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INFORMATIONAL
Network Working Group D. Pinkas
Request for Comments: 3379 Bull
Category: Informational R. Housley
RSA Laboratories
September 2002
Delegated Path Validation and Delegated Path Discovery
Protocol Requirements
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document specifies the requirements for Delegated Path
Validation (DPV) and Delegated Path Discovery (DPD) for Public Key
Certificates. It also specifies the requirements for DPV and DPD
policy management.
1. Introduction
This document specifies the requirements for Delegated Path
Validation (DPV) and Delegated Path Discovery (DPD) for Public Key
Certificates, using two main request/response pairs.
Delegated processing provides two primary services: DPV and DPD.
Some clients require a server to perform certification path
validation and have no need for data acquisition, while some other
clients require only path discovery in support of local path
validation.
The DPV request/response pair, can be used to fully delegate path
validation processing to an DPV server, according to a set of rules,
called a validation policy.
The DPD request/response pair can be used to obtain from a DPD server
all the information needed (e.g., the end-entity certificate, the CA
certificates, full CRLs, delta-CRLs, OCSP responses) to locally
validate a certificate. The DPD server uses a set of rules, called a
path discovery policy, to determine which information to return.
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A third request/response pair allows clients to obtain references for
the policies supported by a DPV or DPD server.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document (in uppercase, as shown) are to be interpreted as described
in [RFC2119].
2. Rationale and Benefits for DPV (Delegated Path Validation)
DPV allows a server to perform a real time certificate validation for
a validation time T, where T may be the current time or a time in the
recent past.
In order to validate a certificate, a chain of multiple certificates,
called a certification path, may be needed, comprising a certificate
of the public key owner (the end entity) signed by one CA, and zero
or more additional certificates of CAs signed by other CAs.
Offloading path validation to a server may be required by a client
that lacks the processing, and/or communication capabilities to fetch
the necessary certificates and revocation information, perform
certification path construction, and perform local path validation.
In constrained execution environments, such as telephones and PDAs,
memory and processing limitations may preclude local implementation
of complete, PKIX-compliant certification path validation [PKIX-1].
In applications where minimum latency is critical, delegating
validation to a trusted server can offer significant advantages. The
time required to send the target certificate to the validation
server, receive the response, and authenticate the response, can be
considerably less than the time required for the client to perform
certification path discovery and validation. Even if a certification
path were readily available to the client, the processing time
associated with signature verification for each certificate in the
path might (especially when validating very long paths or using a
limited processor) be greater than the delay associated with use of a
validation server.
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Another motivation for offloading path validation is that it allows
validation against management-defined validation policies in a
consistent fashion across an enterprise. Clients that are able to do
their own path validation may rely on a trusted server to do path
validation if centralized management of validation policies is
needed, or the clients rely on a trusted server to maintain
centralized records of such activities.
When a client uses this service, it inherently trusts the server as
much as it would its own path validation software (if it contained
such software). Clients can direct the server to perform path
validation in accordance with a particular validation policy.
3. Rationale and Benefits for DPD (Delegated Path Discovery)
DPD is valuable for clients that do much of the PKI processing
themselves and simply want a server to collect information for them.
The server is trusted to return the most current information that is
available to it (which may not be the most current information that
has been issued). The client will ultimately perform certification
path validation.
A client that performs path validation for itself may get benefit in
several ways from using a server to acquire certificates, CRLs, and
OCSP responses [OCSP] as inputs to the validation process. In this
context, the client is relying on the server to interact with
repositories to acquire the data that the client would otherwise have
to acquire using LDAP, HTTP, FTP [LDAP, FTP&HTTP] or another
repository access protocol. Since these data items are digitally
signed, the client need not trust the server any more than the client
would trust the repositories.
DPD provides several benefits. For example, a single query to a
server can replace multiple repository queries, and caching by the
server can reduce latency. Another benefit to the client system is
that it need not incorporate a diverse set of software to interact
with various forms of repositories, perhaps via different protocols,
nor to perform the graph processing necessary to discover
certification paths, separate from making the queries to acquire path
validation data.
