Online Certificate Status Protocol (OCSP) Nonce Extension

Introduction Nonce extension was previously defined in Section 4.4.1 of and updated in . enforces the maximum Nonce length to 32 octets, which restricts the client from supporting the longer Nonce generated by different algorithms. There are a few compliance recommendations that suggest using cryptographical algorithms that results in generating a longer Nonce. This document updates the maximum allowed Nonce length to 128 octets from the existing 32 octets limit to support the longer length of the nonce. This document provides a recommendation to the client and responder to support 32 octets as the minimum length for Nonce. Since Nonce is an optional extension, changes in this document makes Nonce length restrictions applicable to only those OCSP clients and responders that support the Nonce extensions.

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 extends the maximum allowed nonce length to 128 octets and does not change the specifications of any of the other standard extensions defined in [RFC6960].

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 128 octets. A newer OCSP client that supports Nonce and this document SHOULD use a minimum length of 32 octets for Nonce in the Nonce extension. OCSP resonder that supports Nonce and this document MUST reject any OCSP request that has a Nonce in the Nonce extension with a length of either 0 octets or more than 128 octets, with the malformedRequest OCSPResponseStatus as described in Section 4.2.1 of . Responders supporting the Nonce extension 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 or more than 32 octets. 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..128)) 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.

Changes to Appendix B of RFC 6960. This section updates the ASN.1 definitions of the OCSP Nonce extension in Appendices B.1 and B.2 of . Appendix B.1 defines OCSP using ASN.1 - 1998 Syntax; Appendix B.2 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 Appendix B.1 of for the ASN.1 - 1998 Syntax. NEW Syntax: Nonce ::= OCTET STRING(SIZE(1..128))

Changes to Appendix B.2 OCSP in ASN.1 - 2008 Syntax OLD Syntax: re-ocsp-nonce EXTENSION ::= { SYNTAX OCTET STRING IDENTIFIED BY id-pkix-ocsp-nonce } NEW Syntax: re-ocsp-nonce EXTENSION ::= { SYNTAX OCTET STRING(SIZE(1..128)) IDENTIFIED BY id-pkix-ocsp-nonce }