Internationalized Email Addresses in X.509 CertificatesIsode Ltd14 Castle MewsHampton, MiddlesexTW12 2NPUnited KingdomAlexey.Melnikov@isode.comGoogle, Inc.1600 Amphitheater ParkwayMountain ViewCAUnited States of Americaweihaw@google.comDigiCertPittsburghPAUnited States of Americacorey.bonnell@digicert.com
SEC
lampsEAIPKIXemail addressThis 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
.
Copyright Notice
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Provisions Relating to IETF Documents
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Table of Contents
. Introduction
. Conventions Used in This Document
. Name Definitions
. IDNA2008
. Matching of Internationalized Email Addresses in X.509 Certificates
. Name Constraints in Path Validation
. Security Considerations
. Differences from RFC 8398
. IANA Considerations
. References
. Normative References
. Informative References
. ASN.1 Module
. Example of SmtpUTF8Mailbox
Acknowledgments
Authors' Addresses
Introduction defines the rfc822Name subjectAltName name type for
representing email addresses as described in . The syntax
of rfc822Name is restricted to a subset of US-ASCII characters and
thus can't be used to represent internationalized email addresses
. 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 .This document obsoletes . 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, 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.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 when, and only when, they appear in all capitals, as
shown here.
Name DefinitionsThe GeneralName structure 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
, which was derived from Mailbox as defined in
. 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
. Also, domain was extended to support U-labels, as defined
in .This document further refines internationalized Mailbox ABNF rules as
described in and calls this SmtpUTF8Mailbox. In
SmtpUTF8Mailbox, labels that include non-ASCII characters MUST be
stored in A-label (rather than U-label) form . This
restriction reduces complexity for implementations of the certification
path validation algorithm defined in . In
SmtpUTF8Mailbox, domain labels that solely use ASCII characters (meaning
neither A- nor U-labels) SHALL use NR-LDH restrictions as specified by
. 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 ,
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 ,
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) to aid consistency
across implementations, particularly for comparison.
Email Address Formatting
Local-part char
subjectAltName
ASCII-only
rfc822Name
non-ASCII
SmtpUTF8Mailbox
Non-ASCII Local-part values may additionally include ASCII characters.IDNA2008To facilitate comparison between email addresses, all email address
domains in X.509 certificates MUST conform to IDNA2008 (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.Matching of Internationalized Email Addresses in X.509 CertificatesEquivalence 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 () 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 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:
Convert all labels that constitute the domain part that include
non-ASCII characters to A-labels, if not already in that form.
Detect all U-labels present within the domain part using
.
Transform all detected U-labels (Unicode) to A-labels (ASCII)
as specified in .
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 and , 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:
If the domain contains U-labels, transform them to A-labels.
If any NR-LDH or A-label domain label in the domain part
contains uppercase letters, lowercase them.
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.Name Constraints in Path ValidationThis section updates 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, updates,
modifies, and extends rfc822Name name constraints defined in
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
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 . 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 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
. 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.Name Constraints with SmtpUTF8Name and rfc822Name
+-------------------------------------------------------------------+
| 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@elementary.school.example.com (1) |
| SmtpUTF8Mailbox: u+5B66u+751F@elementary.school.example.com |
| (1) |
| |
| rfc822Name: student@xn--pss25c.example.com (2) |
| SmtpUTF8Mailbox: u+533Bu+751F@xn--pss25c.example.com (2) |
| |
+-------------------------------------------------------------------+
Security ConsiderationsUse of SmtpUTF8Mailbox for certificate subjectAltName (and
issuerAltName) will incur many of the same security considerations
described in , but it introduces a new issue by
permitting non-ASCII characters in the email address Local-part.
This issue, as mentioned in and in , 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 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, 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.Differences from RFC 8398This document obsoletes . There are three major changes
defined in this specification:
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.
To accommodate the first change listed above, the mail address
matching algorithm defined in has been modified
to only accept domain labels that are encoded using their A-label
representation.
Additionally, the procedure to process rfc822Name name constraints as defined
in
has been modified to only accept domain labels
that are encoded using their A-label representation.
