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SEC lamps OCSP Nonce backward compatibility RFC 8954 imposed size constraints on the optional Nonce extension for the Online Certificate Status Protocol (OCSP). OCSP is used to check the status of a certificate, and the Nonce extension is used to cryptographically bind an OCSP response message to a particular OCSP request message. Some environments use cryptographic algorithms that generate a Nonce value that is longer than 32 octets. This document also modifies the "Nonce" section of RFC 6960 to clearly define and differentiate the encoding format and values for easier implementation and understanding. This document obsoletes RFC 8954, which includes updated ASN.1 modules for OCSP, and updates RFC 6960.

Introduction The Nonce extension was previously defined in . The Nonce cryptographically binds an OCSP request and a response. It guarantees the freshness of an OCSP response and avoids replay attacks. This extension was updated in . limits the maximum Nonce length to 32 octets. To support cryptographic algorithms that generate a Nonce that is longer than 32 octets, this document updates the maximum allowed size of the Nonce to 128 octets. In addition, this document recommends that the OCSP requester and responder use a Nonce with a minimum length of 32 octets.

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 the 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 to limit 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 extensions defined in .

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 is included as one of the responseExtensions. In both the request and the response, the Nonce is identified by the object identifier id-pkix-ocsp-nonce, while the extnValue is the encoded value of 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. Implementations compliant with will not be able to process nonces generated per the new specification with sizes in excess of the limit (32 octets) specified in . An OCSP requester that implements the extension in this document MUST use a minimum length of 32 octets for Nonce in the Nonce extension. An OCSP responder that supports the Nonce extension MUST accept Nonce lengths of at least 16 octets and up to and including 32 octets. A responder MAY choose to respond without the Nonce extension for requests in which the length of the Nonce is in between 1 octet and 15 octets or 33 octets and 128 octets. Responders that implement the extension in this document MUST reject any OCSP request that has a Nonce with a length of either 0 octets or greater than 128 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 OCSP requesters that follow . The following is an example of an encoded OCSP Nonce extension with a 32-octet Nonce in hexadecimal format. Here is the decoded version of the above example. Offset, Length, and Object Identifier are in decimal.

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 requester's nonce value in the OCSP response ensures that the response is the most recent 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 requester 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 For the ASN.1 modules in Appendixes and , IANA has assigned the following object identifiers (OIDs) in the "SMI Security for PKIX Module Identifier" registry (1.3.6.1.5.5.7.0):

Value	Description
111	id-mod-ocsp-2024-88
112	id-mod-ocsp-2024-08

References Normative References Informative References RFC Errata RFC 6960

This section includes the ASN.1 modules for OCSP and replaces the entirety of . It addresses Errata ID 5891 as well. includes an ASN.1 module that conforms to the 1998 version of ASN.1 for all syntax elements of OCSP. This module replaces the module in . includes an ASN.1 module, corresponding to the module present in , that conforms to the 2008 version of ASN.1. This module replaces the modules in and . Although a 2008 ASN.1 module is provided, the module in remains the normative module per the policy of the PKIX Working Group.

Acknowledgements The authors of this document thank for his work to produce . The authors also thank , , , , and for their feedback and suggestions.