Network Working Group                                         R. Housley
Request for Comments: 4630                                Vigil Security
Updates: 3280                                               S. Santesson
Category: Standards Track                                      Microsoft
                                                             August 2006

              Update to DirectoryString Processing in the
                Internet X.509 Public Key Infrastructure
       Certificate and Certificate Revocation List (CRL) Profile

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2006).

Abstract

   This document updates the handling of DirectoryString in the Internet
   X.509 Public Key Infrastructure Certificate and Certificate
   Revocation List (CRL) Profile, which is published in RFC 3280.  The
   use of UTF8String and PrintableString are the preferred encoding.
   The requirement for exclusive use of UTF8String after December 31,
   2003 is removed.

   1. Introduction ....................................................2
   2. Terminology .....................................................2
   3. Update to RFC 3280, Section 4.1.2.4: Issuer .....................2
   4. Update to RFC 3280, Section 4.1.2.6: Subject ....................3
   5. Update to RFC 3280, Section 4.2.1.7: Subject
      Alternative Name ................................................4
   6. Security Considerations .........................................4
   7. Normative References ............................................5

1. Introduction

   At the time that RFC 3280 [PKIX1] was published, it was very unclear
   how international character sets ought to be supported.
   Implementation experience and deployment experience have made the
   picture much less fuzzy.  This update to RFC 3280 aligns the document
   with this experience and the direction of the IETF PKIX Working
   Group.

   The use of UTF8String and PrintableString are the preferred encoding.
   UTF8String provides support for international character sets, and
   PrintableString preserves support for the vast bulk of the
   certificates that have already been deployed.

2. Terminology

   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 RFC 2119 [STDWORDS].

3.  Update to RFC 3280, Section 4.1.2.4: Issuer1>

   In Section 4.1.2.4, RFC 3280 says:

      The DirectoryString type is defined as a choice of
      PrintableString, TeletexString, BMPString, UTF8String, and
      UniversalString.  The UTF8String encoding [RFC 2279] is the
      preferred encoding, and all certificates issued after December 31,
      2003 MUST use the UTF8String encoding of DirectoryString (except
      as noted below).  Until that date, conforming CAs MUST choose from
      the following options when creating a distinguished name,
      including their own:

         (a)  if the character set is sufficient, the string MAY be
              represented as a PrintableString;

         (b)  failing (a), if the BMPString character set is sufficient
              the string MAY be represented as a BMPString; and

         (c)  failing (a) and (b), the string MUST be represented as a
              UTF8String.  If (a) or (b) is satisfied, the CA MAY still
              choose to represent the string as a UTF8String.

Housley & Santesson         Standards Track                     [Page 2]

   Exceptions to the December 31, 2003 UTF8 encoding requirements
   are as follows:

         (a)  CAs MAY issue "name rollover" certificates to support an
              orderly migration to UTF8String encoding.  Such
              certificates would include the CA's UTF8String encoded
              name as issuer and the old name encoding as subject,
              or vice-versa.

         (b)  As stated in section 4.1.2.6, the subject field MUST be
              populated with a non-empty distinguished name matching the
              contents of the issuer field in all certificates issued by
              the subject CA regardless of encoding.

      The TeletexString and UniversalString are included for backward
      compatibility, and SHOULD NOT be used for certificates for new
      subjects.  However, these types MAY be used in certificates where
      the name was previously established.  Certificate users SHOULD be
      prepared to receive certificates with these types.

      In addition, many legacy implementations support names encoded in
      the ISO 8859-1 character set (Latin1String) [ISO 8859-1] but tag
      them as TeletexString.  TeletexString encodes a larger character
      set than ISO 8859-1, but it encodes some characters differently.
      Implementations SHOULD be prepared to handle both encodings.

   This block of text is replaced with the following:

      The DirectoryString type is defined as a choice of
      PrintableString, TeletexString, BMPString, UTF8String, and
      UniversalString.  CAs conforming to this profile MUST use either
      the PrintableString or UTF8String encoding of DirectoryString,
      with one exception.  When CAs have previously issued certificates
      with issuer fields with attributes encoded using the
      TeletexString, BMPString, or UniversalString, the CA MAY continue
      to use these encodings of the DirectoryString to preserve the
      backward compatibility.

4.  Update to RFC 3280, Section 4.1.2.6: Subject1>

   In Section 4.1.2.6, RFC 3280 says:

      The subject name field is defined as the X.501 type Name.
      Implementation requirements for this field are those defined for
      the issuer field (section 4.1.2.4).  When encoding attribute
      values of type DirectoryString, the encoding rules for the issuer
      field MUST be implemented.

Housley & Santesson         Standards Track                     [Page 3]

   This block of text is replaced with the following:

      The subject name field is defined as the X.501 type Name.
      Implementation requirements for this field are those defined for
      the issuer field (Section 4.1.2.4).  CAs conforming to this
      profile MUST use either the PrintableString or UTF8String encoding
      of DirectoryString, with one exception.  When CAs have previously
      issued certificates with subject fields with attributes encoded
      using the TeletexString, BMPString, or UniversalString, the CA MAY
      continue to use these encodings of the DirectoryString in new
      certificates for the same subject to preserve backward
      compatibility.

      Since name comparison assumes that attribute values encoded in
      different types (e.g., PrintableString and UTF8String) are assumed
      to represent different strings, any name components that appear in
      both the subject field and the issuer field SHOULD use the same
      encoding throughout the certification path.

5.  Update to RFC 3280, Section 4.2.1.7: Subject Alternative Name1>

   In Section 4.2.1.7, RFC 3280 says:

      When the subjectAltName extension contains a DN in the
      directoryName, the DN MUST be unique for each subject entity
      certified by the one CA as defined by the issuer name field.  A CA
      MAY issue more than one certificate with the same DN to the same
      subject entity.

   This block of text is replaced with the following:

      When the subjectAltName extension contains a DN in the
      directoryName, the encoding preference is defined in Section
      4.1.2.4.  The DN MUST be unique for each subject entity certified
      by the one CA as defined by the issuer name field.  A CA MAY issue
      more than one certificate with the same DN to the same subject
      entity.

6. Security Considerations

   The use of consistent encoding for name components will ensure that
   the name constraints specified in [PKIX1] work as expected.

   When strings are mapped from internal representations to visual
   representations, sometimes two different strings will have the same
   or similar visual representations.  This can happen for many
   different reasons, including the use of similar glyphs and use of
   composed characters (such as e + ' equaling U+00E9, the Korean

   composed characters, and vowels above consonant clusters in certain
   languages).  As a result of this situation, people doing visual
   comparisons between to different names may think they are the same
   when in fact they are not.  Also, people may mistake one string for
   another.  Issuers of certificates and relying parties both need to be
   aware of this situation.

7. Normative References

   [PKIX1]     Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
               X.509 Public Key Infrastructure Certificate and
               Certificate Revocation List (CRL) Profile", RFC 3280,
               April 2002.

   [STDWORDS]  Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

Authors' Addresses

   Russell Housley
   Vigil Security, LLC
   918 Spring Knoll Drive
   Herndon, VA 20170
   USA

   EMail: housley@vigilsec.com

   Stefan Santesson
   Microsoft
   Tuborg Boulevard 12
   2900 Hellerup
   Denmark

   EMail: stefans@microsoft.com

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