ASCII characters to A-labels, if not already in that form.

       a.  Detect all U-labels present within the domain part using
           Section 5.1 of [RFC5891].

       b.  Transform all detected U-labels (Unicode) to A-labels (ASCII)
           as specified in Section 5.5 of [RFC5891].

   2.  Convert all uppercase letters found within the NR-LDH and A-label
       labels that constitute the domain part to lowercase letters.

   For the setup of the Local-part, the Local-part MUST be verified to
   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 of an internationalized
   email address is already in UTF-8.  Once setup is complete, they are
   again compared octet for octet.

   To summarize non-normatively, the comparison steps, including setup,
   are:

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

   2.  If any NR-LDH or A-label domain label in the domain part contains
       uppercase letters, lowercase them.

   3.  Compare strings octet for octet for equivalence.

   This specification expressly does not define any wildcard characters,
   and SmtpUTF8Mailbox comparison implementations MUST NOT interpret any
   characters as wildcards.  Instead, to specify multiple email
   addresses through SmtpUTF8Mailbox, the certificate MUST use multiple
   subjectAltNames or issuerAltNames to explicitly carry any additional
   email addresses.

6.  Name Constraints in Path Validation

   This section updates Section 4.2.1.10 of [RFC5280] to extend
   rfc822Name name constraints to SmtpUTF8Mailbox subjectAltNames.
   SmtpUTF8Mailbox-aware path validators will apply name constraint
   comparison to the subject distinguished name and both forms of
   subject alternative names, rfc822Name and SmtpUTF8Mailbox.

   Both rfc822Name and SmtpUTF8Mailbox subject alternative names
   represent the same underlying email address namespace.  Since legacy
   Certification Authorities (CAs) constrained to issue certificates for
   a specific set of domains would lack corresponding UTF-8 constraints,
   [RFC9549] updates, modifies, and extends rfc822Name name constraints
   defined in [RFC5280] to cover SmtpUTF8Mailbox subject alternative
   names.  This ensures that the introduction of SmtpUTF8Mailbox does
   not violate existing name constraints.  Since it is not valid to
   include non-ASCII UTF-8 characters in the Local-part of rfc822Name
   name constraints, and since name constraints that include a Local-
   part are rarely, if at all, used in practice, name constraints
   updated in [RFC9549] allow the forms that represent all addresses at
   a host, or all mailboxes in a domain and deprecates rfc822Name name
   constraints that represent a particular mailbox.  That is, rfc822Name
   constraints with a Local-part SHOULD NOT be used.

   Constraint comparison with SmtpUTF8Mailbox subjectAltName starts with
   the setup steps defined in Section 5.  Setup converts the inputs of
   the comparison (which is one of a subject distinguished name, an
   rfc822Name, or an SmtpUTF8Mailbox subjectAltName, and one of an
   rfc822Name name constraint) to constraint comparison form.  For both
   the name constraint and the subject, this will convert all A-labels
   and NR-LDH labels to lowercase.  Strip the Local-part and "@"
   separator from each rfc822Name and SmtpUTF8Mailbox, which leaves just
   the domain part.  After setup, follow the comparison steps defined in
   Section 4.2.1.10 of [RFC5280] as follows.  If the resulting name
   constraint domain starts with a "." character, then for the name
   constraint to match, a suffix of the resulting subject alternative
   name domain MUST match the name constraint (including the leading
   ".") octet for octet.  If the resulting name constraint domain does
   not start with a "." character, then for the name constraint to
   match, the entire resulting subject alternative name domain MUST
   match the name constraint octet for octet.

   Certificate Authorities that wish to issue CA certificates with email
   address name constraints MUST use rfc822Name subject alternative
   names only.  These MUST be IDNA2008-conformant names with no mappings
   and with non-ASCII domains encoded in A-labels only.

