Online Certificate Status Protocol (OCSP) Nonce Extension

Introduction This document updates the OCSP server behavior for the OCSP requests, which have Nonce length that exceeds the maximum length limit defined in . This extension was previously defined in Section 4.4.1 of and updated in . defines OCSP extensions as an optional extension that provides the freedom to OCSP responders to ignore the Nonce extension regardless of its existence or value in OCSP request, whereas language directs the OCSP responders to MUST take action based on the Nonce value in OCSP requests. For example, directs OCSP responder to MUST reject all the OCSP requests that have NONCE longer than 32 OCTET string and MUST respond with the malformedRequest OCSPResponseStatus. The OCSP responder SHOULD be able to ignore the Nonce values (valid or invalid) as it is an optional extension. Ignoring the longer Nonce value helps the OCSP responder achieve better backward compatibility, as it won't impact the existing OCSP clients that have already been using longer Nonce values. This document updates the by replacing the section 2.1 of . This document updates the OCSP responder's behavior in case of Nonce length presented in OCSP request exceeds the maximum Nonce length of 32 OCTETS. This update also provides an example of Nonce value in OCTET string.

Requirements Language 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 and Nonce extension was updated in . also defines the standard extensions for OCSP messages based on the extension model employed in X.509 version 3 certificates (see ). replaces this section to enforce the minimum and maximum length for the Nonce value. This document adds flexibility to OCSP servers to either reject or ignore the OCSP request that has Nonce longer than 32 OCTETS. This document doesn't change the length requirement for Nonce value and does not change the specifications of any of the other standard extensions defined in or .

Nonce Extension This section replaces the entirety of Section 2.1 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; 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 either 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 Section 4.2.1 of [RFC6960], or the server MAY choose to ignore the Nonce extension with a length of more than 32 octets and in that case MUST leave out the Nonce extension in the response. 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 . id-pkix-ocsp OBJECT IDENTIFIER ::= { id-ad-ocsp } id-pkix-ocsp-Nonce OBJECT IDENTIFIER ::= { id-pkix-ocsp 2 } Nonce ::= OCTET STRING(SIZE(1..32)) Raw Nonce Example: 1FA16041C0E2D739898EC69972A4B967DFD3488148E6DA61CF3AF2DC32383383 OCSP Nonce Octet String: 04201FA16041C0E2D739898EC69972A4B967DFD3488148E6DA61CF3AF2DC32383383

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.

IANA Considerations This memo includes no request to IANA.

Security Considerations This document should not affect the security of the Internet.