RFC4630

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.

5. Update to RFC 3280, Section 4.2.1.7: Subject

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.

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].

Update to RFC 3280, Section 4.1.2.4: Issuer

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 [[[RFC2279|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.

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.

Update to RFC 3280, Section 4.1.2.6: Subject

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.

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.

Update to RFC 3280, Section 4.2.1.7: Subject Alternative Name

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.

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.

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: [email protected]

Stefan Santesson Microsoft Tuborg Boulevard 12 2900 Hellerup Denmark

EMail: [email protected]

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