LAMPS A. Melnikov, Ed.
Internet-Draft Isode Ltd
Intended status: Standards Track W. Chuang, Ed.
Expires: August 5, 2017 Google, Inc.
February 1, 2017
Internationalized Email Addresses in X.509 certificates
draft-ietf-lamps-eai-addresses-06
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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 2
3. Name Definitions . . . . . . . . . . . . . . . . . . . . . . 2
4. IDNA2008 . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Matching of Internationalized Email Addresses in X.509
certificates . . . . . . . . . . . . . . . . . . . . . . . . 4
6. Name constraints in path validation . . . . . . . . . . . . . 5
7. Deployment Considerations . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. 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 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 is defined in
Section 3.3 of [RFC6531] which is itself derived from SMTP Mailbox
from Section 4.1.2 of [RFC5321]. [RFC6531] defines the following
ABNF rules for Mailbox whose parts are modified for
internationalization: <Local-part>, <Dot-string>, <Quoted-string>,
<QcontentSMTP>, <Domain>, and <Atom>. In particular, <Local-part>
was updated to also support UTF8-non-ascii. UTF8-non-ascii is
described by Section 3.1 of [RFC6532]. Also, sub-domain is 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 solely ASCII character labels SHALL use NR-LDH
restrictions as specified by section 2.3.1 of [RFC5890] and
restricted to lower case letters. Note that a SmtpUtf8Mailbox 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.
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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 can be straightforward. No setup work
is needed and it can be an octet for octet comparison. For other
email address forms such as Internationalized email address or
rfc822Name, the comparison requires additional setup to convert the
format for comparison. 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 already being a string in UTF-8
form. In particular, the local-part MUST NOT be transformed in any
way, such as by doing case folding or normalization of any kind. The
<Local-part> part of an Internationalized email address is already in
UTF-8. For rfc822Name the local-part, which is IA5String (ASCII),
trivially maps to UTF-8 without change. Once setup is completed,
comparison is again checking for octet for octet equivalence.
To summarize non-normatively the domain setup steps are:
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1. if the domain contains A-labels, transform them to U-label
2. if the domain contains ASCII NR-LDH labels, lowercase them
This enables an octet for octet comparison.
This specification expressly does not define any wildcards characters
and SmtpUtf8Name comparison implementations MUST NOT interpret any
character as wildcards. Instead, to specify multiple specifying
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 [RFC5280] is specified
with SmtpUtf8Name name are only done on the subjectAltName (and
issuerAltName) SmtpUtf8Name name, and says nothing more about
constraints on other email address forms such as rfc822Name.
Consequently it may be necessary to include other name constraints
such as rfc822Name in addition to SmtpUtf8Name to constrain all
potential email addresses. For example a domain with both ASCII and
non-ASCII local-part email addresses may require both rfc822Name and
SmtpUtf8Name name constraints. This can be illustrated in the
following non-normative diagram Figure 1 which shows a name
constraint set in the intermediate CA certificate, which then applies
to the children entity certificates. Note that a constraint on
rfc822Name does not apply to SmtpUtf8Name and vice versa as is shown
in non-normative diagram Figure 2.
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+--------------------------------------------------------------+
| Root CA Cert |
+--------------------------------------------------------------+
|
v
+--------------------------------------------------------------+
| Intermediate CA Cert |
| Name Constraint Extension |
| Permitted |
| rfc822Name: allowed.example.com |
| SmtpUtf8Name: allowed.example.com |
| Excluded |
| rfc822Name: ignored.allowed.example.com |
+--------------------------------------------------------------+
|
v
+--------------------------------------------------------------+
| Entity Cert (w/explicitly permitted subjects) |
| SubjectAltName Extension |
| rfc822Name: student@allowed.example.com |
| SmtpUtf8Name: u+8001u+5E2B@allowed.example.com |
+--------------------------------------------------------------+
Figure 1
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+--------------------------------------------------------------+
| Root CA Cert |
+--------------------------------------------------------------+
|
v
+--------------------------------------------------------------+
| Intermediate CA Cert |
| Name Constraint Extension |
| Permitted |
| rfc822Name: allowed.example.com |
| SmtpUtf8Name: allowed.example.com |
| Excluded |
| rfc822Name: ignored.allowed.example.com |
+--------------------------------------------------------------+
|
v
+--------------------------------------------------------------+
| Entity Cert (w/permitted subject i.e. excluded rfc822Name |
| does not exclude SmtpUtf8Name) |
| SubjectAltName Extension |
| SmtpUtf8Name: u+4E0Du+5C0D@ignored.allowed.example.com |
+--------------------------------------------------------------+
Figure 2
7. Deployment Considerations
For email addresses whose local-part is ASCII it may be more
reasonable to continue using rfc822Name instead of SmtpUtf8Name. 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 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.
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9. IANA Considerations
This document makes use of object identifiers for the SmtpUtf8Name
defined 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>.
[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>.
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[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>.
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.
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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 3
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".
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The hexidecimal 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 4
The example was encoded on the OSS Nokalva ASN.1 Playground and the
above text decoding is an output of Peter Gutmann's "dumpasn1"
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, Viktor Dukhovni and Patrik
Falstrom for their feedback. Also special thanks to John Klensin for
his valuable input on internationalization, Unicode and ABNF
formatting, and to Jim Schaad for his help with the ASN.1 example and
his helpful feedback.
Authors' Addresses
Alexey Melnikov (editor)
Isode Ltd
14 Castle Mews
Hampton, Middlesex TW12 2NP
UK
Email: Alexey.Melnikov@isode.com
Weihaw Chuang (editor)
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Email: weihaw@google.com
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