Internet DRAFT - draft-ietf-wpkops-trustmodel
draft-ietf-wpkops-trustmodel
Internet Engineering Task Force I. Barreira, Ed.
Internet-Draft Izenpe
Intended status: Best Current Practice B. Morton, Ed.
Expires: October 31, 2015 Entrust
April 29, 2015
Trust models of the Web PKI
draft-ietf-wpkops-trustmodel-04
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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 31, 2015.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 3
2. Trust model . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Root store provider . . . . . . . . . . . . . . . . . . . 4
2.2. CA Infrastructure . . . . . . . . . . . . . . . . . . . . 4
2.2.1. Registration Authority . . . . . . . . . . . . . . . 5
2.2.2. Certificate status . . . . . . . . . . . . . . . . . 5
2.3. Subscriber . . . . . . . . . . . . . . . . . . . . . . . 5
2.4. Browser . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Trust Model variants . . . . . . . . . . . . . . . . . . . . 5
3.1. Root store provider variations . . . . . . . . . . . . . 6
3.1.1. Browser adopts root store . . . . . . . . . . . . . . 6
3.2. CA Infrastructure variations . . . . . . . . . . . . . . 6
3.2.1. One root CA cross-certifies another root CA . . . . . 6
3.2.2. Issuing CA is a third party to the root CA . . . . . 6
3.2.3. Registration authority is a third party to the
issuing CA . . . . . . . . . . . . . . . . . . . . . 7
3.2.4. Root CA is operated by the government . . . . . . . . 7
3.2.5. Subscriber operates issuing CA . . . . . . . . . . . 7
3.2.6. Subscriber sources management of issuing CA . . . . . 7
3.2.7. Subscriber manages registration authority . . . . . . 7
3.2.8. Subscriber certificate issued by a root CA . . . . . 8
3.3. Subscriber . . . . . . . . . . . . . . . . . . . . . . . 8
3.3.1. Subscriber uses agent . . . . . . . . . . . . . . . . 8
3.4. Browser . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.4.1. Browser directly trusts issuing CA key . . . . . . . 8
3.4.2. Browser directly trusts subscriber entity key . . . . 8
3.4.3. Browser makes root CA public key unusable . . . . . . 8
3.4.4. Browser supports public key pinning . . . . . . . . . 8
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
5.1. HTTPS is optional . . . . . . . . . . . . . . . . . . . . 9
5.2. automatic update of root certificates . . . . . . . . . . 9
5.3. Naming of subscribers . . . . . . . . . . . . . . . . . . 9
5.4. Root CA compromise . . . . . . . . . . . . . . . . . . . 10
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. IETF Normative References . . . . . . . . . . . . . . . . 10
6.2. IETF Informative References . . . . . . . . . . . . . . . 10
Appendix A. Other references . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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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. It refers also to the current
processing behaviours of cryptolibraries with the browsers they
support, related to the communication between the server and the
browser as indicated in the "Browser processing of server
certificates" document. This document does not address future
changes to the implemented trust model.
1.1. Requirements Language
The key words "REQUIRED", "MUST", "MUST NOT" and "MAY" in this
document are to be interpreted as described in RFC 2119 [RFC2119]
1.2. Definitions
The use of PKI terminology is used as defined in RFC 5280 [RFC5280].
Additional definitions are provided below for interpretation of this
document.
Certificate policy - per RFC 3647. [RFC3647]
Intermediate CA - is a non-root CA which issues certificates to
issuing CAs.
Issuing CA - in relation to a particular subscriber certificate,
the CA that issued the certificate.
Root CA - a CA with a self signed certificate and whose public key
is included as a trust anchor in a root certificates store.
Root certificate - typically a self-signed certificate that
identifies the root CA. The root certificate is a type of trust
anchor.
Root certificates store - a set of root certificates which can be
trusted by the operating system and/or a browser. Within the
context of the present document the more general term Root Store
is used in preference.
Root store policy - the governance policy provided by the root
store provider.
Subscriber - per RFC 3647. [RFC3647]
Subscriber agreement - per RFC 3647. [RFC3647]
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Trust Anchor - per RFC 5914. [RFC5914]
2. Trust model
This section describes the basic Web PKI trust model. Variants to
the trust model are discussed in section 3.
