Internet DRAFT - draft-moses-webpki-trustmodel
draft-moses-webpki-trustmodel
Internet Engineering Task Force T. Moses, Ed.
Internet-Draft Entrust Ltd.
Intended status: BCP September 15, 2012
Expires: March 19, 2013
Trust models of the Web PKI
draft-moses-webpki-trustmodel-01
Abstract
This is one of a set of drafts that document the operation of the Web
PKI. It describes common variants of the 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
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This Internet-Draft will expire on March 19, 2013.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . . 3
1.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Basic trust model . . . . . . . . . . . . . . . . . . . . . . . 4
3. Trust model variants . . . . . . . . . . . . . . . . . . . . . 4
3.1. Certificate using product adopts root store . . . . . . . . 4
3.2. Certificate using product uses OS root store . . . . . . . 4
3.3. Certificate holder certificates issued by root CA . . . . . 4
3.4. One root CA cross-certifies another root CA . . . . . . . . 5
3.5. Issuing CA is an affiliate . . . . . . . . . . . . . . . . 5
3.6. Registration authority is an affiliate . . . . . . . . . . 5
3.7. Root CA is operated by a government . . . . . . . . . . . . 5
3.8. Certificate user directly trusts issuing CA key . . . . . . 6
3.9. Certificate user directly trusts certificate holder key . . 6
3.10. Certificate holder operates issuing CA . . . . . . . . . . 6
3.11. Certificate holder manages issuing CA . . . . . . . . . . . 6
3.12. Certificate holder manages RA . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
6. Normative References . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
1.1. Requirements Language
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].
1.2. Definitions
Certificate holder - A natural or legal person who is identified
as the subject in a certificate.
Certificate subject - The certificate holder as represented in the
certificate.
Certificate user - A natural person who operates a certificate
using product.
Certificate using product - A product that evaluates a certificate
or certificate chain and adjusts its behavior according to the
result.
Intermediate CA - A CA that issues certificates to issuing CAs
and/or other intermediate CAs.
Issuing CA - A CA that issues certificates to certificate holders.
Policy management authority - A natural or legal person who
administers the certificate policy by which one or more
certification authorities operate.
Root CA - A CA whose public key is included in a root store.
Root store - A set of certification authority public keys that is
embedded in a certificate using product and used in the
certificate evaluation procedure.
Trust model - The roles, and the relationships between those
roles, that are relevant to the management and evaluation of
certificates.
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2. Basic trust model
In the basic Web PKI trust model, a certificate-using product
includes a root store that contains one or more root certification
authority public keys, each of which is under the control of a
commercial CA and managed in conformance with the certificate policy
prescribed and administered by the certificate using product
supplier. Each such root certification authority issues a
certificate to one or more issuing CAs that are under the control of
the same commercial CA. Each issuing CA accepts and responds to
certificate requests from one or more certificate applicants via one
or more registration authorities that are under the control of the
same commercial CA. If the request is granted, then the certificate
applicant becomes a certificate holder. The role of the registration
authority is to confirm the accuracy of the information provided in
the certificate request.
The certificate user implicitly accepts the policy of the policy
management authority by choosing to use a particular certificate-
using product.
All functions of the commercial CA are subject to the audit process
prescribed by the certificate policy.
3. Trust model variants
There are several variants of the basic trust model in common use.
3.1. Certificate using product adopts root store
In this variant, the supplier of the certificate using product adopts
the policies of other suppliers by copying their root stores, without
seeking independent evidence of conformance.
3.2. Certificate using product uses OS root store
In this variant, the certificate using product does not use its own
root store. Instead, it uses the platform operating system root
store and certificate processing functions to evaluate the
certificate holder's certificate. It may then apply additional
checks, such as checking that the certificate subject's domain name
matches that requested by the certificate user.
3.3. Certificate holder certificates issued by root CA
Some legacy situations demand that the certificate holder certificate
be issued directly by the root CA, without the involvement of
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intermediate or issuing CAs. This model is now deprecated, but the
practice will remain in effect indefinitely.
3.4. One root CA cross-certifies another root CA
A small but significant portion of the certificate using products in
active use does not possess the capability to be updated in the
field. Consequently, these products do not accept certificates
issued by CAs that came into existence after they were first
deployed. Although their certificates are accepted by newer products
and ones that can be updated in the field, 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.
Because the cross-certified root CA is also recognized directly by a
policy management authority, it operates in accordance with the
requirements of that certificate policy, regardless of any
requirements placed upon it by the contract between it and the cross-
certifying root CA.
3.5. Issuing CA is an affiliate
The issuing CA may operate at arm's length to the root CA.
The issuing CA's behavior is governed by its contract with the root
CA, which commonly stipulates adherence to the policies of the policy
management authority.
3.6. Registration authority is an affiliate
The registration authority may operate at arm's length to the issuing
CA.
The registration authority's behavior is governed by its contract
with the issuing CA. The issuing CA commonly performs a subset of
the checks normally performed by a registration authority, such as
identifying unacceptable names.
3.7. Root CA is operated by a government
In the case where the root CA is operated by a government department,
the policy authority may relax the requirement for a fully-
independent third-party audit, relying instead upon an audit
conducted in accordance with the government's own internal audit
process.
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3.8. Certificate user directly trusts issuing CA key
The certificate using product may allow the certificate user to
designate a CA key as trusted, a priori, for the purpose of
evaluating certificate holder certificates.
3.9. Certificate user directly trusts certificate holder key
The certificate using product may allow the certificate user to
designate a certificate holder key as trusted, a priori.
3.10. Certificate holder operates issuing CA
A certificate holder may operate its own issuing CA. Typically, the
certificate holder 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 RFC 5280 [RFC5280] name constraints
certificate extension to limit the region of the name-space in which
the issuing CA can issue valid certificates.
3.11. Certificate holder manages issuing CA
A root CA may host an issuing CA on behalf of a certificate holder.
Typically, the certificate holder 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 certificate holder.
3.12. Certificate holder manages RA
An issuing CA may host a registration authority on behalf of a
certificate holder. Typically, the certificate holder is approved to
issue certificates only within a specific region of the name-space,
and this limitation is enforced by the host issuing CA. Examination
of the certificate chain would indicate that the registration
authority was owned and operated by the issuing CA.
4. IANA Considerations
This memo includes no request to IANA.
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5. Security Considerations
The trust models described here exhibit several vulnerabilities that
could adversely affect the reliability of the authentication they
provide. The first concerns the naming of certificate holders. The
second concerns controllability and observability of issued
certificates.
Certificate holder 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 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 2016.
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.
In the event of a compromise of a root CA, its key is blacklisted by
certificate using products by means of a software update. This has
the effect of invalidating every otherwise-valid certificate that
chains to that root, whether or not it was issued while the
compromise existed. This step would have a severe impact upon the CA
and its certificate holders; a step not likely to be taken without
very careful deliberation and (perhaps) hesitation.
6. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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
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(CRL) Profile", RFC 5280, May 2008.
Author's Address
Tim Moses (editor)
Entrust Ltd.
Phone: +1 613 270 3183
Email: tim.moses@entrust.com
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