OCSP Nonce Extension

This document updates the usage and format of the Nonce extension used in OCSP request and response messages. This extension was previously defined in section 4.4.1 of . does not mention any minimum and maximum length of nonce in the Nonce extension. Lacking limits on the length of nonce in the Nonce extension, an OCSP responders that follow may be vulnerable to various attacks like Denial of Service attacks , chosen prefix attacks to get a desired signature, and possible evasions using the Nonce extension data. This document specifies a lower limit of 1 and an upper limit of 32 to the length of nonce in the Nonce extension. This document updates .

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

The message format for OCSP request and response is defined in . also defines the standard extensions for OCSP messages based on the extension model employed in X.509 version 3 certificates (see ). This document only specifies the new format for Nonce extension and does not change specification of any of the other standard extensions defined in .

This section replaces the entirety of the Section 4.4.1 of which describes the OCSP Nonce extension. The nonce cryptographically binds a request and a response to prevent replay attacks. The nonce is included as one of the requestExtensions in requests, while in responses it would be included as one of the responseExtensions. In both the request and the response, the nonce will be identified by the object identifier id-pkix-ocsp-nonce, while the extnValue is the value of the nonce. If Nonce extension is present then the length of nonce MUST be at least 1 octet and can be up to 32 octets. A server MUST reject any OCSP request having a nonce in the Nonce extension with length of 0 octets or more than 32 octets with the malformedRequest OCSPResponseStatus as described in section 4.2.1 of . The value of the nonce MUST be generated using a cryptographically strong pseudorandom number generator (see ). The minimum nonce length of 1 octet is defined to provide backward compatibility with older clients that follow [RFC6960]. Newer OCSP clients that support this document MUST use a length of 32 octets for the nonce in Nonce extension. OCSP responders MUST accept lengths of at least 16 octets, and MAY choose to ignore the Nonce extension for requests where the length of the nonce is less than 16 octets

The security considerations of OCSP, in general, are described in . During the interval in which the previous OCSP response for a certificate is not expired but the responder has a changed status for that certificate, a copy of that OCSP response can be used to indicate that the status of the certificate is still valid. Including client's Nonce value in the OCSP response makes sure that the response is the latest response from the server and not an old copy.

The Nonce extension is used to avoid replay attacks. Since the OCSP responder may choose to not send the Nonce extension in the OCSP response even if the client has sent the Nonce extension in the request , an on-path attacker can intercept the OCSP request and respond with an earlier response from the server without the Nonce extension. This can be mitigated by configuring the server to use a short time interval between the thisUpdate and nextUpdate fields in the OCSP response.

If the value of nonce used by a client in OCSP request is predictable, then an attacker may prefetch responses with the predicted nonce and can replay them, thus defeating the purpose of using nonce. Therefore the value of Nonce extension in the OCSP request MUST contain cryptographically strong randomness and MUST be freshly generated at the time of creating the OCSP request. Also if the length of nonce is too small e.g. 1 octet then an on-path attacker can prefetch responses with all the possible values of nonce and replay a matching nonce.

This document does not call for any IANA actions.

This section updates the ASN.1 definitions of the OCSP Nonce extension in Appendix B.1 and Appendix B.2 of The Appendix B.1 defines OCSP using ASN.1 - 1998 Syntax and Appendix B.2 defines OCSP using ASN.1 - 2008 Syntax

OLD Syntax: The definition of OCSP Nonce Extension is not provided in Appendix B.1 of for the ASN.1 - 1998 Syntax. NEW Syntax:

OLD Syntax:

NEW Syntax: