Internet DRAFT - draft-hallambaker-ocspagility
draft-hallambaker-ocspagility
Internet Engineering Task Force P. Hallam-Baker
Internet-Draft VeriSign Inc
Intended status: Informational December 2, 2008
Expires: June 5, 2009
OCSP Algorithm Agility
draft-hallambaker-ocspagility-02
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Abstract
The OSCP specification defined in RFC 2560 requires server responses
to be signed but does not specify a mechanism for selecting the
signature algorithm to be used leading to possible interoperability
failures in contexts where multiple signature algorithms are in use.
This document specifies an algorithm for server signature algorithm
selection and an extension that allows a client to advise a server
that specific signature algorithms are supported.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . . 3
2. OCSP Algorithm Agility Requirements . . . . . . . . . . . . . . 3
3. Client Indication of Preferred Signature Algorithms . . . . . . 4
4. Responder Signature Algorithm Selection . . . . . . . . . . . . 5
4.1. Dynamic Response . . . . . . . . . . . . . . . . . . . . . 5
4.2. Static Response . . . . . . . . . . . . . . . . . . . . . . 5
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
6.1. Use of insecure algorithms . . . . . . . . . . . . . . . . 6
6.2. Man in the Middle Downgrade Attack . . . . . . . . . . . . 6
7. Normative References . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . . . 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].
2. OCSP Algorithm Agility Requirements
OCSP RFC 2560 [RFC2560] defines a protocol for obtaining certificate
status information from an online service. A particular OCSP server
may or may not be provided by the CA that issued the certificate
whose status is being queried and may or may provide a realtime
indication of the certificate status or a time delayed status
indication.
RFC 2560 [RFC2560] specifies a means for an OCSP responder to
indicate the signature and digest algorithms used in a response but
not how those algorithms are specified. The only algorithm mandated
by the protocol specification is that the OCSP client SHALL support
the DSA sig-alg-oid specified in section 7.2.2 of [RFC2459] and
SHOULD be capable of processing RSA signatures as specified in
section 7.2.1 of [RFC2459]. The only requirement placed on
responders is that they SHALL support the SHA1 hashing algorithm.
This requirement is clearly insufficient to ensure interoperabilty.
While the responder may apply heuristics such as using the signature
algorithm employed by the certificate issuer, such heuristics fail in
many common real-world situations where multiple signature algorithms
are employed:
o The algorithm used to sign the certificate may differ from the
subject key algorithm
o The properties of the OCSP responder certificate chain are
frequently only known to the responder after the fact.
o In an extended PKI deployment, the task of verifying certificate
status may be separated from the task(s) that make use of the
certificate.
o A responder cannot infer anything if a request for an unknown
certificate is issued.
The last criterion is significant as it occurs frequently in real
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world PKI deployments and cannot be resolved through the information
available from in-band signalling using the RFC 2560 [RFC2560]
protocol without modification.
In addition, a system that employs a signature algorithm other than
the de-facto default is frequently doing so to achieve very specific
security properties that may not be captured by a heuristic
assumptuion designed to facilitate interoperability rather than
performance. In particular:
o An implementation may intentionally employ an algorithm for
certificate status response that is less computationally demanding
than for signing the certificate itself, thus allowing for more
frequent certificate status validation.
o An implementation may intentionally wish to guard against the
possibility of a compromise resulting from a signature algorithm
compromise by employing two separate encryption algorithms.
This document describes:
o A mechanism that allows a client to indicate the set of preferred
signature algorithms.
o An algorithm for signature algorithm selection that maximizes the
probability of successful operation in the case that no supported
prefered algorithm(s) are specified.
3. Client Indication of Preferred Signature Algorithms
A client MAY declare a preferred set of algorithms algorithms in a
request using the preferred signature algorithm extension.
id-pkix-ocsp-preferred-signature-algorithms OBJECT IDENTIFIER ::= {
id-pkix-ocsp x }
PreferredSignatureAlgorithms ::= SEQUENCE {
Algorithms SEQUENCE OF AlgorithmIdentifier
}
If a set of preferred signature algorithms is declared the client
MUST support each of the specified algorithms.
If a set of preferred algorithms is declared the OCSP responder
SHOULD use one of the specified signing algorithms.
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4. Responder Signature Algorithm Selection
RFC 2560 [RFC2560] does not specify a mechanism for deciding the
signature algorithm to be used in an OCSP response. As previously
noted this does not provide a sufficient degree of certainty as to
the algorithm selected to guarantee interoperation.
4.1. Dynamic Response
A responder MAY maximize the potential for ensuring interoperability
by selecting a supported signature algorithm using the following
order of precedence where the first method has the highest
precedence:
1. Using an algorithm specified as a preferred signing algorithm in
the client request.
2. Using the signing algorithm used to sign the CertID specified in
the query.
3. Using the signing algorithm used to sign a CRL issued by the
certificate issuer providing status information for the
certificate specified by CertID.
4. Using a signature algorithm that has been advertised as being the
default signature algorithm for the signing service using an out
of band mechanism
5. Using a mandatory signing algorithm specified for the version of
the OCSP protocol in use.
A responder SHOULD always apply the lowest numbered selection
mechanism that is known, supported and meets the responder's criteria
for cryptographic algorithm strength.
4.2. Static Response
For purposes of efficiency, an OCSP responder is permitted to
generate static responses in advance of a request. Although this
case does not permit the responder to make use of the client data
directly, the responder may anticipate the client request and
generate a set of signed responses so as to maximize the probability
that it is possible to generate a response that is assigned the
highest preference weighting.
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5. Acknowledgements
The author acknowleges the helpful comments made on earlier drafts of
this work by Santosh Chokhani and Stefan Santesson
6. Security Considerations
The mechanism used to choose the response signing algorithm MUST be
considered to be sufficiently secure against cryptanalytic attack for
the intended application.
In most applications it is sufficient for the signing algorithm to be
at least as secure as the signing algorithm used to sign the original
certificate whose status is being queried. This criteria may not
hold in long term archival applications however in which the status
of a certificate is being queried for a date in the distant past,
long after the signing algorithm has ceased being considered
trustworthy.
6.1. Use of insecure algorithms
It is not always possible for a responder to generate a response that
the client is expected to understand and meets contemporary standards
for cryptographic security. In such cases an application MUST
balance the risk of employing a compromised security solution and the
cost of mandating an upgrade, including the risk that the alternative
chosen by end users will offer even less security or no security.
In archival applications it is quite possible that an OCSP responder
might be asked to report the validity of a certificate on a date in
the distant past. Such a certificate might employ a signing method
that is no longer considered acceptably secure. In such
circumstances the responder MUST NOT generate a signature for a
signing mechanism that is considered unacceptably insecure.
A client MUST accept any signing algorithm in a response that it
specified as a preferred signing algorithm in the request. It
follows therefore that a client MUST NOT specify as a preferred
signing algorithm any signing algorithm that is either not supported
or not considered acceptably secure.
6.2. Man in the Middle Downgrade Attack
The mechanism to support client indication of preferred signature
algorithms is not protected against a man in the middle downgrade
attack. This constraint is not considered to be a significant
security concern as the client MUST NOT accept any signing algorithm
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that does not meet its own criteria for acceptable cryptographic
security no matter what mechanism is used to determine the signing
algorithm of the response.
7. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2560] 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.
Author's Address
Phillip Hallam-Baker
VeriSign Inc
Email: pbaker@verisign.com
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