Internet Engineering Task Force P. Hallam-Baker
Internet-Draft VeriSign Inc
Intended status: Informational July 2, 2008
Expires: January 3, 2009
OCSP Algorithm Agility
draft-hallambaker-ocspagility-01
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Abstract
The behavior of an OCSP server is specified for cases in which the
OCSP server is capable of supporting more than one signature
algorithm.
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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 . . . . . . . . . . . . 4
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
6.1. Use of insecure algorithms . . . . . . . . . . . . . . . . 5
6.2. Man in the Middle Downgrade Attack . . . . . . . . . . . . 6
7. Normative References . . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6
Intellectual Property and Copyright Statements . . . . . . . . . . 7
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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
protocol 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] requires the implementation but not the use of a
set of mandatory cryptographic algorithms. The OCSP protocol
provides a means for an OCSP responder to indicate the signature and
digest algorithms used in a response but not how that algorithm is to
be chosen or how the client might influence the choice of signature
algorithm.
In practice this approach has proved insufficient to ensure
interoperability of implementations, in particular when a transition
from the use of one cryptographic algorithm to another is in
progress. The mandatory cryptographic algorithms are the only ones
that a responder can expect a client to implement according to the
specification but this is a clearly unacceptable choice if a
different signing algorithm has been chosen for the certificate whose
status is being querried precisely because the mandatory algorithm is
not acceptably secure.
Although security concerns may lead to the use of a different signing
algorithm in a certificate there is little security advantage to be
used from using a stronger signing algorithm in an OCSP response than
in the certificate whose status is being reported. A possible
exception being in the case where a certificate issuer determines
that a certificate signing algorithm has become regarded as
unacceptably secure during the period of validity for a certificate
and thus requires a means of revoking the certificates that does not
depend on the compromised algorithm.
While use of a non-mandated signing algorithm is often motivated by
security concerns, this is not always the case: performance, key and
signature value sizes are also possible motivations.
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In most cases an OCSP reponder may assume that the client is capable
of verifying any signature algorithm that is used in either the
certificate whose status is being verified or the certificate of the
OCSP responder.
There are however circumstances in which it may be desirable to use a
different form of signature or demonstration of authenticity for the
OCSP response. In a wireless application, the use of a signature
algorithm that provides a compact signature value such as DSA or ECC
might be desirable.
The introduction of delegated certificate path discovery and
validation (e.g. XKMS, SCVP) gives rise to further exceptions. A
certificate path discovery service may be required to establish a
certificate path for a public key algorithm that the discovery
service itself does not support. In many embedded applications a
relying party application may use an OCSP service to verify the
current status of a certificate that has previously (but not
necessarily contemporaneously) been subject to path validation by
another component in the system.
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.
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.
A responder MAY maximize the potential for ensuring interoperability
by selecting the OCSP signature algorithm using the following order
of precedence where the first method has the highest precedence:
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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.
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
The security of the signing algorithm used by the responder MUST take
precedence over all other considerations. A responder MUST NOT
generate a signature for a signing mechanism that is considered
unacceptably insecure regardless of the other circumstances.
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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
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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