4. Delegated Path Validation Protocol Requirements
4.1. Basic Protocol
The Delegated Path Validation (DPV) protocol allows a server to
validate one or more public key certificates on behalf of a client
according to a validation policy.
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If the DPV server does not support the client requested validation
policy, then the DPV server MUST return an error.
If the DPV request does not specify a validation policy, the server
response MUST indicate the validation policy that was used.
Policy definitions can be quite long and complex, and some policies
may allow for the setting of a few parameters (such as root self-
signed certificates). The protocol MUST allow the client to include
these policy dependent parameters in the DPV request; however, it is
expected that most clients will simply reference a validation policy
for a given application or accept the DPV server's default validation
policy.
The client can request that the server determines the certificate
validity at a time other than the current time. The DPV server MUST
obtain revocation status information for the validation time in the
client request.
In order to obtain the revocation status information of any
certificate from the certification path, the DPV server might use, in
accordance with the validation policy, different sources of
revocation information. For example, a combination of OCSP
responses, CRLs, and delta CRLs could be used. Alternatively, a
response from another DPV server could be used.
If the revocation status information for the requested validation
time is unavailable, then the DPV server MUST return a status
indicating that the certificate is invalid. Additional information
about the reason for invalidity MAY also be provided.
The certificate to be validated MUST either be directly provided in
the request or unambiguously referenced, such as the CA distinguished
name, certificate serial number, and the hash of the certificate,
like ESSCertID as defined in [ESS] or OtherSigningCertificate as
defined in [ES-F].
The DPV client MUST be able to provide to the validation server,
associated with each certificate to be validated, useful
certificates, as well as useful revocation information. Revocation
information includes OCSP responses, CRLs, and delta CRLs. As an
example, an S/MIME message might include such information, and the
client can simply copy that information into the DPV request.
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The DPV server MUST have the certificate to be validated. When the
certificate is not provided in the request, the server MUST obtain
the certificate and then verify that the certificate is indeed the
one being unambiguous referenced by the client. The DPV server MUST
include either the certificate or an unambiguous reference to the
certificate (in case of a CA key compromise) in the DPV response.
The DPV response MUST indicate one of the following status
alternatives:
1) the certificate is valid according to the validation policy.
2) the certificate is not valid according to the validation policy.
3) the validity of the certificate is unknown according to the
validation policy.
4) the validity could not be determined due to an error.
When the certificate is not valid according to the validation policy,
then the reason MUST also be indicated. Invalidity reasons include:
a) the DPV server cannot determine the validity of the certificate
because a certification path cannot be constructed.
b) the DPV server successfully constructed a certification path, but
it was not valid according to the validation algorithm in
[PKIX-1].
c) the certificate is not valid at this time. If another request
could be made later on, the certificate could possibly be
determined as valid. This condition may occur before a
certificate validity period has begun or while a certificate is
suspended.
The protocol MUST prevent replay attacks, and the replay prevention
mechanism employed by the protocol MUST NOT rely on synchronized
clocks.
The DPV request MUST allow the client to request that the server
include in its response additional information which will allow
relying parties not trusting the DPV server to be confident that the
certificate validation has correctly been performed. Such
information may (not necessarily exclusively) consist of a
certification path, revocation status information from authorized CRL
issuers or authorized OCSP responders, revocation status information
from CRL issuers or OCSP responders trusted under the validation
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policy, time-stamp tokens from TSAs responders trusted under the
validation policy, or a DPV response from a DPV server that is
trusted under the validation policy. When the certificate is valid
according to the validation policy, the server MUST, upon request,
include that information in the response. However, the server MAY
omit that information when the certificate is invalid or when it
cannot determine the validity.
The DPV server MUST be able, upon request, copy a text field provided
by the client into the DPV response. As an example, this field may
relate to the nature or reason for the DPV query.
The DPV response MUST be bound to the DPV request so that the client
can be sure that all the parameters from the request have been taken
into consideration by the DPV server to build the response. This can
be accomplished by including a one-way hash of the request in the
response.
In some environments it may be necessary to present only a DPV
response to another relying party without the corresponding request.
In this case the response MUST be self contained. This can be
accomplished by repeating only the important components from the
request in the response.
