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Versions: 00 01 02 03 04                                                
Internet Engineering Task Force                         I. Barreira, Ed.
Internet-Draft                                                    Izenpe
Intended status: Best Current Practice                    B. Morton, Ed.
Expires: April 24, 2014                                          Entrust
                                                        October 21, 2013


                      Trust models of the Web PKI
                    draft-ietf-wpkops-trustmodel-00

Abstract

   This is one of a set of documents to define the operation of the Web
   PKI.  It describes the currently deployed Web PKI trust model.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 24, 2014.

Copyright Notice

   Copyright (c) 2013 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
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   (http://trustee.ietf.org/license-info) in effect on the date of
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   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 Simplified BSD License.





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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Trust model . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Root store provider . . . . . . . . . . . . . . . . . . .   3
     2.2.  CA Infrastructure . . . . . . . . . . . . . . . . . . . .   4
       2.2.1.  Registration Authority  . . . . . . . . . . . . . . .   4
       2.2.2.  Certificate status  . . . . . . . . . . . . . . . . .   4
     2.3.  Subscriber  . . . . . . . . . . . . . . . . . . . . . . .   4
     2.4.  Browser . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Trust Model variants  . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Root store provider variations  . . . . . . . . . . . . .   5
       3.1.1.  Browser adopts root store . . . . . . . . . . . . . .   5
     3.2.  CA Infrastructure variations  . . . . . . . . . . . . . .   5
       3.2.1.  One root CA cross-certifies another root CA . . . . .   5
       3.2.2.  Issuing CA is a third party to the root CA  . . . . .   5
       3.2.3.  Registration authority is a third party to the
               issuing CA  . . . . . . . . . . . . . . . . . . . . .   6
       3.2.4.  Root CA is operated by the government . . . . . . . .   6
       3.2.5.  Subscriber operates issuing CA  . . . . . . . . . . .   6
       3.2.6.  Subscriber sources management of issuing CA . . . . .   6
       3.2.7.  Subscriber manages registration authority . . . . . .   6
       3.2.8.  Subscriber certificate issued by a root CA  . . . . .   7
     3.3.  Subscriber  . . . . . . . . . . . . . . . . . . . . . . .   7
       3.3.1.  Subscriber uses agent . . . . . . . . . . . . . . . .   7
     3.4.  Browser . . . . . . . . . . . . . . . . . . . . . . . . .   7
       3.4.1.  Browser directly trusts issuing CA key  . . . . . . .   7
       3.4.2.  Browser directly trusts subscriber entity key . . . .   7
       3.4.3.  Browser supports public key pinning . . . . . . . . .   7
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
     5.1.  HTTPS is optional . . . . . . . . . . . . . . . . . . . .   8
     5.2.  Naming of subscribers . . . . . . . . . . . . . . . . . .   8
     5.3.  Root CA compromise  . . . . . . . . . . . . . . . . . . .   8
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   This document defines the Web PKI trust model as it is currently
   implemented.  The trust model is to support communications between
   the subscriber and the browser.  This document does not address
   future changes to the implemented trust model.





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1.1.  Definitions

   The use of PKI terminology is used as defined in RFC 5280.
   Additional definitions are provided below for interpretation of this
   document.

      Certificate policy - per RFC 3647.

      Root CA - a CA with a self-signed certificate and whose public key
      is included as a trust anchor in a root store.

      Root certificate - typically a self-signed certificate that
      identifies the root CA.  The root certificate is a type of trust
      anchor.

      Root store - a set of root certificates which can be trusted by a
      browser.

      Root store policy - the governance policy provided by the root
      store provider.

      Subscriber - per RFC 3647.

      Subscriber agreement - per RFC 3647.

      Trust anchor - per RFC 5914.

2.  Trust model

   In the Web PKI trust model, a browser uses a root store that contains
   one or more root CA public keys.  Each entry in a browser's root
   store has been installed on an evaluation made by the browser vendor.
   Each such root CA issues a certificate to one or more issuing CAs
   that are under the control of the same CA entity.  Each issuing CA
   accepts and responds to certificate requests from one or more
   subscribers via one or more registration authorities.

2.1.  Root store provider

   A root store provider (e.g., Microsoft or Mozilla) determines a root
   store policy.  This policy must be met by a candidate root CA in
   order to be included in the root store.  The root store provider
   installs and manages root certificates in its operating system or
   browser to support certificate chain validation.  The root store
   provider establishes requirements for accepting a root certificate.
   These requirements may include legal agreements, security or audit
   reports by third parties or acceptance by another root store
   provider.



