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.

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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

   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 }

   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.

   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

   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.

   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.

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: stephen.farrell@cs.tcd.ie