OCSP Algorithm Agility

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.

OCSP RFC 2560 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 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: The algorithm used to sign the certificate may differ from the subject key algorithm The properties of the OCSP responder certificate chain are frequently only known to the responder after the fact. In an extended PKI deployment, the task of verifying certificate status may be separated from the task(s) that make use of the certificate. 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 world PKI deployments and cannot be resolved through the information available from in-band signalling using the RFC 2560 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: 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. 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: A mechanism that allows a client to indicate the set of preferred signature algorithms. An algorithm for signature algorithm selection that maximizes the probability of successful operation in the case that no supported prefered algorithm(s) are specified.

A client MAY declare a preferred set of algorithms algorithms in a request using the preferred signature algorithm extension.

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.

RFC 2560 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 a supported signature algorithm using the following order of precedence where the first method has the highest precedence: Using an algorithm specified as a preferred signing algorithm in the client request. Using the signing algorithm used to sign the CertID specified in the query. Using the signing algorithm used to sign a CRL issued by the certificate issuer providing status information for the certificate specified by CertID. Using a signature algorithm that has been advertised as being the default signature algorithm for the signing service using an out of band mechanism 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.

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.

The author acknowleges the helpful comments made on earlier drafts of this work by Santosh Chokhani and Stefan Santesson

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.

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.

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.