   The name constraint requirement with an SmtpUTF8Mailbox subject
   alternative name is illustrated in the non-normative diagram in
   Figure 1.  The first example (1) illustrates a permitted rfc822Name
   ASCII-only host name name constraint and the corresponding valid
   rfc822Name subjectAltName and SmtpUTF8Mailbox subjectAltName email
   addresses.  The second example (2) illustrates a permitted rfc822Name
   host name name constraint with an A-label, and the corresponding
   valid rfc822Name subjectAltName and SmtpUTF8Mailbox subjectAltName
   email addresses.  Note that an email address with an ASCII-only
   Local-part is encoded as rfc822Name despite also having Unicode
   present in the domain.

   +-------------------------------------------------------------------+
   |  Root CA Cert                                                     |
   +-------------------------------------------------------------------+
                                     |
                                     v
   +-------------------------------------------------------------------+
   |  Intermediate CA Cert                                             |
   |      Permitted                                                    |
   |        rfc822Name: elementary.school.example.com (1)              |
   |                                                                   |
   |        rfc822Name: xn--pss25c.example.com (2)                     |
   |                                                                   |
   +-------------------------------------------------------------------+
                                     |
                                     v
   +-------------------------------------------------------------------+
   |  Entity Cert (w/explicitly permitted subjects)                    |
   |    SubjectAltName Extension                                       |
   |      rfc822Name: student@elementary.school.example.com (1)        |
   |      SmtpUTF8Mailbox: u+5B66u+751F@elementary.school.example.com  |
   |        (1)                                                        |
   |                                                                   |
   |      rfc822Name: student@xn--pss25c.example.com (2)               |
   |      SmtpUTF8Mailbox: u+533Bu+751F@xn--pss25c.example.com (2)     |
   |                                                                   |
   +-------------------------------------------------------------------+

        Figure 1: Name Constraints with SmtpUTF8Name and rfc822Name

7.  Security Considerations

   Use of SmtpUTF8Mailbox for certificate subjectAltName (and
   issuerAltName) will incur many of the same security considerations
   described in Section 8 of [RFC5280], but it introduces a new issue by
   permitting non-ASCII characters in the email address Local-part.
   This 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 that can be exploited to deceive the
   recipient.  The former document references some means to mitigate
   against these attacks.  See [WEBER] for more background on security
   issues with Unicode.

   Additionally, it is possible to encode a string of Unicode user-
   perceived characters in multiple ways.  While various Unicode
   normalization forms exist, [RFC6531] does not mandate the use of any
   such forms for the encoding of the Local-part.  Thus, it may be
   possible to encode a Local-part value in multiple ways.  To mitigate
   against attacks where different encodings are used by the mail system
   and the Certification Authority issues certificates containing
   SmtpUTF8Mailbox values, this specification requires an octet-for-
   octet comparison of the Local-part.  However, requiring the use of
   binary comparison may raise interoperability concerns where the mail
   system employs one encoding and the Certification Authority employs
   another.

8.  Differences from RFC 8398

   This document obsoletes [RFC8398].  There are three major changes
   defined in this specification:

   1.  In all cases, domain labels in mail addresses SHALL be encoded as
       LDH labels.  In particular, domain names SHALL NOT be encoded
       using U-Labels; instead, use A-Labels.

   2.  To accommodate the first change listed above, the mail address
       matching algorithm defined in Section 5 of [RFC8398] has been
       modified to only accept domain labels that are encoded using
       their A-label representation.

   3.  Additionally, the procedure to process rfc822Name name
       constraints as defined in Section 6 of [RFC8398] has been
       modified to only accept domain labels that are encoded using
       their A-label representation.

9.  IANA Considerations

   IANA has updated the reference for the id-mod-lamps-eai-
   addresses-2016 module in the "SMI Security for PKIX Module
   Identifier" (1.3.6.1.5.5.7.0) registry to refer to this document
   instead of [RFC8398].

   IANA has updated the reference for the SmtpUTF8Mailbox otherName in
   the "SMI Security for PKIX Other Name Forms" (1.3.6.1.5.5.7.8)
   registry to refer to this document instead of [RFC8398].

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,
              <https://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, <https://www.rfc-editor.org/info/rfc3629>.

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

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

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

   [RFC6530]  Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", RFC 6530, DOI 10.17487/RFC6530,
              February 2012, <https://www.rfc-editor.org/info/rfc6530>.