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   A root store provider requires the root CA to be subject to an annual
   compliance audit performed by a third party auditor.  The audit
   requirements are defined by the root store policy.  The audit is
   based on an accepted schema of the standards (e.g., WebTrust or
   ETSI).  A third party auditor generates an audit report which is
   provided to the root store provider.  If the audit report states the
   root CA did not comply with the auditing standards, then the root CA
   will be required to take corrective actions.  Once the corrective
   actions are completed, then an updated report is submitted to the
   root store provider.  If the status of the root CA is not acceptable
   to the root store provider, then the root CA certificates may be
   removed from the root store or the indications from the browser may
   change for certificates verified under that root CA.

2.2.  CA Infrastructure

   The CA infrastructure consists of a PKI hierarchy.  Each organization
   acting as a CA entity is represented by one or more self-signed
   certificates.  The self-signed certificate is called the root
   certificate of a root CA.  The root CAs sign certificates for
   subordinate issuing CAs.  The root CA may have subordinate
   intermediate CAs to manage groups of subordinate issuing CAs.  The CA
   entity manages root, intermediate, and issuing CAs and oversees
   operation of the certificate issuance and management system in
   accordance with a certificate policy.

2.2.1.  Registration Authority

   Each issuing CA operates a registration authority that authenticates
   requests for certificates in accordance with the certificate policy.

2.2.2.  Certificate status

   Each CA provides certificate status in the form of a certificate
   revocation list (CRL) and/or an online certificate status protocol
   (OCSP) response.  Updates and validity periods of the certificate
   status are provided in accordance with the certificate policy of the
   CA.  The location of the CRL is provided in the certificate CRL
   distribution point (CDP) OID and the location of the OCSP response is
   provided in the authority info access (AIA) OID of the issued
   certificate.

2.3.  Subscriber

   Each subscriber provides services through the browsers to relying
   parties.  The subscriber identifies the online location of its
   service using a domain name contained in a certificate.  The
   subscriber submits certificate requests in accordance with a CA's



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   certificate policy.  Once the certificate request has been accepted,
   the subscriber will receive the certificate and will manage the
   certificate in accordance with the subscriber agreement.

2.4.  Browser

   The browser accepts and manages certificates and performs related
   functions in accordance with the root store policy.

3.  Trust Model variants

   This section defines variants to the roles of the parties as defined
   in section 2.

3.1.  Root store provider variations

3.1.1.  Browser adopts root store

   The browser does not use its own root store, but uses the root store
   managed by a separate root store provider.

3.2.  CA Infrastructure variations

3.2.1.  One root CA cross-certifies another root CA

   Some browsers in active use do not possess the capability to be
   updated with new root certificates in the field.  Consequently, these
   products do not accept certificates issued by CAs that came into
   existence after they were first deployed; although the certificates
   of these CAs are accepted by newer products and ones that can be
   updated in the field.  As such newer CAs operate at a disadvantage to
   older CAs, and they commonly address this disadvantage by having
   their public key cross-certified by an older CA.

   As the cross-certified root CA is also recognized directly by the
   root store provider, it operates in accordance with the requirements
   of that certificate policy, in addition to any requirements placed
   upon it by the contract between it and the cross-certifying root CA.

3.2.2.  Issuing CA is a third party to the root CA

   An issuing CA may operate as a third party subordinate to the root
   CA.  The issuing CA's behavior is governed by its contract with the
   root CA, which commonly stipulates adherence to the root store
   policy.  Unlike the situation in section 3.2.1, the subordinate
   issuing CA is not recognized independently by any relationship with
   the root store provider.




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3.2.3.  Registration authority is a third party to the issuing CA

   A registration authority may operate as a third party to an issuing
   CA.  A registration authority's behavior is governed by its contract
   with the issuing CA, which commonly stipulates adherence to the root
   store policy to which the CA adheres.  A third party registration
   authority is not identified in a CA certificate.

3.2.4.  Root CA is operated by the government

   In the case where a root CA is operated by a government department, a
   root store provider may rely upon an audit conducted in accordance
   with the government's own internal audit process.

3.2.5.  Subscriber operates issuing CA

   A subscriber may operate its own issuing CA.  Typically, the
   subscriber is approved to issue certificates only within a specific
   region of the name-space, and this limitation is enforced by
   contract.  The root CA may use the name constraints certificate
   extension to limit the region of the name-space in which the issuing
   CA can issue valid certificates.  This is often referred to as an
   enterprise-based subordinate CA relationship.

