Other Certificates Extension
draft-farrell-pkix-other-certs-02

Status of this Memo

By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress.”

The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt.

The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html.

This Internet-Draft will expire on October 6, 2008.

Abstract

Some applications that associate state information with public key certificates can benefit from a way to link together a set of certificates belonging to the same end entity that can safely be considered to be equivalent for the purposes of referencing that application state information. This memo defines a certificate extension that supports such linkage that can allow applications to establish the required linkage without introducing a new application protocol data unit.

Table of Contents

1.  Introduction
2.  A Use Case
3.  Other Certificates Extension
4.  Acknowledgements
5.  IANA Considerations
6.  Security Considerations
7.  References
    7.1.  Normative References
    7.2.  Informative References
Appendix A.  ASN.1 Module
§  Author's Address
§  Intellectual Property and Copyright Statements

1.  Introduction

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 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].

RFC 3280 (Housley, R., Polk, W., Ford, W., and D. Solo, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” April 2002.) [RFC3280] defines a profile for the use of public key certificates for Internet applications. If an application associates application state information with a public key certificate, then that association may be disrupted if the end entity changes its public key certificate. Such disruption can occur due to renewals or if the end entity changes its certificate issuer. Similarly, if the end entity is actually a distributed system, where each instance has a different private key, then the relying party has no way to assoicate the different public key certificates with the relevant application state information.

For example, assume a web browser retains state information (perhaps passwords) about a web site, indexed (possibly indirectly) via values contained in the web server's public key certificate (perhaps a DNS name). When the web server certificate expires, and a new certifiate is acquired (perhaps with a different DNS name), then the browser cannot safely map the new certificate to the relevant state information.

This memo defines a new public key certificate extension that supports such linkage.

Other than the issuer asserting that the set of certificates belong to the same end entity for use with the same application, the fine-detail of the semantics of the linkage of certifcates is not defined here, since that is a matter for application developers and the operators of certification authorities (CAs). In particular we do not define how a CA can validate that the same end entity is the holder of the various private keys, nor how the application should make use of this information. Nor do we define what kinds of state information may be shared.

2.  A Use Case

Public key certificates expire, typically about a year after they are created. Some applications might need to know that the same entity is the subject of this certificate and a previously used certificate.

For example, if a web server certificate expires, it could be useful for a web browser to know that the server currently presenting a certificate in a TLS (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.1,” April 2006.) [RFC4346] handshake represents the same web server that previously presented a certificate. This could be used for example to allow the browser to automatically fill in form fields for the server in question, even if the server certificate has been replaced. While the same effect can be achieved based on the use of the same issuer and subject fields in a certificate there could be security issues involved in such comparisons, e.g. if the subject name includes a DNS name and the ownership of that DNS domain has changed.

The use of the new extension provides a way for the CA to signal to the application that the same end entity is involved, regardless of name changes. The new extension could also allow the web site operator to more easily change CA when renewing its certificate.

3.  Other Certificates Extension

This section defines the syntax for the other certificates extension.

The new extension is simply a list of references to the linked certificates. The references make use of the SCVPCertID structure from the SCVP (Freeman, T., Housley, R., Malpani, A., Cooper, D., and W. Polk, “Server-Based Certificate Validation Protocol (SCVP),” December 2007.) [RFC5055] protocol which contains a hash over the relevant certificate and the certificate's issuer and serial number.

When this extension is present the CA is asserting that the same end entity is the subject of the relevant certificates.

This extension MUST NOT be marked critical.

id-ce-otherCerts OBJECT IDENTIFIER ::== { id-ce XXX }

OtherCertificates ::= SEQUENCE OF SCVPCertID

CAs MUST only issue certificates containing this extension where the links created are such that the relevant consumers of the certificates can safely make use of those links. This will typically be the case where the certificates are only used by a single application. CAs MUST NOT issue certificates that link to certificates issued for a different purpose, for example, a CA SHOULD NOT link a web server certificate to a VPN gateway certificate (unless those can be the same, which might occur for some embedded devices).

Applications MUST validate certificates according to the rules specified in RFC 3280 (Housley, R., Polk, W., Ford, W., and D. Solo, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” April 2002.) [RFC3280], and MUST NOT assume that because certficates are linked, that they are therefore valid. This means of course that both certificates must chain up to some local trust point(s).

If an application imposes further checks on certificate validity (e.g. as is done in RFC 2818 (Rescorla, E., “HTTP Over TLS,” May 2000.) [RFC2818] for web server certificates), then both certificates MUST be valid according to those application specific rules.

It is NOT REQUIRED that two linked certificates are both simultaneously valid. For example, an application can validate certificate1 and cache that information. When it is subsequently presented with certificate2 (linked back to certificate1) then, if it considers the cached information about certificate1 trustworthy, then it can validate certificate2, and use the linkage to associate certificate2 with the relevant application state information. (Just as it would have done had certificate1 been re-presented.) As a second example, if certificate1 is expired, but would otherwise be valid, then the linkage from certificate2 can also be used once certificate2 has been validated.

If the application checks certificate status for the certificates in question, and any of the certificates concerned has been revoked, then the linkage MUST NOT be used. [Is this needed?]

4.  Acknowledgements

The use case motivating this was contributed to the W3C web security context (WSC) working group by Tyler Close. See http://www.w3.org/2006/WSC/wiki/SafeWebFormEditor for details.

5.  IANA Considerations

This memo includes no request to IANA.

6.  Security Considerations

As stated above, relying parties MUST validate any certificates per the algorithm given in RFC 3280 (Housley, R., Polk, W., Ford, W., and D. Solo, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” April 2002.) [RFC3280] before making any use of those certificates.

Relying parties similarly MUST NOT assume that any other fields in the relevant certificates have common values. For example, linked certificates might have non-overlapping key usage extensions.

More TBD no doubt.

7.  References

7.1. Normative References

7.2. Informative References

Appendix A.  ASN.1 Module

TBD

Author's Address

Full Copyright Statement

Copyright © The IETF Trust (2008).

This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights.

This document and the information contained herein are provided on an “AS IS” basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org.