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