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 root CA issues a certificate to one or more issuing CAs that are
under the control of the same CA entity with the variant stated in
3.2.2. 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 or acceptance by another root store provider.
The root store provider may require the root CA to be subject to an
annual compliance audit performed by a third party auditor as
specified in [BR-certs]. 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 (e.g., removal of https indicator) 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 root
certificates. The root CAs sign certificates for subordinate issuing
CAs. The root CA may have subordinate intermediate CAs to manage
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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, with variations in
3.2.3 and 3.2.7, which authenticates requests for certificates in
accordance with the certificate policy of the CA.
2.2.2. Certificate status
Each CA provides certificate status in the form of a certificate
revocation list (CRL) and/or an on-line 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 CDP (CRL
Distribution Point) OID and the location of the OCSP response is
provided in the AIA (Authority Info Access) OID of the issued
certificate.
2.3. Subscriber
Each subscriber provides services through the browsers to relying
parties. The subscriber identifies the on-line web location of its
service using a domain name or IP address contained in a certificate.
The subscriber submits certificate requests in accordance with a CA's
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 (e.g., [Mozilla-
CP]).
3. Trust Model variants
This section defines variants to the roles of the parties as defined
in section 2.
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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. For example, the Google
Chrome browser operated on Windows uses the Windows root store.
The browser will provide its own trust and security indications. The
browser may determine whether it will provide additional validation
indications. The browser may also provide its own services to verify
the status of the certificates.
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
browsers do not accept new root certificates issued by CAs that came
into existence after they were first deployed. The new root
certificates are accepted by newer browsers and other browsers that
can be updated in the field. As such newer CAs operate at a
disadvantage to older CAs.
The disadvantage can be addressed by having trust extended to the new
root certificate (that can belong to the older CA or be another CA),
by having the public key of the new root certificate cross-signed by
an older root CA which is already accepted in the older browsers. 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 to which the root CA conforms. In addition,
the cross-certified CA complies to any requirements placed upon it by
the contract between it and the cross-certifying root CA.
This contract should indicate also the adherence to the root store
policy.
The [BR-certs] may be used as guidance for clarification.
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 behaviour 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 behaviour 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 legal
means or it can be also technically constrained. For example, 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 either technically or by legal means.
Examination of the certificate chain would indicate that the issuing
CA was owned by the subscriber by viewing the organization name in
the subject field.
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.
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This is often referred to an enterprise-based registration authority
relationship with the issuing CA.
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 makes root CA public key unusable
A browser may allow a relying party to remove the trust of a root CA
by deleting the root certificate from the root store. In some cases
the trust removal may only be temporary as the browser or operating
system may update the root store and restore the trust of the root
CA.
3.4.4. 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.
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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.
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
[RFC6797]. HSTS allows the subscriber to declare to the browser that
interactions shall only be done using HTTPS connections.
5.2. automatic update of root certificates
The end user may remove or add some or all root certificates provided
in a root store provider and then when an automatic update takes
place it may be reinstated the removed ones and remove the added ones
causing a posible denial of service and introducing some
vulnerabilities.
5.3. 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 per [BR-certs].
Technically, unless constrained by an upstream CA to issue
certificates only in a specific region of the name-space, any CA in
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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.
5.4. Root CA compromise
In the event of a detected compromise of a root CA, its key is
blacklisted 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 being very
careful.
6. References
6.1. IETF 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. IETF Informative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
Appendix A. Other references
[BR-certs] - CA/Browser Forum, Baseline Requirements for the
Issuance and Management of Publicly-Trusted Certificates.
https://cabforum.org/baseline-requirements-documents/
[Mozilla-CP] - Mozilla CA Certificate Policy.
https://www.mozilla.org/projects/security/certs/policy/
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Authors' Addresses
Inigo Barreira (editor)
Izenpe
Beato Tomas de Zumarraga 71, 1. 01008 Vitoria-Gasteiz. Spain
Email: i-barreira@izenpe.net
Bruce Morton (editor)
Entrust
1000 Innovation Drive. Ottawa, Ontario. Canada K2K 3E7
Email: bruce.morton@entrust.com
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