Internationalized Email Addresses in X.509 certificates
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Security
lampsEAIPKIXemail address
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.
defines rfc822Name subjectAltName choice for representing
email addresses. This form is restricted to a subset of US-ASCII
characters and thus can't be used to represent Internationalized Email addresses
.
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
.The formal syntax use the Augmented
Backus-Naur Form (ABNF) notation.
This section defines the smtputf8Name name as a form of otherName from the
GeneralName structure in SubjectAltName defined in .
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 which is
itself derived from SMTP Mailbox from Section 4.1.2 of .
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 . Also sub-domain is extended to support U-label, as
defined in
This document further refines Internationalized 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 .
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 , U-label use
UTF-8 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 . 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 ,
the smtputf8Mailbox rules are modified to allow partial productions to allow
for additional forms required by . 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.
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
specify transforming to A-label, this document RECOMMENDS transforming to UTF-8
U-label even in place of those other specifications. As more implementations
natively support U-label domain, requiring 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 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 process specified in section 7.5 and 7.2 in )
The first step is to detect A-label by using section 5.1 of .
Next if necessary, transform the A-label to U-label Unicode as specified in
section 5.2 of . Finally if necessary convert the
Unicode to UTF-8 as specified in section 3 of . 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.
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 with the extension as noted there
for partial productions.
Constraint comparison on complete email address with
smtputf8Name name uses the matching procedure defined by .
As with rfc822Name name constraints as specified in Section 4.2.1.10 of , 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 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. 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
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.
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 encoding with otherName and the additional OID
needed. This document RECOMMENDS using smtputf8Name when local-part contains
non-ASCII characters, and otherwise rfc822Name.
Just need a new OID.
Use for smtputf8Name for certificate subjectAltName will incur many of the
same security considerations of Section 8 in but further
complicated by permitting non-ASCII characters in the email address local-part.
As mentioned in Section 4.4 of and in Section 4 of
Unicode introduces the
risk for visually similar characters which can be exploited to deceive the recipient.
The former document references some means to mitigate against these attacks.
Thank you to Magnus Nystrom for motivating this document. Thanks to
Nicolas Lidzborski, Laetitia Baudoin, Ryan Sleevi and Sean Leonard for their
early feedback. Also thanks to John Klensin for his valuable input on
internationalization, Unicode and ABNF formatting.