Other Certificates Extension
draft-farrell-pkix-other-certs-01

Status of this Memo

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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 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

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 certficate. 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, the detailed 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.

CAs MUST however 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.

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].

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

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

While it should go without saying, relying parties MUST validdate 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. Some warnings for CAs and applications needed.

7.  References

7.1. Normative References

7.2. Informative References

Appendix A.  ASN.1 Module

TBD

Author's Address

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