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IETF RFC 8399
Internationalization Updates to RFC 5280
Last modified on Thursday, May 24th, 2018
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Internet Engineering Task Force (IETF) R. Housley
Request for Comments: 8399 Vigil Security
Updates: 5280 May 2018
Category: Standards Track
ISSN: 2070-1721
Internationalization Updates to RFC 5280
 Abstract
The updates to RFC 5280 described in this document provide alignment
with the 2008 specification for Internationalized Domain Names (IDNs)
and add support for internationalized email addresses in X.509
certificates.
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/RFC 8399.
Housley Standards Track PAGE 1 
RFC 8399 I18n Updates to RFC 5280 May 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
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publication of this document. Please review these documents
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than English.
Table of Contents
1. Introduction ....................................................3
1.1. Terminology ................................................3
2. Updates to RFC 5280 .............................................3
2.1. Update in the Introduction (Section 1) .....................4
2.2. Update in Name Constraints (Section 4.2.1.10) ..............4
2.3. Update in IDNs in GeneralName (Section 7.2) ................5
2.4. Update in IDNs in Distinguished Names (Section 7.3) ........6
2.5. Update in Internationalized Electronic Mail
Addresses (Section 7.5) ....................................6
3. Security Considerations .........................................7
4. IANA Considerations .............................................8
5. References ......................................................8
5.1. Normative References .......................................8
5.2. Informative References .....................................9
Acknowledgements ...................................................9
Author's Address ...................................................9
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RFC 8399 I18n Updates to RFC 5280 May 2018
1. Introduction
This document updates the Introduction in Section 1, the Name
Constraints certificate extension discussion in Section 4.2.1.10, and
the Processing Rules for Internationalized Names in Section 7 of RFC
5280 [RFC 5280] to provide alignment with the 2008 specification for
Internationalized Domain Names (IDNs) and add support for
internationalized email addresses in X.509 certificates.
An IDN in Unicode (native character) form contains at least one
U-label [RFC 5890]. With one exception, IDNs are carried in
certificates in ACE-encoded form. That is, all U-labels within an
IDN are converted to A-labels. Conversion of a U-label to an A-label
is described in [RFC 5891].
The GeneralName structure supports many different name forms,
including otherName for extensibility. RFC 8398 [RFC 8398] specifies
the SmtpUTF8Mailbox for internationalized email addresses, which
includes IDNs with U-labels.
Note that Internationalized Domain Names in Applications
specifications published in 2003 (IDNA2003) [RFC 3490] and 2008
(IDNA2008) [RFC 5890] both refer to the Punycode algorithm for
conversion [RFC 3492].
1.1. Terminology
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 [RFC 2119] [RFC 8174] when, and only when, they appear in all
capitals, as shown here.
2. Updates to RFC 5280
This section provides updates to several paragraphs of RFC 5280
[RFC 5280]. For clarity, if the entire section is not replaced, then
the original text and the replacement text are shown.
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RFC 8399 I18n Updates to RFC 5280 May 2018
2.1. Update in the Introduction (Section 1)
This update provides references for IDNA2008.
OLD
* Enhanced support for internationalized names is specified in
Section 7, with rules for encoding and comparing
Internationalized Domain Names, Internationalized Resource
Identifiers (IRIs), and distinguished names. These rules are
aligned with comparison rules established in current RFCs,
including [RFC 3490], [RFC 3987], and [RFC 4518].
NEW
* Enhanced support for internationalized names is specified in
Section 7, with rules for encoding and comparing
Internationalized Domain Names, Internationalized Resource
Identifiers (IRIs), and distinguished names. These rules are
aligned with comparison rules established in current RFCs,
including [RFC 3987], [RFC 4518], [RFC 5890], and [RFC 5891].
2.2. Update in Name Constraints (Section 4.2.1.10)
This update removes the ability to include constraints for a
particular mailbox. This capability was not used, and removing it
allows name constraints to apply to email addresses in rfc822Name and
SmtpUTF8Mailbox [RFC 8398] within otherName.
OLD
A name constraint for Internet mail addresses MAY specify a
particular mailbox, all addresses at a particular host, or all
mailboxes in a domain. To indicate a particular mailbox, the
constraint is the complete mail address. For example,
"root@example.com" indicates the root mailbox on the host
"example.com". To indicate all Internet mail addresses on a
particular host, the constraint is specified as the host name. For
example, the constraint "example.com" is satisfied by any mail
address at the host "example.com". To specify any address within a
domain, the constraint is specified with a leading period (as with
URIs). For example, ".example.com" indicates all the Internet mail
addresses in the domain "example.com", but not Internet mail
addresses on the host "example.com".