For the client to be confident that the certificate validation was
handled by the expected DPV server, the DPV response MUST be
authenticated, unless an error is reported (such as a badly formatted
request or unknown validation policy).
For the client to be able prove to a third party that trusts the same
DPV server that the certificate validation was handled correctly, the
DPV response MUST be digitally signed, unless an error is reported.
The DPV server's certificate MUST authenticate the DPV server.
The DPV server MAY require client authentication, therefore, the DPV
request MUST be able to be authenticated.
When the DPV request is authenticated, the client SHOULD be able to
include a client identifier in the request for the DPV server to copy
into the response. Mechanisms for matching this identifier with the
authenticated identity depends on local DPV server conditions and/or
the validation policy. The DPV server MAY choose to blindly copy the
identifier, omit the identifier, or return an error response.
There are no specific confidentiality requirements within this
application layer protocol. However, when confidentiality is needed,
it can be achieved with a lower-layer security protocol.
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4.2. Relaying, Re-direction and Multicasting
In some network environments, especially ones that include firewalls,
a DPV server might not be able to obtain all of the information that
it needs to process a request. However, the DPV server might be
configured to use the services of one or more other DPV servers to
fulfill all requests. In such cases, the client is unaware that the
queried DPV server is using the services of other DPV servers, and
the client-queried DPV server acts as a DPV client to another DPV
server. Unlike the original client, the DPV server is expected to
have moderate computing and memory resources, enabling the use of
relay, re-direct or multicasting mechanisms. The requirements in
this section support DPV server-to-DPV server exchanges without
imposing them on DPV client-to-DPV server exchanges.
Protocols designed to satisfy these requirements MAY include optional
fields and/or extensions to support relaying, re-direction or
multicasting. However, DPV clients are not expected to support
relay, re-direct or multicast. If the protocol supports such
features, the protocol MUST include provisions for DPV clients and
DPV servers that do not support such features, allowing them to
conform to the basic set of requirements.
- When a server supports a relay mechanism, a mechanism to detect
loops or repetition MUST be provided.
- When a protocol provides the capability for a DPV server to re-
direct a request to another DPV server (that is, the protocol
chooses to provide a referral mechanism), a mechanism to provide
information to be used for the re-direction SHOULD be supported.
If such re-direction information is sent back to clients, then the
protocol MUST allow conforming clients to ignore it.
- Optional parameters in the protocol request and/or response MAY be
provide support for relaying, re-direction or multicasting. DPV
clients that ignore any such optional parameters MUST be able to
use the DPV service. DPV servers that ignore any such optional
parameters MUST still be able to offer the DPV service, although
they might not be able to overcome the limitations imposed by the
network topology. In this way, protocol implementers do not need
to understand the syntax or semantics of any such optional
parameters.
5. Delegated Path Discovery Protocol Requirements
The Delegated Path Discovery (DPD) protocol allows the client to use
a single request to collect at one time from a single server the data
elements available at the current time that might be collected using
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different protocols (such as LDAP, HTTP, FTP, or OCSP) or by querying
multiple servers, to locally validate a public key certificate
according to a single path discovery policy. The returned
information can be used to locally validate one or more certificates
for the current time.
Clients MUST be able to specify whether they want, in addition to the
certification path, the revocation information associated with the
path, for the end-entity certificate, for the CA certificates, or for
both.
If the DPD server does not support the client requested path
discovery policy, the DPD server MUST return an error. Some forms of
path discovery policy can be simple. In that case it is acceptable
to pass the parameters from the path discovery policy with each
individual request. For example, the client might provide a set of
trust anchors and separate revocation status conditions for the end-
entity certificate and for the other certificates. The DPD request
MUST allow more elaborated path discovery policies to be referenced.
However, it is expected that most of the time clients will only be
aware of the referenced path discovery policy for a given
application.
The DPD server response includes zero, one, or several certification
paths. Each path consists of a sequence of certificates, starting
with the certificate to be validated and ending with a trust anchor.
If the trust anchor is a self-signed certificate, that self-signed
certificate MUST NOT be included. In addition, if requested, the
revocation information associated with each certificate in the path
MUST also be returned.
