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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.

 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 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: stephen.farrell@cs.tcd.ie

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