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RFC 8399 I18n Updates to RFC 5280 May 2018
NEW
A name constraint for Internet mail addresses MAY specify all
addresses at a particular host or all mailboxes in a domain. To
indicate all Internet mail addresses on a particular host, the
constraint is specified as the host name. For example, the
constraint "example.com" is satisfied by any mail address at the
host "example.com". To specify any address within a domain, the
constraint is specified with a leading period (as with URIs). For
example, ".example.com" indicates all the Internet mail addresses
in the domain "example.com" but not Internet mail addresses on
the host "example.com".
2.3. Update in IDNs in GeneralName (Section 7.2)
This update aligns with IDNA2008. Since all of Section 7.2 is
replaced, the OLD text is not provided.
NEW
Internationalized Domain Names (IDNs) may be included in certificates
and CRLs in the subjectAltName and issuerAltName extensions, name
constraints extension, authority information access extension,
subject information access extension, CRL distribution points
extension, and issuing distribution point extension. Each of these
extensions uses the GeneralName type; one choice in GeneralName is
the dNSName field, which is defined as type IA5String.
IA5String is limited to the set of ASCII characters. To accommodate
IDNs, U-labels are converted to A-labels. The A-label is the
encoding of the U-label according to the Punycode algorithm [RFC 3492]
with the ACE prefix "xn--" added at the beginning of the string.
When comparing DNS names for equality, conforming implementations
MUST perform a case-insensitive exact match on the entire DNS name.
When evaluating name constraints, conforming implementations MUST
perform a case-insensitive exact match on a label-by-label basis. As
noted in Section 4.2.1.10, any DNS name that may be constructed by
adding labels to the left-hand side of the domain name given as the
constraint is considered to fall within the indicated subtree.
Implementations SHOULD convert IDNs to Unicode before display.
Specifically, conforming implementations convert A-labels to U-labels
for display.
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RFC 8399 I18n Updates to RFC 5280 May 2018
Implementation consideration: There are increased memory requirements
for IDNs. An IDN ACE label will begin with the four additional
characters "xn--", and an IDN can require as many as five ASCII
characters to specify a single international character.
2.4. Update in IDNs in Distinguished Names (Section 7.3)
This update aligns with IDNA2008.
OLD
Domain Names may also be represented as distinguished names using
domain components in the subject field, the issuer field, the
subjectAltName extension, or the issuerAltName extension. As with
the dNSName in the GeneralName type, the value of this attribute is
defined as an IA5String. Each domainComponent attribute represents a
single label. To represent a label from an IDN in the distinguished
name, the implementation MUST perform the "ToASCII" label conversion
specified in Section 4.1 of RFC 3490. The label SHALL be considered
a "stored string". That is, the AllowUnassigned flag SHALL NOT be
set.
NEW
Domain names may also be represented as distinguished names using
domain components in the subject field, the issuer field, the
subjectAltName extension, or the issuerAltName extension. As with
the dNSName in the GeneralName type, the value of this attribute is
defined as an IA5String. Each domainComponent attribute represents a
single label. To represent a label from an IDN in the distinguished
name, the implementation MUST convert all U-labels to A-labels.
2.5. Update in Internationalized Electronic Mail Addresses
(Section 7.5)
This update aligns with IDNA2008 and RFC 8398 [RFC 8398]. Since all
of Section 7.5 is replaced, the OLD text is not provided.
NEW
Electronic Mail addresses may be included in certificates and CRLs in
the subjectAltName and issuerAltName extensions, name constraints
extension, authority information access extension, subject
information access extension, issuing distribution point extension,
or CRL distribution points extension. Each of these extensions uses
the GeneralName construct. If the email address includes an IDN but
the local-part of the email address can be represented in ASCII, then
the email address is placed in the rfc822Name choice of GeneralName,
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RFC 8399 I18n Updates to RFC 5280 May 2018
which is defined as type IA5String. If the local-part of the
internationalized email address cannot be represented in ASCII, then
the internationalized email address is placed in the otherName choice
of GeneralName using the conventions in RFC 8398 [RFC 8398].