By default, the DPD server MUST return a single certification path
for each end-entity certificate in the DPD request. However, the
returned path may need to match some additional local criteria known
only to the client. For example, the client might require the
presence of a particular certificate extension or a particular name
form. Therefore, the DPD client MUST have a means of obtaining more
than one certification path for each end-entity certificate in the
DPD request. At the same time, the mechanism for obtaining
additional certification paths MUST NOT impose protocol state on the
DPD server. Avoiding the maintenance of state information associated
with previous requests minimizes potential denial of service attacks
and other problems associated with server crashes.
Path discovery MUST be performed according to the path discovery
policy. The DPD response MUST indicate one of the following status
alternatives:
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1) one or more certification paths was found according to the path
discovery policy, with all of the requested revocation information
present.
2) one or more certification paths was found according to the path
discovery policy, with a subset of the requested revocation
information present.
3) one or more certification paths was found according to the path
discovery policy, with none of the requested revocation
information present.
4) no certification path was found according to the path discovery
policy.
5) path construction could not be performed due to an error.
When no errors are detected, the information that is returned
consists of one or more certification paths and, if requested, its
associated revocation status information for each certificate in the
path.
For the client to be confident that all of the elements from the
response originate from the expected DPD server, an authenticated
response MAY be required. For example, the server might sign the
response or data authentication might also be achieved using a
lower-layer security protocol.
The DPD server MAY require client authentication, allowing the DPD
request MUST to be authenticated.
There are no specific confidentiality requirement within the
application layer protocol. However, when confidentiality is needed,
it can be achieved with a lower-layer security protocol.
6. DPV and DPD Policy Query
Using a separate request/response pair, the DPV or DPD client MUST be
able to obtain references for the default policy or for all of the
policies supported by the server. The response can include
references to previously defined policies or to a priori known
policies.
7. Validation Policy
A validation policy is a set of rules against which the validation of
the certificate is performed.
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A validation policy MAY include several trust anchors. A trust
anchor is defined as one public key, a CA name, and a validity time
interval; a trust anchor optionally includes additional constraints.
The use of a self-signed certificate is one way to specify the public
key to be used, the issuer name, and the validity period of the
public key.
Additional constraints for each trust anchor MAY be defined. These
constraints might include a set of certification policy constraints
or a set of naming constraints. These constraints MAY also be
included in self-signed certificates.
Additional conditions that apply to the certificates in the path MAY
also be specified in the validation policy. For example, specific
values could be provided for the inputs to the certification path
validation algorithm in [PKIX-1], such as user-initial-policy-set,
initial-policy-mapping-inhibit, initial-explicit-policy, or initial-
any-policy-inhibit.
Additional conditions that apply to the end-entity certificate MAY
also be specified in the validation policy. For example, a specific
name form might be required.
In order to succeed, one valid certification path (none of the
certificates in the path are expired or revoked) MUST be found
between an end-entity certificate and a trust anchor and all
constraints that apply to the certification path MUST be verified.
7.1. Components for a Validation Policy
A validation policy is built from three components:
1. Certification path requirements,
2. Revocation requirements, and
3. End-entity certificate specific requirements.
Note: [ES-P] defines ASN.1 data elements that may be useful while
defining the components of a validation policy.
7.2. Certificate Path Requirements
The path requirements identify a sequence of trust anchors used to
start certification path processing and initial conditions for
certification path validation as defined in [PKIX-1].
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7.3. Revocation Requirements
Revocation information might be obtained through CRLs, delta CRLs or
OCSP responses. Certificate revocation requirements are specified in
terms of checks required on the end-entity certificate and CA
certificates.
Revocation requirements for the end-entity certificate may not be the
same as the requirements for the CA certificates. For example, an
OCSP response may be needed for the end-entity certificate while CRLs
may be sufficient for the CA certificates.
The validation policy MUST specify the source of revocation
information:
- full CRLs (or full Authority Revocation Lists) have to be
collected.
- OCSP responses, using [OCSP], have to be collected.
- delta CRLs and the relevant associated full CRLs (or full Authority
Revocation Lists) are to be collected.
