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rfc:rfc6818

Internet Engineering Task Force (IETF) P. Yee Request for Comments: 6818 AKAYLA Updates: 5280 January 2013 Category: Standards Track ISSN: 2070-1721

      Updates to the Internet X.509 Public Key Infrastructure
     Certificate and Certificate Revocation List (CRL) Profile

Abstract

 This document updates RFC 5280, the "Internet X.509 Public Key
 Infrastructure Certificate and Certificate Revocation List (CRL)
 Profile".  This document changes the set of acceptable encoding
 methods for the explicitText field of the user notice policy
 qualifier and clarifies the rules for converting internationalized
 domain name labels to ASCII.  This document also provides some
 clarifications on the use of self-signed certificates, trust anchors,
 and some updated security considerations.

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 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6818.

Yee Standards Track [Page 1] RFC 6818 RFC 5280 Clarifications January 2013

Copyright Notice

 Copyright (c) 2013 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
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1. Introduction ....................................................2
    1.1. Terminology ................................................3
 2. Update to RFC 5280, Section 3.2: "Certification Paths and Trust" 3
 3. Update to RFC 5280, Section 4.2.1.4: "Certificate Policies" .....3
 4. Update to RFC 5280, Section 6.2: "Using the Path Validation
    Algorithm" ......................................................4
 5. Update to RFC 5280, Section 7.3: "Internationalized
    Domain Names in Distinguished Names" ............................5
 6. Security Considerations .........................................5
 7. Update to RFC 5280, Section 11.1: "Normative References" ........7
 8. Update to RFC 5280, Section 11.2: "Informative References" ......7
 9. References ......................................................7
    9.1. Normative References .......................................7
    9.2. Informative References .....................................7
10. Acknowledgements ................................................8

1. Introduction

 This document updates the "Internet X.509 Public Key Infrastructure
 Certificate and Certificate Revocation List (CRL) Profile" [RFC5280].
 This document makes a recommendation that self-signed certificates
 used to convey trust anchor data be marked as certificate authority
 (CA) certificates, which is not always current practice.
 The use of self-signed certificates as trust anchors in Section 6.2
 of [RFC5280] is clarified.  While it is optional to use additional
 information in these certificates in the path validation process,
 [RFC5937] is noted as providing guidance in that regard.

Yee Standards Track [Page 2] RFC 6818 RFC 5280 Clarifications January 2013

 The acceptable and unacceptable encodings for the explicitText field
 of the user notice policy qualifier are updated to bring them in line
 with existing practice.
 The rules in Section 7.3 of [RFC5280] for ASCII encoding of
 Internationalized Domain Names (IDNs) as Distinguished Names are
 aligned with the rules in Section 7.2 of that document that govern
 IDN encoding as GeneralNames.
 In light of some observed attacks [Prins], the Security
 Considerations section now gives added depth to the consequences of
 CA key compromise.  This section additionally notes that collision
 resistance is not a required property of one-way hash functions when
 used to generate key identifiers.
 This document also adds normative and informative references for
 Trust Anchor formats and how they may be used to initialize the path
 validation inputs.  These are needed as a result of the changes made
 in Section 4 of this document.

1.1. 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 [RFC2119].

2. Update to RFC 5280, Section 3.2: "Certification Paths and Trust"

 Add the following paragraph to the end of RFC 5280, Section 3.2:

3. Update to RFC 5280, Section 4.2.1.4: "Certificate Policies"

 RFC 5280, Section 4.2.1.4, the tenth paragraph says:
An explicitText field includes the textual statement directly in
the certificate.  The explicitText field is a string with a
maximum size of 200 characters.  Conforming CAs SHOULD use the

| UTF8String encoding for explicitText, but MAY use IA5String.

Yee Standards Track [Page 3] RFC 6818 RFC 5280 Clarifications January 2013

the UTF8String encoding is used, all character sequences SHOULD be
normalized according to Unicode normalization form C (NFC) [NFC].
 This paragraph is replaced with:
An explicitText field includes the textual statement directly in
the certificate.  The explicitText field is a string with a
maximum size of 200 characters.  Conforming CAs SHOULD use the

4. Update to RFC 5280, Section 6.2: "Using the Path Validation

  Algorithm"
 RFC 5280, Section 6.2, the third paragraph says:
Where a CA distributes self-signed certificates to specify trust
anchor information, certificate extensions can be used to specify
recommended inputs to path validation.  For example, a policy
constraints extension could be included in the self-signed
certificate to indicate that paths beginning with this trust anchor
should be trusted only for the specified policies.  Similarly, a name
constraints extension could be included to indicate that paths
beginning with this trust anchor should be trusted only for the
specified name spaces.  The path validation algorithm presented in
Section 6.1 does not assume that trust anchor information is provided
in self-signed certificates and does not specify processing rules for

| additional information included in such certificates.