3.2.6.  Subscriber sources management of issuing CA

   A root CA may host an issuing CA on behalf of a subscriber.
   Typically, the subscriber is approved to issue certificates only
   within a specific region of the name-space, and this limitation is
   enforced by the host root CA.  Examination of the certificate chain
   would indicate that the issuing CA was owned and operated by the
   subscriber.

   This may also be an enterprise-based CA relationship; however, the
   entity operating the CA (rather than the enterprise subscriber) has
   immediate control of the CA and physical possession of the CA private
   key.

3.2.7.  Subscriber manages registration authority

   A subscriber may manage a registration authority.  The subscriber is
   approved to issue certificates only within a specific region of the
   name-space, and this limitation is enforced by the issuing CA through
   technical or legal means.

   This is often referred to an enterprise-based registration authority
   relationship with the issuing CA.




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3.2.8.  Subscriber certificate issued by a root CA

   Some legacy situations demand that a certificate be issued directly
   by a root CA, without the involvement of intermediate issuing CAs.

3.3.  Subscriber

3.3.1.  Subscriber uses agent

   A subscriber may use a third party agent to manage its certificates.
   The third party will request certificates from a registration
   authority and manage the certificates in accordance with the
   subscriber agreement on the subscriber's behalf.

3.4.  Browser

3.4.1.  Browser directly trusts issuing CA key

   A browser may allow a relying party to designate a CA key as a trust
   anchor for the purpose of evaluating subscriber certificates.

3.4.2.  Browser directly trusts subscriber entity key

   A browser may allow a relying party to designate a subscriber's
   certificate as a trust anchor.

3.4.3.  Browser supports public key pinning

   A browser may limit the set of public keys used to verify a
   certificate containing a domain name.  Limitation can be done by
   including the set of accepted public keys in the browser or by
   respecting an HTTP header provided by the subscriber.

4.  IANA Considerations

   This memo includes no request to IANA.

5.  Security Considerations

   The trust models described here exhibit several vulnerabilities that
   could adversely affect the reliability of the authentication they
   provide.









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5.1.  HTTPS is optional

   The subscriber does not have to support HTTPS for the web site.  The
   subscriber may provide HTTPS in some cases and not in other cases.
   As such, the trust model is optional for each web site.  In the event
   of no HTTPS, the browser could more easily be attacked.  This attack
   can be mitigated by supporting HSTS in accordance with RFC 6797.
   HSTS allows the subscriber to declare to the browser that
   interactions shall only be done using HTTPS connections.

5.2.  Naming of subscribers

   Subscriber names with any of the following characteristics can be
   used in an impersonation attack.

   o  homographic name

   o  mixed-alphabet name

   o  name that contains a string termination character

   o  Internet non-unique name (e.g. an internal server name)

   With the exception of non-unique names, CAs in the Web PKI are
   required to screen out requests for certificates with any of these
   characteristics.  CAs are required to phase out the practice of
   issuing non-unique names by 2015.

   Technically, unless constrained by an upstream CA to issue
   certificates only in a specific region of the name-space, any CA in
   the Web PKI can issue an apparently legitimate certificate for any
   name, whether or not the legitimate holder of that name is aware of
   or approves the issuance.  Furthermore, the legitimate holder of that
   name may not discover that such a certificate has been issued.

5.3.  Root CA compromise

   In the event of a detected compromise of a root CA, its key is
   blacklisted by the root store provider by means of a software update.
   This has the effect of invalidating every certificate that is
   subordinate to that root CA, whether or not the certificate was
   issued while the compromise existed.  This step would have a severe
   impact upon the CA and its subscribers; this is a step not likely to
   be taken without very careful.

6.  References





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6.1.  Normative References

   [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.

   [RFC5914]  Housley, R., Ashmore, S., and C. Wallace, "Trust Anchor
              Format", RFC 5914, June 2010.

   [RFC6797]  Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
              Transport Security (HSTS)", RFC 6797, November 2012.

6.2.  Informative References

   [RFC3647]  Chokhani, S., Ford, W., Sabett, R., Merrill, C., and S.
              Wu, "Internet X.509 Public Key Infrastructure Certificate
              Policy and Certification Practices Framework", RFC 3647,
              November 2003.

Authors' Addresses

   Inigo Barreira (editor)
   Izenpe
   Beato Tomas de Zumarraga 71, 1.  01008 Vitoria-Gasteiz.  Spain

   Phone: +34 945067705
   Email: i-barreira@izenpe.net


   Bruce Morton (editor)
   Entrust
   1000 Innovation Drive.  Ottawa, Ontario.  Canada K2K 3E7

   Phone: +1 613 2703743
   Email: bruce.morton@entrust.com















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