Internet Engineering Task Force (IETF) A. Melnikov

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
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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: [email protected] (1) |
| SmtpUTF8Mailbox: [email protected] |
| (1) |
| |
| rfc822Name: [email protected] (2) |
| SmtpUTF8Mailbox: [email protected] (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
"[email protected]".

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 '[email protected]'
: }
: }

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: [email protected]

Wei Chuang
Google, Inc.
1600 Amphitheater Parkway
Mountain View, CA
United States of America
Email: [email protected]

Corey Bonnell
DigiCert
Pittsburgh, PA
United States of America
Email: [email protected]