IANA ConsiderationsIANA 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 .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 .ReferencesNormative ReferencesKey words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.UTF-8, a transformation format of ISO 10646ISO/IEC 10646-1 defines a large character set called the Universal Character Set (UCS) which encompasses most of the world's writing systems. The originally proposed encodings of the UCS, however, were not compatible with many current applications and protocols, and this has led to the development of UTF-8, the object of this memo. UTF-8 has the characteristic of preserving the full US-ASCII range, providing compatibility with file systems, parsers and other software that rely on US-ASCII values but are transparent to other values. This memo obsoletes and replaces RFC 2279.Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) ProfileThis memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK]Simple Mail Transfer ProtocolThis document is a specification of the basic protocol for Internet electronic mail transport. It consolidates, updates, and clarifies several previous documents, making all or parts of most of them obsolete. It covers the SMTP extension mechanisms and best practices for the contemporary Internet, but does not provide details about particular extensions. Although SMTP was designed as a mail transport and delivery protocol, this specification also contains information that is important to its use as a "mail submission" protocol for "split-UA" (User Agent) mail reading systems and mobile environments. [STANDARDS-TRACK]Internationalized Domain Names for Applications (IDNA): Definitions and Document FrameworkThis document is one of a collection that, together, describe the protocol and usage context for a revision of Internationalized Domain Names for Applications (IDNA), superseding the earlier version. It describes the document collection and provides definitions and other material that are common to the set. [STANDARDS-TRACK]Internationalized Domain Names in Applications (IDNA): ProtocolThis document is the revised protocol definition for Internationalized Domain Names (IDNs). The rationale for changes, the relationship to the older specification, and important terminology are provided in other documents. This document specifies the protocol mechanism, called Internationalized Domain Names in Applications (IDNA), for registering and looking up IDNs in a way that does not require changes to the DNS itself. IDNA is only meant for processing domain names, not free text. [STANDARDS-TRACK]Overview and Framework for Internationalized EmailFull use of electronic mail throughout the world requires that (subject to other constraints) people be able to use close variations on their own names (written correctly in their own languages and scripts) as mailbox names in email addresses. This document introduces a series of specifications that define mechanisms and protocol extensions needed to fully support internationalized email addresses. These changes include an SMTP extension and extension of email header syntax to accommodate UTF-8 data. The document set also includes discussion of key assumptions and issues in deploying fully internationalized email. This document is a replacement for RFC 4952; it reflects additional issues identified since that document was published. [STANDARDS-TRACK]SMTP Extension for Internationalized EmailThis document specifies an SMTP extension for transport and delivery of email messages with internationalized email addresses or header information. [STANDARDS-TRACK]Internationalized Email HeadersInternet mail was originally limited to 7-bit ASCII. MIME added support for the use of 8-bit character sets in body parts, and also defined an encoded-word construct so other character sets could be used in certain header field values. However, full internationalization of electronic mail requires additional enhancements to allow the use of Unicode, including characters outside the ASCII repertoire, in mail addresses as well as direct use of Unicode in header fields like "From:", "To:", and "Subject:", without requiring the use of complex encoded-word constructs. This document specifies an enhancement to the Internet Message Format and to MIME that allows use of Unicode in mail addresses and most header field content.This specification updates Section 6.4 of RFC 2045 to eliminate the restriction prohibiting the use of non-identity content-transfer- encodings on subtypes of "message/". [STANDARDS-TRACK]Ambiguity of Uppercase vs Lowercase in RFC 2119 Key WordsRFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.Internationalized Email Addresses in X.509 CertificatesThis 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.Internationalization Updates to RFC 5280The updates to RFC 5280 described in this document provide alignment with the 2008 specification for Internationalized Domain Names (IDNs) and includes support for internationalized email addresses in X.509 certificates. The updates ensure that name constraints for email addresses that contain only ASCII characters and internationalized email addresses are handled in the same manner. This document obsoletes RFC 8399.Informative ReferencesNew ASN.1 Modules for the Public Key Infrastructure Using X.509 (PKIX)The Public Key Infrastructure using X.509 (PKIX) certificate format, and many associated formats, are expressed using ASN.1. The current ASN.1 modules conform to the 1988 version of ASN.1. This document updates those ASN.1 modules to conform to the 2002 version of ASN.1. There are no bits-on-the-wire changes to any of the formats; this is simply a change to the syntax. This document is not an Internet Standards Track specification; it is published for informational purposes.Unraveling Unicode: A Bag of Tricks for Bug HuntingASN.1 ModuleThe following ASN.1 module normatively specifies the SmtpUTF8Mailbox
structure. This specification uses the ASN.1 definitions from
with the 2002 ASN.1 notation used in that document.
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
Example of SmtpUTF8MailboxThis non-normative example demonstrates using SmtpUTF8Mailbox as an
otherName in GeneralName to encode the email address
"u+533Bu+751F@xn--pss25c.example.com".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 'u+533Bu+751F@xn--pss25c.example.com'
: }
: }
The example was encoded using Google's "der-ascii" program and the
above text decoding is an output of Peter Gutmann's "dumpasn1"
program.AcknowledgmentsThe 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 :
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' AddressesIsode Ltd14 Castle MewsHampton, MiddlesexTW12 2NPUnited KingdomAlexey.Melnikov@isode.comGoogle, Inc.1600 Amphitheater ParkwayMountain ViewCAUnited States of Americaweihaw@google.comDigiCertPittsburghPAUnited States of Americacorey.bonnell@digicert.com