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

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

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8398]  Melnikov, A., Ed. and W. Chuang, Ed., "Internationalized
              Email Addresses in X.509 Certificates", RFC 8398,
              DOI 10.17487/RFC8398, May 2018,
              <https://www.rfc-editor.org/info/rfc8398>.

   [RFC9549]  Housley, R., "Internationalization Updates to RFC 5280",
              RFC 9549, DOI 10.17487/RFC9549, March 2024,
              <https://www.rfc-editor.org/info/rfc9549>.

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,
              <https://www.rfc-editor.org/info/rfc5912>.

   [WEBER]    Weber, C., "Unraveling Unicode: A Bag of Tricks for Bug
              Hunting", July 2009, <https://www.lookout.net/files/
              Chris_Weber_Character%20Transformations%20v1.7_IUC33.pdf>.

Appendix A.  ASN.1 Module

   The following ASN.1 module normatively specifies the SmtpUTF8Mailbox
   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.

   LAMPS-EaiAddresses-2016
   { iso(1) identified-organization(3) dod(6)
     internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
     id-mod-lamps-eai-addresses-2016(92) }

   DEFINITIONS IMPLICIT TAGS ::=
   BEGIN

   IMPORTS
   OTHER-NAME
   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) }

   id-pkix
   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-SmtpUTF8Mailbox, ... }

   on-SmtpUTF8Mailbox OTHER-NAME ::= {
       SmtpUTF8Mailbox IDENTIFIED BY id-on-SmtpUTF8Mailbox
   }

   id-on-SmtpUTF8Mailbox OBJECT IDENTIFIER ::= { id-on 9 }

   SmtpUTF8Mailbox ::= UTF8String (SIZE (1..MAX))
   -- SmtpUTF8Mailbox conforms to Mailbox as specified
   -- in Section 3.3 of RFC 6531. Additionally, all domain
   -- labels included in the SmtpUTF8Mailbox value are
   -- encoded as LDH Labels. In particular, domain labels
   -- are not encoded as U-Labels and instead are encoded
   -- using their A-label representation.

   END

Appendix B.  Example of SmtpUTF8Mailbox

   This non-normative example demonstrates using SmtpUTF8Mailbox as an
   otherName in GeneralName to encode the email address
   "u+533Bu+751F@xn--pss25c.example.com".

   The hexadecimal DER encoding of the block is:

   a02b0608 2b060105 05070809 a01f0c1d e58cbbe7 949f4078 6e2d2d70
   73733235 632e6578 616d706c 652e636f 6d

   The text decoding is:

   0  43: [0] {
   2   8:   OBJECT IDENTIFIER '1 3 6 1 5 5 7 8 9'
   12  31:   [0] {
   14  29:     UTF8String 'u+533Bu+751F@xn--pss25c.example.com'
         :     }
         :   }

   The example was encoded using Google's "der-ascii" program and the
   above text decoding is an output of Peter Gutmann's "dumpasn1"
   program.

Acknowledgments

   The authors thank David Benjamin for providing the motivation for
   this document.  Additionally, the authors thank Éric Vyncke, John
   Levine, Peter van Dijk, Rich Salz, Russ Housley, and Tim Hollebeek
   for their reviews and feedback, which meaningfully improved the
   document.

   The authors also recognize and appreciate the following individuals
   for their contributions to [RFC8398]:

   |  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 especially to Viktor Dukhovni for
   |  helping us with name constraints and his many detailed document
   |  reviews.

Authors' Addresses

   Alexey Melnikov
   Isode Ltd
   14 Castle Mews
   Hampton, Middlesex
   TW12 2NP
   United Kingdom
   Email: Alexey.Melnikov@isode.com


   Wei Chuang
   Google, Inc.
   1600 Amphitheater Parkway
   Mountain View, CA
   United States of America
   Email: weihaw@google.com


   Corey Bonnell
   DigiCert
   Pittsburgh, PA
   United States of America
   Email: corey.bonnell@digicert.com
