Internationalization Updates to RFC 5280
RFC 8399

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/rfc8399.

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Copyright Notice

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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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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 [RFC5280] 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 [RFC5890].  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 [RFC5891].

   The GeneralName structure supports many different name forms,
   including otherName for extensibility.  RFC 8398 [RFC8398] 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) [RFC3490] and 2008
   (IDNA2008) [RFC5890] both refer to the Punycode algorithm for
   conversion [RFC3492].

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 [RFC2119] [RFC8174] 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
   [RFC5280].  For clarity, if the entire section is not replaced, then
   the original text and the replacement text are shown.

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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 [RFC3490], [RFC3987], and [RFC4518].

   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 [RFC3987], [RFC4518], [RFC5890], and [RFC5891].

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 [RFC8398] 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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   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 [RFC3492]
   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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   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 [RFC8398].  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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   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 [RFC8398].

   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 [RFC8398].  Note that the UTF8
   encoding of the internationalized email address MUST NOT contain a
   Byte-Order-Mark (BOM) [RFC3629] to aid comparison.

   The comparison of two internationalized email addresses is specified
   in Section 5 of RFC 8398 [RFC8398].

   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 [RFC5892].
   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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4.  IANA Considerations

   This document has no IANA actions.

5.  References

5.1.  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>.

   [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
              for Internationalized Domain Names in Applications
              (IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003,
              <https://www.rfc-editor.org/info/rfc3492>.

   [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>.

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987,
              January 2005, <https://www.rfc-editor.org/info/rfc3987>.

   [RFC4518]  Zeilenga, K., "Lightweight Directory Access Protocol
              (LDAP): Internationalized String Preparation", RFC 4518,
              DOI 10.17487/RFC4518, June 2006,
              <https://www.rfc-editor.org/info/rfc4518>.

   [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>.

   [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>.

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   [RFC5892]  Faltstrom, P., Ed., "The Unicode Code Points and
              Internationalized Domain Names for Applications (IDNA)",
              RFC 5892, DOI 10.17487/RFC5892, August 2010,
              <https://www.rfc-editor.org/info/rfc5892>.

   [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",
              DOI 10.17487/RFC8398, May 2016,
              <http://www.rfc-editor.org/info/rfc8398>.

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/>.

   [RFC3490]  Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, DOI 10.17487/RFC3490, March 2003,
              <https://www.rfc-editor.org/info/rfc3490>.

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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