Network Working Group A. Melnikov, Ed.
Internet-Draft Isode Ltd
Intended status: Standards Track W. Chuang, Ed.
Expires: January 2, 2017 Google, Inc.
July 1, 2016
Internationalized Email Addresses in X.509 certificates
draft-melnikov-spasm-eai-addresses-02
Abstract
This document defines a new name form for inclusion in the otherName
field of an X.509 Subject Alternative Name extension that allows a
certificate subject to be associated with an Internationalized Email
Address.
Status of This Memo
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This Internet-Draft will expire on January 2, 2017.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 2
3. Name Definitions . . . . . . . . . . . . . . . . . . . . . . 2
4. Matching of Internationalized Email Addresses in X.509
certificates . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Name constraints in path validation . . . . . . . . . . . . . 4
6. Resource Considerations . . . . . . . . . . . . . . . . . . . 4
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
8. Security Considerations . . . . . . . . . . . . . . . . . . . 4
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
9.1. Normative References . . . . . . . . . . . . . . . . . . 5
9.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
[RFC5280] defines rfc822Name subjectAltName choice for representing
[RFC5322] 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].
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
This section defines the SMTPUTF8Name name as a form of otherName
from the GeneralName structure in SubjectAltName defined in
[RFC5280].
id-on-smtputf8Name OBJECT IDENTIFIER ::= { id-on XXX }
SMTPUTF8Name ::= UTF8String (SIZE (1..MAX))
When the subjectAltName 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
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[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 SHALL use U-label Unicode native character labels and MUST NOT
use A-label [RFC5890] to encode non-ascii characters. This
restriction prevents having to determine which label encoding A- or
U-label is present in the Domain. Further U-label SHALL use UTF-8
Unicode [RFC3629] for encoding efficiency. 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 ">".
4. 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 normalization and local-part normalization. The normalized
form for local-part always is UTF-8. The normalized form for domain
depends on context. While some contexts such as certificate path
validation in [RFC5280] specify normalizing to A-label, this document
RECOMMENDS normalizing to UTF-8 U-label even in place of those other
specifications. As more implementations natively support UTF-8
U-label domain, normalizing to UTF-8 U-label reduces conversions
required, which then reduces likelihood of errors caused by bugs in
implementation.
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 normalize the
format for comparison. Domain normalization is particularly
important for forms that may contain A- or U-label such as
International email address, or A-label only such as rfc822Name.
This document specifies the process to normalize domain to UTF-8
U-label. (To normalize to domain to A-label, follow the process
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
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section 5.2 of [RFC5891]. Finally if necessary convert the Unicode
to UTF-8 as specified in section 3 of [RFC3629]. In setup for
smtputf8Mailbox, the email address local-part MUST be converted to
UTF-8 if it is not already. The <Local-part> part of an
Internationalized email address is already in UTF-8. For the
rfc822Name local-part is IA5String (ASCII), and conversion to UTF-8
is trivial since ASCII octets maps to UTF-8 without change. Once the
setup is completed, comparison is an octet for octet comparison.
5. 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.
Name constraint comparisons in the context [RFC5280] is specified
with smtputf8Name name are only done on the subjectAltName
smtputf8Name name, and says nothing more about constaints on other
email address forms such as rfc822Name. Constraint comparison on
complete email address with smtputf8Name name uses the matching
procedure defined by Section 4. 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.
6. Resource Considerations
For email addresses whose local-part is ASCII it may be more
reasonable to continue using rfc822Name instead of smtputf8Name. Use
of smtputf8Name incurs higher byte representation overhead due to the
use of otherName and the additiona OID needed. This document
RECOMMENDS using smtputf8Name when local-part contains non-ascii
characters, and otherwise rfc822Name.
7. IANA Considerations
[[CREF1: Just need a new OID.]]
8. Security Considerations
Use for smtputf8Name for certificate subjectAltName will incur many
of the same security considerations of Section 8 [RFC5280] but
further complicated by permitting non-ASCII characters to the email
address local-part. As mentioned in Section 4.4 of [RFC5890] and in
Section 4 of [RFC6532] Unicode introduces the risk for visually
similar characters which can be exploited to deceive the recipient.
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The former document references some means to mitigate against these
attacks.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<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>.
[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>.
[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>.
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9.2. Informative References
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<http://www.rfc-editor.org/info/rfc5322>.
[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>.
Appendix A. Acknowledgements
Thank you to Magnus Nystrom for motivating this document. Thanks to
Nicolas Lidzborski, Laetitia Baudoin, Ryan Sleevi and the reviewers
for their early 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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