Network Working Group P. Hallam-Baker
Internet-Draft VeriSign Inc
Intended status: Informational S. Santesson
Expires: February 13, 2010 3xA Security
August 12, 2009
OCSP Algorithm Agility
draft-ietf-pkix-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. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
7.1. Use of insecure algorithms . . . . . . . . . . . . . . . . 6
7.2. Man in the Middle Downgrade Attack . . . . . . . . . . . . 7
8. Referencess . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 9
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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. An OCSP Responder may or
may not be issued an OCSP Responder certificate by the CA that issued
the certificate whose status is being queried. An OCSP Responder may
provide pre-signed OCSP responses or may sign responses when queried.
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 interoperability.
While an OCSP Responder may apply rules such as using the signature
algorithm employed by the CA for signing CRLs and certificates, such
rules may fail in common situations such as:
o Algorithm used to sign the CRLs and certificates may not be
consistent with key pair being used by the OCSP Responder to sign
responses.
o A responder cannot draw any conclusions from a request for an
unknown certificate.
The last criterion cannot be resolved through the information
available from in-band signaling using the RFC 2560 [RFC2560]
protocol without modification.
In addition, an OCSP Responder may wish to employ different signature
algorithms than the the one used by the CA to sign certificates and
CRLs for several reasons:
o The responder may intentionally employ an algorithm for
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certificate status response that is less computationally demanding
than for signing the certificate itself.
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
preferred algorithm(s) are specified.
3. Client Indication of Preferred Signature Algorithms
A client MAY declare a preferred set of algorithms in a request using
the preferred signature algorithms extension.
id-pkix-ocsp-preferred-signature-algorithms OBJECT IDENTIFIER ::= {
id-pkix-ocsp x }
PreferredSignatureAlgorithms ::= SEQUENCE {
Algorithms SEQUENCE OF AlgorithmIdentifier
}
The client MUST support each of the specified preferred signature
algorithms.
The server SHOULD use one of the preferred signature algorithms for
signing OCSP responses to the requesting client.
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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 interoperability.
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, as long as the selected algorithm meets all
security requirements of the OCSP responder, 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 a CRL issued by the
certificate issuer providing status information for the
certificate specified by CertID.
3. Using the signing algorithm used to sign the CertID specified in
the query.
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 following the selection logic in 4.1.
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5. Acknowledgements
The author acknowledges the helpful comments made on earlier drafts
of this work by Santosh Chokhani and Stefan Santesson
6. IANA Considerations
This document requires no actions by IANA.
7. 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.
7.1. 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.
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7.2. 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 since the OCSP Responder MUST NOT sign OCSP
Responses using weak algorithms even if requested by the client. In
addition, the client can reject OCSP responses that do not meet its
own criteria for acceptable cryptographic security no matter what
mechanism is used to determine the signing algorithm of the response.
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8. Referencess
8.1. 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.
8.2. Informative References
[RFC2459] R. Housley, W. Ford, W. Polk, D. Solo, "Internet X.509
Public Key Infrastructure - Certificate and CRL Profile",
January 1999
Author's Address
Phillip Hallam-Baker
VeriSign Inc
Email: pbaker@verisign.com
Stefan Santesson
3xA Security AB
Sweden
Email: sts@aaa-sec.com
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