Online Certificate Status Protocol (OCSP) Nonce Extension

Introduction This document updates the usage and format of the Nonce extension in OCSP request and response messages. This extension was previously defined in . does not mention any minimum or maximum length of the nonce in the Nonce extension. Lacking limits on the length of the nonce in the Nonce extension, OCSP responders that follow may be vulnerable to various attacks, like Denial-of-Service attacks or 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 for the length of the nonce in the Nonce extension. This document updates .

Terminology 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.

OCSP Extensions The message formats for OCSP requests and responses are 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 the Nonce extension and does not change the specifications of any of the other standard extensions defined in .

Nonce Extension This section replaces the entirety 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; 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 the Nonce extension is present, then the length of the nonce MUST be at least 1 octet and can be up to 32 octets. A server MUST reject any OCSP request that has a nonce in the Nonce extension with a length of either 0 octets or more than 32 octets with the malformedRequest OCSPResponseStatus, as described in . 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 . Newer OCSP clients that support this document MUST use a length of 32 octets for the nonce in the 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.

Security Considerations 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 a 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.

Replay Attack The Nonce extension is used to avoid replay attacks. Since the OCSP responder may choose not to 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.

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

IANA Considerations This document has no IANA actions.

Changes to Appendix B of RFC 6960 This section updates the ASN.1 definitions of the OCSP Nonce extension in Appendices and of . Appendix defines OCSP using ASN.1 - 1998 Syntax; Appendix defines OCSP using ASN.1 - 2008 Syntax.

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

Changes to Appendix B.2 OCSP in ASN.1 - 2008 Syntax OLD Syntax: NEW Syntax: