Internet DRAFT - draft-ietf-lamps-eai-addresses
draft-ietf-lamps-eai-addresses
LAMPS A. Melnikov, Ed.
Internet-Draft Isode Ltd
Updates: 5280 (if approved) W. Chuang, Ed.
Intended status: Standards Track Google, Inc.
Expires: September 5, 2018 March 4, 2018
Internationalized Email Addresses in X.509 certificates
draft-ietf-lamps-eai-addresses-18
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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on September 5, 2018.
Copyright Notice
Copyright (c) 2018 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
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include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 9
Appendix B. Example of SmtpUTF8Mailbox . . . . . . . . . . . . . 10
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
[RFC5280] defines the rfc822Name subjectAltName name type for
representing [RFC5321] email addresses. 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].
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 uses the Augmented Backus-Naur Form (ABNF)
[RFC5234] notation.
3. Name Definitions
The GeneralName structure is defined in [RFC5280], and 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
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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.
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 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, domain was extended to support
U-labels, as defined in [RFC5890].
This document further refines Internationalized [RFC6531] Mailbox
ABNF rules and calls this SmtpUTF8Mailbox. In SmtpUTF8Mailbox,
labels that include non-ASCII characters MUST be stored in U-label
(rather than A-label) [RFC5890] form. This restriction removes the
need 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 are encoded
as UTF-8 [RFC3629] in Normalization Form C and other properties
specified there. In SmtpUTF8Mailbox, domain labels that solely use
ASCII characters (meaning not A- nor U-labels) SHALL use NR-LDH
restrictions as specified by Section 2.3.1 of [RFC5890] and SHALL be
restricted to lower case letters. 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 position, which
excludes "tagged domain names" such as A-labels. 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 ">".
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
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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 | domain char | domain label | subjectAltName |
+-----------------+-------------+--------------+-----------------+
| ASCII-only | ASCII-only | NR-LDH label | rfc822Name |
| non-ASCII | ASCII-only | NR-LDH label | SmtpUTF8Mailbox |
| ASCII-only | non-ASCII | A-label | rfc822Name |
| non-ASCII | non-ASCII | U-label | SmtpUTF8Mailbox |
+-----------------+-------------+--------------+-----------------+
non-ASCII may additionally include ASCII characters.
Table 1: Email address formatting
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
In equivalence comparison with SmtpUTF8Mailbox, there may be some
setup work on one or both inputs depending of whether the input is
already in comparison form. Comparing SmtpUTF8Mailboxs consists 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
depends on context. While some contexts such as certificate path
validation in [RFC5280] specify transforming domain to A-label
(Section 7.5 and 7.2 in [RFC5280] as updated by
[ID-lamps-rfc5280-i18n-update]), 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 SmtpUTF8Mailbox is straightforward with no setup
work needed. They are considered equivalent if there is an exact
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octet-for-octet match. Comparison with email addresses such as
Internationalized email address or rfc822Name requires additional
setup steps for domain part and local-part. The initial preparation
for the email addresses is to remove any phrases or comments, as well
as "<" and ">" present. This document calls for comparison of domain
labels that include non-ASCII characters be transformed to U-label if
not already in that form. The first step is to detect use of the
A-label by using Section 5.1 of [RFC5891]. Next if necessary,
transform any A-labels to U-labels 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
are:
1. If the domain contains A-labels, transform them to U-labels.
2. If the domain contains ASCII NR-LDH labels, 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
character 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. A
SmtpUTF8Mailbox aware path validators will apply name constraint
comparison to the subject distinguished name and both forms of
subject alternative name rfc822Name and SmtpUTF8Mailbox.
Both rfc822Name and SmtpUTF8Mailbox subject alternative names
represent the same underlying email address namespace. Since legacy
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CAs constrained to issue certificates for a specific set of domains
would lack corresponding UTF-8 constraints,
[ID-lamps-rfc5280-i18n-update] 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
[ID-lamps-rfc5280-i18n-update] admit 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 by Section 5. Setup converts the inputs of
the comparison which is one of a subject distinguished name or a
rfc822Name or SmtpUTF8Mailbox subjectAltName, and one of a rfc822Name
name constraint, to constraint comparison form. For rfc822Name name
constraint, this will convert any domain A-labels to U-labels. For
both the name constraint and the subject, this will lower case any
domain NR-LDH labels. Strip the local-part and "@" separator from
each rfc822Name and SmtpUTF8Mailbox, leaving just the domain-part.
After setup, this follows the comparison steps defined in 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 constraint 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 SmtpUTF8Mailbox subject
alternative name is illustrated in the non-normative diagram
Figure 1. The first example (1) illustrates a permitted rfc822Name
ASCII only hostname name constraint, and the corresponding valid
rfc822Name subjectAltName and SmtpUTF8Mailbox subjectAltName email
addresses. The second example (2) illustrates a permitted rfc822Name
hostname name constraint with A-label, and the corresponding valid
rfc822Name subjectAltName and SmtpUTF8Mailbox subjectAltName email
addresses. Note that an email address with ASCII only local-part is
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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@elemenary.school.example.com (1) |
| SmtpUTF8Mailbox: u+5B66u+751F@elementary.school.example.com |
| (1) |
| |
| rfc822Name: student@xn--pss25c.example.com (2) |
| SmtpUTF8Mailbox: u+533Bu+751F@u+5927u+5B66.example.com (2) |
| |
+-------------------------------------------------------------------+
Name constraints with SmtpUTF8Name and rfc822Name
Figure 1
7. Security Considerations
Use of SmtpUTF8Mailbox for certificate subjectAltName (and
issuerAltName) will incur many of the same security considerations as
in Section 8 in [RFC5280], but 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 which 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.
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8. IANA Considerations
In Section 3 and the ASN.1 module identifier defined in 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 SmtpUTF8Mailbox otherName in the "PKIX Other Name Forms"
registry (1.3.6.1.5.5.7.8). {{ Note to IANA: id-on-smtputf8Name
was assigned based on an earlier version of this document. Please
change that entry to id-on-SmtpUTF8Mailbox. }}
9. References
9.1. Normative References
[ID-lamps-rfc5280-i18n-update]
Housley, R., "Internationalization Updates to RFC 5280",
June 2017, <https://datatracker.ietf.org/doc/
draft-housley-rfc5280-i18n-update/>.
[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>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://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,
<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>.
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[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>.
9.2. 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., "Attacking Software Globalization", March 2010,
<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.
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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-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.
END
Appendix B. Example of SmtpUTF8Mailbox
This non-normative example demonstrates using SmtpUTF8Mailbox as an
otherName in GeneralName to encode the email address
"u+8001u+5E2B@example.com".
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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 2
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
especially to Viktor Dukhovni for helping us with name constraints
and his many detailed document reviews.
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 Amphitheater Parkway
Mountain View, CA 94043
US
Email: weihaw@google.com
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