IETF S. Farrell
Internet-Draft Trinity College Dublin
Intended status: Informational February 23, 2008
Expires: August 26, 2008
Other Certificates Extension
draft-farrell-pkix-other-certs-01
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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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. A Use Case . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Other Certificates Extension . . . . . . . . . . . . . . . . . 4
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . . 5
Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 5
Intellectual Property and Copyright Statements . . . . . . . . . . 7
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1. Introduction
RFC 3280 [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 [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
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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 [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 [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.
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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 [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
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3280] 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.
[RFC5055] Freeman, T., Housley, R., Malpani, A., Cooper, D., and W.
Polk, "Server-Based Certificate Validation Protocol
(SCVP)", RFC 5055, December 2007.
7.2. Informative References
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
Appendix A. ASN.1 Module
TBD
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Author's Address
Stephen Farrell
Trinity College Dublin
Department of Computer Science
Trinity College
Dublin, 2
Ireand
Phone: +353-1-896-1761
Email: stephen.farrell@cs.tcd.ie
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