 This paragraph is replaced with:
Where a CA distributes self-signed certificates to specify trust
anchor information, certificate extensions can be used to specify
recommended inputs to path validation.  For example, a policy
constraints extension could be included in the self-signed
certificate to indicate that paths beginning with this trust anchor
should be trusted only for the specified policies.  Similarly, a name
constraints extension could be included to indicate that paths
beginning with this trust anchor should be trusted only for the
specified name spaces.  The path validation algorithm presented in

Yee Standards Track [Page 4] RFC 6818 RFC 5280 Clarifications January 2013

Section 6.1 does not assume that trust anchor information is provided

| in self-signed certificates and does not specify processing rules for

5. Update to RFC 5280, Section 7.3: "Internationalized Domain Names in

  Distinguished Names"
 RFC 5280, Section 7.3, the first paragraph says:
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

 This paragraph is replaced with:
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 with the UseSTD3ASCIIRules flag

6. Security Considerations

 This document modifies the Security Considerations section of RFC
 5280 as follows.  The fifth paragraph of the Security Considerations
 section of RFC 5280 says:

Yee Standards Track [Page 5] RFC 6818 RFC 5280 Clarifications January 2013

The protection afforded private keys is a critical security factor.
On a small scale, failure of users to protect their private keys will
permit an attacker to masquerade as them or decrypt their personal
information.  On a larger scale, compromise of a CA's private signing

| key may have a catastrophic effect. If an attacker obtains the

 This paragraph is replaced with:
The protection afforded private keys is a critical security factor.
On a small scale, failure of users to protect their private keys will
permit an attacker to masquerade as them or decrypt their personal
information.  On a larger scale, compromise of a CA's private signing
key may have a catastrophic effect.

|

Yee Standards Track [Page 6] RFC 6818 RFC 5280 Clarifications January 2013

 The following text is added to the end of the Security Considerations
 section of 5280:

7. Update to RFC 5280, Section 11.1: "Normative References"

 [RFC5914]   Housley, R., Ashmore, S., and C. Wallace, "Trust Anchor
             Format", RFC 5914, June 2010.

8. Update to RFC 5280, Section 11.2: "Informative References"

 [RFC5937]   Ashmore, S. and C. Wallace, "Using Trust Anchor
             Constraints during Certification Path Processing",
             RFC 5937, August 2010.

9. References

9.1. Normative References

 [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
 [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, May 2008.
 [RFC5914]   Housley, R., Ashmore, S., and C. Wallace, "Trust Anchor
             Format", RFC 5914, June 2010.
 [X.509]     ITU-T Recommendation X.509 (2008) | ISO/IEC 9594-8:2008,
             Information Technology - Open Systems Interconnection -
             The Directory: Public-key and attribute certificate
             frameworks.

9.2. Informative References

 [RFC5937]   Ashmore, S. and C. Wallace, "Using Trust Anchor
             Constraints during Certification Path Processing", RFC
             5937, August 2010.

Yee Standards Track [Page 7] RFC 6818 RFC 5280 Clarifications January 2013

 [Prins]     Prins, J. R., "DigiNotar Certificate Authority breach
             'Operation Black Tulip'", September 2011,
             <http://www.rijksoverheid.nl/bestanden/
             documenten-en-publicaties/rapporten/2011/
             09/05/diginotar-public-report-version-1/
             rapport-fox-it-operation-black-tulip-v1-0.pdf>.
 [NFC]       Davis, M. and M. Duerst, "Unicode Standard Annex #15:
             Unicode Normalization Forms", October 2006,
             <http://www.unicode.org/reports/tr15/>.

10. Acknowledgements

 David Cooper is acknowledged for his fine work in editing previous
 versions of this document.

Author's Address

 Peter E. Yee
 AKAYLA
 7150 Moorland Drive
 Clarksville, MD 21029
 USA
 EMail: peter@akayla.com

Yee Standards Track [Page 8]

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