- any available revocation information has to be collected.
- no revocation information need be collected.
7.4. End-entity Certificate Specific Requirements
The validation policy might require the end-entity certificate to
contain specific extensions with specific types or values (it does
not matter whether they are critical or non-critical). For example,
the validation policy might require an end-entity certificate that
contains an electronic mail address (either in the rfc822 subject alt
name or in the emailAddress naming attribute in the subject name).
8. Path Discovery Policy
A path discovery policy is a set of rules against which the discovery
of a certification path is performed. A path discovery policy is a
subset of a validation policy. A path discovery policy MAY either be
a reference to a validation policy or contain only some major
elements from a validation policy, such as the trust anchors.
Since the DPD client is "PKI aware", it can locally apply additional
selection criteria to the certification paths returned by the server.
Thus, a simpler policy can be defined and used for path discovery.
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8.1. Components for a Path Discovery Policy
The path discovery policy includes certification path requirements,
revocation requirements, and end-entity certificate specific
requirements. These requirements are the same as those specified in
sections 7.2, 7.3, and 7.4, respectively.
9. Security Considerations
A DPV client must trust a DPV server to provide the correct answer.
However, this does not mean that all DPV clients will trust the same
DPV servers. While a positive answer might be sufficient for one DPV
client, that same positive answer will not necessarily convince
another DPV client.
Other clients may trust their own DPV servers, or they might perform
certification path validation themselves. DPV clients operating
under an organizational validation policy must ensure that each of
the DPV servers they trust is operating under that organizational
validation policy.
When no policy reference is present in the DPV request, the DPV
client ought to verify that the policy selected by the DPV server is
appropriate.
The revocation status information is obtained for the validation
time. In case of a digital signature, it is not necessarily
identical to the time when the private key was used. The validation
time ought to be adjusted by the DPV client to compensate for:
1) time for the end-entity to realize that its private key has been
or could possibly be compromised, and/or
2) time for the end-entity to report the key compromise, and/or
3) time for the revocation authority to process the revocation
request from the end-entity, and/or
4) time for the revocation authority to update and distribute the
revocation status information.
10. Acknowledgments
These requirements have been refined after some valuable inputs from
Trevor Freeman, Paul Hoffman, Ambarish Malpani, Mike Myers, Tim Polk,
and Peter Sylvester.
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11. References
11.1. Normative References
[PKIX-1] Housley, R., Ford, W., Polk, W. and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and CRL
Profile", RFC 3280, April 2002.
[OCSP] Myers, M., Ankney, R., Malpani, A., Galperin, S. and C.
Adams, "X.509 Internet Public Key Infrastructure Online
Certificate Status Protocol - OCSP", RFC 2560, June 1999.
11.2. Informative References
[ES-F] Pinkas, D., Ross, J. and N. Pope, "Electronic Signature
Formats for long term electronic signatures", RFC 3126,
September 2001.
[ES-P] Pinkas, D., Ross, J. and N. Pope, "Electronic Signature
Policies", RFC 3125, September 2001.
[ESS] Hoffman, P., "Enhanced Security Services for S/MIME", RFC
2634, June 1999.
[ISO-X509] ISO/IEC 9594-8/ITU-T Recommendation X.509, "Information
Technology - Open Systems Interconnection: The Directory:
Authentication Framework," 1997 edition.
[FTP&HTTP] Housley, R. and P. Hoffman, "Internet X.509 Public Key
Infrastructure. Operational Protocols: FTP and HTTP", RFC
2585, May 1999.
[LDAP] Boeyen, S., Howes, T. and P. Richard, "Internet X.509
Public Key Infrastructure Operational Protocols LDAPv2",
RFC 2559, April 1999.
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12. Authors' Addresses
Denis Pinkas
Bull
Rue Jean-Jaures - BP 68
78340 Les Clayes-sous-Bois
FRANCE
EMail: Denis.Pinkas@bull.net
Russell Housley
RSA Laboratories
918 Spring Knoll Drive
Herndon, VA 20170
USA
EMail: rhousley@rsasecurity.com
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13. Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
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Internet organizations, except as needed for the purpose of
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English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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