Which of the following is used to validate a certificate when it is presented to a user?

The Online Certificate Status Protocol (OCSP) is a protocol used by clients to query a

known OCSP responder (typically run by the Certificate Authority that issued the

certificate) to determine the revocation status of an X.509 digital certificate. When a user

is presented with a certificate (e.g., when visiting a secure website), their system can use

OCSP to check if the certificate is still valid and has not been revoked by the issuing CA

before its scheduled expiration date. This provides more timely revocation information

than traditional Certificate Revocation Lists (CRLs).

B. CSR (Certificate Signing Request): A CSR is a message sent from an applicant to a

Certificate Authority to apply for a digital certificate. It is part of the certificate issuance

process, not its subsequent validation.

C. CA (Certificate Authority): A CA is an entity that issues digital certificates. While

the CA is responsible for maintaining and publishing revocation information (which

OCSP uses), the CA itself is not the protocol or direct mechanism a user employs for

real-time validation at the point of presentation.

D. CRC (Cyclic Redundancy Check): A CRC is an error-detecting code used to detect

accidental changes to raw data during transmission or storage. It is not related to the

cryptographic validation or revocation status checking of digital certificates.

OCSP:

Internet Engineering Task Force (IETF). RFC 6960: "X.509 Internet Public Key

Infrastructure Online Certificate Status Protocol - OCSP". June 2013. Section 1

(Introduction) states, "This document specifies a protocol for determining the current

status of a digital certificate without requiring CRLs."

URL: https://datatracker.ietf.org/doc/html/rfc6960

Specific: Section 1, Paragraph 1.

National Institute of Standards and Technology (NIST). SP 800-32: "Introduction to

Public Key Technology and the Federal PKI Infrastructure". February 2001. Section

5.4.2 describes OCSP as a way to obtain certificate status information.

URL: https://csrc.nist.gov/publications/detail/sp/800-32/final

Specific: Section 5.4.2 "Online Certificate Status Protocol (OCSP)".

CSR:

Internet Engineering Task Force (IETF). RFC 2986: "PKCS #10: Certification Request

Syntax Specification Version 1.7". November 2000. Section 1 (Introduction) states,

"This document describes a syntax for certification requests."

URL: https://datatracker.ietf.org/doc/html/rfc2986

Specific: Section 1, Paragraph 1.

CA:

National Institute of Standards and Technology (NIST). SP 800-32: "Introduction to

Public Key Technology and the Federal PKI Infrastructure". February 2001. Section

3.1 defines a CA as "The authority trusted to create and assign public key certificates."

URL: https://csrc.nist.gov/publications/detail/sp/800-32/final

Specific: Section 3.1 "Certification Authority (CA)".

CRC:

Koopman, P. "Cyclic Redundancy Code (CRC) Polynomial Selection For Embedded

Networks". Proceedings The International Conference on Dependable Systems and

Networks, DSN 2004. June 2004. Abstract describes CRCs for error detection.

DOI: https://doi.org/10.1109/DSN.2004.1311885

Specific: Abstract.