7.5.1. Local-Part Contains Only ASCII Characters
Where the host-part contains an IDN, conforming implementations MUST
convert all U-labels to A-labels.
Two email addresses are considered to match if:
1) the local-part of each name is an exact match, AND
2) the host-part of each name matches using a case-insensitive
ASCII comparison.
Implementations SHOULD convert the host-part of internationalized
email addresses specified in these extensions to Unicode before
display. Specifically, conforming implementations convert A-labels
to U-labels for display.
7.5.2. Local-Part Contains Non-ASCII Characters
When the local-part contains non-ASCII characters, conforming
implementations MUST place the internationalized email address in the
SmtpUTF8Mailbox within the otherName choice of GeneralName as
specified in Section 3 of RFC 8398 [RFC 8398]. Note that the UTF8
encoding of the internationalized email address MUST NOT contain a
Byte-Order-Mark (BOM) [RFC 3629] to aid comparison.
The comparison of two internationalized email addresses is specified
in Section 5 of RFC 8398 [RFC 8398].
Implementations SHOULD convert the host-part of internationalized
email addresses specified in these extensions to Unicode before
display. Specifically, conforming implementations convert A-labels
to U-labels for display.
3. Security Considerations
Conforming CAs SHOULD ensure that IDNs are valid. This can be done
by validating all code points according to IDNA2008 [RFC 5892].
Failure to use valid A-labels and valid U-labels may yield a domain
name that cannot be correctly represented in the Domain Name System
(DNS). In addition, the CA/Browser Forum offers some guidance
regarding internal server names in certificates [CABF].
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RFC 8399 I18n Updates to RFC 5280 May 2018
4. IANA Considerations
This document has no IANA actions.
5. References
5.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, DOI 10.17487/RFC 3492, March 2003,
<https://www.rfc-editor.org/info/RFC 3492>.
[RFC 3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC 3629, November
2003, <https://www.rfc-editor.org/info/RFC 3629>.
[RFC 3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC 3987,
January 2005, <https://www.rfc-editor.org/info/RFC 3987>.
[RFC 4518] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP): Internationalized String Preparation", RFC 4518,
DOI 10.17487/RFC 4518, June 2006,
<https://www.rfc-editor.org/info/RFC 4518>.
[RFC 5280] 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/RFC 5280, May 2008,
<https://www.rfc-editor.org/info/RFC 5280>.
[RFC 5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC 5890, August 2010,
<https://www.rfc-editor.org/info/RFC 5890>.
[RFC 5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891,
DOI 10.17487/RFC 5891, August 2010,
<https://www.rfc-editor.org/info/RFC 5891>.
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RFC 8399 I18n Updates to RFC 5280 May 2018
[RFC 5892] Faltstrom, P., Ed., "The Unicode Code Points and
Internationalized Domain Names for Applications (IDNA)",
RFC 5892, DOI 10.17487/RFC 5892, August 2010,
<https://www.rfc-editor.org/info/RFC 5892>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
[RFC 8398] Melnikov, A., Ed. and W. Chuang, Ed., "Internationalized
Email Addresses in X.509 Certificates",
DOI 10.17487/RFC 8398, May 2016,
<http://www.rfc-editor.org/info/RFC 8398>.
5.2. Informative References
[CABF] CA/Browser Forum, "Internal Server Names and IP Address
Requirements for SSL: Guidance on the Deprecation of
Internal Server Names and Reserved IP Addresses provided
by the CA/Browser Forum", Version 1.0, June 2012,
<https://cabforum.org/internal-names/>.
[RFC 3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, DOI 10.17487/RFC 3490, March 2003,
<https://www.rfc-editor.org/info/RFC 3490>.
Acknowledgements
Thanks to Alexey Melnikov for the encouragement to write this update.
Thanks to John Klensin and Patrik Falstrom for confirming many of the
details in this update. Thanks to Ben Campbell, Wei Chuang, Spencer
Dawkins, Phillip Hallam-Baker, Warren Kumari, Alexey Melnikov, Adam
Roach, Tim Ruehsen, and Sean Turner for their careful review and
comments.
Author's Address
Russ Housley
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
United States of America
Email: housley@vigilsec.com
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Internationalization Updates to RFC 5280
RFC TOTAL SIZE: 19372 bytes
PUBLICATION DATE: Thursday, May 24th, 2018
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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