LAMPS A. Melnikov, Ed.
Internet-Draft Isode Ltd
Intended status: Standards Track W. Chuang, Ed.
Expires: September 13, 2017 Google, Inc.
March 12, 2017
Internationalized Email Addresses in X.509 certificates
draft-ietf-lamps-eai-addresses-08
Abstract
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.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 13, 2017.
Copyright Notice
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document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 12
Appendix B. Example of SmtpUTF8Name . . . . . . . . . . . . . . 13
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
The formal syntax use the Augmented Backus-Naur Form (ABNF) [RFC5234]
notation.
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
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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 2.3.2.1 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.
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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
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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
are:
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
addresses.
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 4.2.1.10 of
[RFC5280], SmtpUTF8Name name can specify a particular mailbox, all
addresses at a host, or all mailboxes in a domain by specifying the
complete email address, a host name, or a domain. 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 subject alternative
names, the constraints are specified by the following changes to the
path validator to prevent bypass of the name constraints. The email
address path validator in Section 6 of [RFC5280] is modified to
consider:
1. When neither rfc822Name nor SmtpUTF8Name name constraints are
present in any issuer CA certificate, then path validation does
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not add restrictions on children certificates with rfc822Name or
SmtpUTF8Name subject alternative names. That is any combination
of rfc822Name or SmtpUTF8Name subject alternative names may be
present.
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 are
present in all issuer CA certificates that have either form, then
the path verifier applies the constraint of the subject
alternative name form in children certificates. This allows any
combination of rfc822Name or SmtpUTF8Name subject alternative
names to be present and implies that the issuer has applied
appropriate name constraints. While commonly the alternative
forms will be equivalent, they need not be, as the forms can
represent features not present in its counterpart. One instance
of this is when the issuer wants to name constrain domain or
hostname using the rules of a particular form.
4. If some issuer CA certificates contain only SmtpUTF8Name name
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 both
rfc822Name and SmtpUTF8Name constraint 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 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 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 processing procedures for host or 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
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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 subject alternative
name is illustrated in the non-normative diagram Figure 1 with
several examples. (3a) shows an issuer constraining a NR-LDH
hostname with rfc822Name and SmtpUTF8Name 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 a hostname
constraint with 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 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 using a SmtpUTF8Name name constraint but requires a
rfc822Name 'invalid' constraint because it lacks any other rfc822Name
constraints needed to initialize the name constraint path
verification. The next non-normative diagram Figure 3 illustrates
legacy name constraints that contrasts the changes this document
specifies. The legacy approach (2) has only a single rfc822Name name
email address name constraint.
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+-------------------------------------------------------------------+
| Root CA Cert |
+-------------------------------------------------------------------+
|
v
+-------------------------------------------------------------------+
| Intermediate CA Cert |
| Permitted |
| rfc822Name: nr.ldh.host.example.com (3a) |
| SmtpUTF8Name: nr.ldh.host.example.com (3a) |
| |
| rfc822Name: u+5C0Fu+5B66.host.example.com (3b) |
| SmtpUTF8Name: xn--48s3o.host.example.com (3b) |
| |
| rfc822Name: xn--pss25c.a.label.example.com (3c) |
| |
| SmtpUTF8Name: u+4E2Du+5B66.u.label.example.com (3d) |
| |
| SmtpUTF8Name: u+8001u+5E2B@i18n.email.example.com (3e) |
+-------------------------------------------------------------------+
|
v
+-------------------------------------------------------------------+
| Entity Cert (w/explicitly permitted subjects) |
| SubjectAltName Extension |
| rfc822Name: student@nr.ldh.host.example.com (3a) |
| SmtpUTF8Name: u+5B66u+751F@nr.ldh.host.example.com (3a) |
| |
| rfc822Name: student@u+5C0Fu+5B66.host.example.com (3b) |
| SmtpUTF8Name: u+5B66u+751F@xn--48s3o.host.example.com (3b) |
| |
| rfc822Name: student@xn--pss25c.a.label.example.com (3c) |
| |
| SmtpUTF8Name: student@u+4E2Du+5B66.u.label.example.com (3d) |
| |
| SmtpUTF8Name: u+8001u+5E2B@i18n.email.example.com (3e) |
+-------------------------------------------------------------------+
Name constraints with SmtpUTF8Name and rfc822Name
Figure 1
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+-------------------------------------------------------------------+
| Root CA Cert |
+-------------------------------------------------------------------+
|
v
+-------------------------------------------------------------------+
| Intermediate CA Cert |
| Name Constraint Extension |
| Permitted |
| rfc822Name: invalid (3f) |
| SmtpUTF8Name: u+8001u+5E2B@i18n.email.example.com (3f) |
+-------------------------------------------------------------------+
|
v
+-------------------------------------------------------------------+
| Entity Cert (w/explicitly permitted subjects) |
| SubjectAltName Extension |
| SmtpUTF8Name: u+8001u+5E2B@i18n.email.example.com (3f) |
+-------------------------------------------------------------------+
Name constraints with SmtpUTF8Name email address and empty rfc822Name
Figure 2
+-------------------------------------------------------------------+
| Root CA Cert |
+-------------------------------------------------------------------+
|
v
+-------------------------------------------------------------------+
| Intermediate CA Cert |
| Name Constraint Extension |
| Permitted |
| rfc822Name: student@email.example.com (2) |
+-------------------------------------------------------------------+
|
v
+-------------------------------------------------------------------+
| Entity Cert (w/explicitly permitted subjects) |
| SubjectAltName Extension |
| rfc822Name: student@email.example.com (2) |
+-------------------------------------------------------------------+
Legacy name constraints with rfc822Name
Figure 3
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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 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 (1.3.6.1.5.5.7.0).
The SmtpUTF8Name otherName in the "PKIX Other Name Forms" registry
(1.3.6.1.5.5.7.8).
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,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <http://www.rfc-editor.org/info/rfc3629>.
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[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<http://www.rfc-editor.org/info/rfc5321>.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<http://www.rfc-editor.org/info/rfc5890>.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891,
DOI 10.17487/RFC5891, August 2010,
<http://www.rfc-editor.org/info/rfc5891>.
[RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 6530, DOI 10.17487/RFC6530,
February 2012, <http://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,
<http://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, <http://www.rfc-editor.org/info/rfc6532>.
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,
<http://www.rfc-editor.org/info/rfc5912>.
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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.
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(TBD) }
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-SmtpUTF8Name, ... }
on-SmtpUTF8Name OTHER-NAME ::= {
SmtpUTF8Name IDENTIFIED BY id-on-SmtpUTF8Name
}
id-on-SmtpUTF8Name OBJECT IDENTIFIER ::= { id-on 9 }
SmtpUTF8Name ::= UTF8String (SIZE (1..MAX))
END
Figure 4
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Appendix B. Example of SmtpUTF8Name
This non-normative example demonstrates using SmtpUTF8Name as an
otherName in GeneralName to encode the email address
"u+8001u+5E2B@example.com".
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 '..@example.com'
: }
: }
Figure 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"
program.
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
UK
Email: Alexey.Melnikov@isode.com
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Weihaw Chuang (editor)
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Email: weihaw@google.com
Melnikov & Chuang Expires September 13, 2017 [Page 14]