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.
Table of Contents
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
Housley & Santesson Standards Track [Page 1]
RFC 4630 DirectoryString Update to RFC 3280 August 2006
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: 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 [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]
RFC 4630 DirectoryString Update to RFC 3280 August 2006
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: 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.
Housley & Santesson Standards Track [Page 3]
RFC 4630 DirectoryString Update to RFC 3280 August 2006
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 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.
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
Housley & Santesson Standards Track [Page 4]
RFC 4630 DirectoryString Update to RFC 3280 August 2006
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
Housley & Santesson Standards Track [Page 5]
RFC 4630 DirectoryString Update to RFC 3280 August 2006
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