Network Working Group S. Farrell
Request for Comments: 5697 Trinity College Dublin
Category: Experimental November 2009
Other Certificates Extension
Abstract
Some applications that associate state information with public key
certificates can benefit from a way to link together a set of
certificates that belong to the same end entity and that can safely
be considered equivalent to one another for the purposes of
referencing that application-state information. This memo defines a
certificate extension that allows applications to establish the
required linkage without introducing a new application protocol data
unit.
Status of This Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
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the Trust Legal Provisions and are provided without warranty as
described in the BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. A Use Case . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Other Certificates Extension . . . . . . . . . . . . . . . . . 3
4. Another Approach Using Permanent Identifiers . . . . . . . . . 5
5. A Possible Optimisation . . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . . 7
Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
RFC 5280 [RFC5280] 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 (RP) has no way to
associate 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 certificate
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, allowing the certificate issuer to attest that
the end entity that holds the private key for the certificate in
question also holds other private keys corresponding to other
identified certificates.
Other than the issuer asserting that the set of certificates belongs
to the same end entity for use with the same application, the fine
detail of the semantics of the linkage of certificates 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
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make use of this information. Nor do we define what kinds of state
information may be shared.
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
is the subject of the current 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 Transport Layer Security (TLS) [RFC5246] 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 the CA when replacing 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 Server-Based Certificate Validation Protocol (SCVP)
[RFC5055], 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-pe-otherCerts OBJECT IDENTIFIER ::= { id-pe 19 }
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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). The purpose for which the certificate is intended may be
determined by certificate policy or other means (e.g., extended key
usage object identifiers) that are out of the scope of this
specification.
CAs MUST NOT issue certificates containing this extension unless they
have validated that the end entity is the holder of all of the
relevant private keys.
Applications MUST validate certificates according to the rules
specified in RFC 5280 [RFC5280] and MUST NOT assume that because
certificates 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 [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 be simultaneously
valid. For example, an application can validate certificate1 and
cache that information. When the application is subsequently
presented with certificate2 (linked back to certificate1), 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 has 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.
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Note that there are no constraints on the contents of the certificate
to which the link points. The consequence is that the CA issuing the
new certificate can link back to legacy certificates of all kinds,
once the relevant RP supports this extension.
This extension MUST only be used in end-entity certificates, that is,
it MUST NOT be used in CA certificates or other similar certificates.
Since CA certificates are only used for certificate validation and
this extension has no effect on the validation procedure, this
extension would generally be meaningless in a CA certificate. In
addition, it may be wise to gain some deployment experience with this
extension before using it for more security-sensitive certificates,
like CA certificates.
4. Another Approach Using Permanent Identifiers
RFC 4043 [RFC4043] defines a new name form (a "Permanent Identifier"
or PI) for public key certificates that supports similar
functionality to the new extension defined here. If two certificates
have the same PI and that PI form is globally unique, then the end
entities involved can be considered to be the same.
The main difference between the PI and the other certificates
extension is that (when more than one CA is involved) PI requires a
globally unique identifier, whereas the other certificates extension
only requires that the issuer of the new certificate be able to link
back to the old certificate(s).
As a consequence, the other certificates extension can be deployed
"reactively" to link certificates that may not match "ideal"
application-naming requirements. If the old certificate did make use
of PI, then presumably application-naming issues have already been
handled, and then the new certificate can contain the same PI. In
this latter case, there would be no need for the other certificates
extension.
5. A Possible Optimisation
The SCVPCertID structure used here contains the issuer name for the
CA of the linked certificate. It may happen that this issuer is also
the issuer of the certificate containing the other certificates
extension. If a new certificate were linked back to a number of old
certificates from that same CA, then there would be considerable
redundancy since there would be many copies of the same issuer name.
One suggestion raised was to have a convention where if the X.500
Name in the SCVPCertID is an "empty" DN (see RFC5280), then that
would indicate that the same CA issued both the current and the
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linked certificates. However, that scheme is not adopted in this
version. A future, Standards Track version of this specification
might adopt that optimisation.
6. 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.
Denis Pinkas pointed out that the PI extension is an alternative to
this one.
James Manger suggested the optimisation to reduce the number of
copies of the issuer name.
7. Security Considerations
As stated above, relying parties MUST validate any certificates per
the algorithm given in RFC 5280 [RFC5280] 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.
Since the issuer of the new certificate (or some superior CA) is
trusted by the RP, and the RP has validated the new certificate, the
RP is basically as reliant on the proper operation of that CA as
always -- if the CA wished to "cheat" on the RP, the other
certificates extension simply provides a new way to do that, but one
that is equivalent to existing vulnerabilities. In many cases, such
a bad CA could simply issue a new certificate that is identical in
all respects (other than the key pair) and the RP would accept the
identity contained in that new certificate.
However, if the issuer of the new certificate is limited in some way
(e.g., via a name constraint in a superior CA certificate), and if
the old certificate doesn't match those limitations (e.g., the
subject of the old certificate doesn't fit under the name constraints
of the issuer of the new certificate), then the new certificate could
be linked back to an identity that doesn't meet the constraints
intended to be imposed on the issuer of the new certificate.
Applications for which this is an unacceptable risk SHOULD NOT make
use of the other certificates extension.
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Since the SCVPCertID structure includes a hash of the other
certificate and hash algorithm weaknesses that produce collisions are
becoming more of an issue, CAs and relying parties MUST ensure that
currently acceptable hash functions are used. In particular, the
default use of SHA-1 for SCVPCertID may or may not currently be
considered acceptable. CAs might be wise to use SHA-256 instead, but
will typically use whatever hash function they use as part of
certificate signing.
In some application contexts, if the old certificate has expired (and
perhaps any associated certificate revocation list (CRL) entries are
no longer on the latest CRL), it may be unsafe to link the new and
old certificates. Application developers SHOULD carefully consider
whether to make use of the other certificates extension in such
contexts.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5055] Freeman, T., Housley, R., Malpani, A., Cooper, D., and W.
Polk, "Server-Based Certificate Validation Protocol
(SCVP)", RFC 5055, December 2007.
[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.
8.2. Informative References
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC4043] Pinkas, D. and T. Gindin, "Internet X.509 Public Key
Infrastructure Permanent Identifier", RFC 4043, May 2005.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
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Appendix A. ASN.1 Module
PKIX OID registrations may be viewed at:
http://www.imc.org/ietf-pkix/pkix-oid.asn
PKIXOtherCertsModule
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0) 44 }
DEFINITIONS EXPLICIT TAGS ::=
BEGIN
-- EXPORTS ALL
IMPORTS
-- From [RFC5055]
SCVPCertID
FROM SCVP { iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0) 21 } ;
-- The one and only new thing, a new certificate extension
id-pe-otherCerts OBJECT IDENTIFIER ::=
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-pe(1) 19 }
-- The value is a sequence of cert ids.
OtherCertificates ::= SEQUENCE OF SCVPCertID
END
Author's Address
Stephen Farrell
Trinity College Dublin
Department of Computer Science
Trinity College
Dublin, 2
Ireland
Phone: +353-1-896-2354
EMail:
[email protected]
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