What Is Certificate Status Checking?

Updated on June 3, 2025

Digital certificates secure online communication and verify identities, but their validity must be ensured. Certificate Status Checking confirms if a certificate is still valid or has been revoked by the Certificate Authority (CA), ensuring trust in secure interactions and transactions.

Definition and Core Concepts 

To better understand Certificate Status Checking, it’s important to grasp the key components involved in the process:

• Digital Certificate: A credential used in PKI systems to validate the identity of an entity (such as a website, application, or individual). Contains the entity’s public key, details about the issuing CA, and a validity period. 
• Public Key Infrastructure (PKI): A framework that enables secure communication between entities. Uses asymmetric cryptography to manage, distribute, and verify digital certificates and public-private key pairs. 
• Certificate Authority (CA): An entity that issues digital certificates. Acts as a trusted third party, ensuring the information within certificates is accurate and legitimate. 
• Validity Period: Digital certificates have a predefined validity period. Becomes invalid after expiration, requiring renewal or reissuance. 
• Certificate Revocation: Certificates can be revoked before expiration for reasons like key compromise, incorrect information, or entity status changes. Revoked certificates should not be trusted. 
• Revocation Reasons: Private key compromise, certificate misuse, or inaccuracies found by the CA. 
• Online Checking: Involves querying a CA’s real-time databases, such as through the Online Certificate Status Protocol (OCSP). 
• Offline Checking: Relies on periodically updated data, such as Certificate Revocation Lists (CRLs), to verify certificate status.

How It Works 

Certificate Status Checking relies on several mechanisms to determine the validity of a certificate. These mechanisms include Online Certificate Status Protocol (OCSP), Certificate Revocation Lists (CRLs), and OCSP Stapling. 

Online Certificate Status Protocol (OCSP) 

OCSP is a protocol used to obtain real-time revocation information. When a certificate’s validity is checked, the client queries an OCSP responder, which is maintained by the CA. The OCSP responder replies with the certificate’s current status (valid, revoked, or unknown). 

OCSP offers several advantages, including real-time response and reduced bandwidth usage compared to CRLs. However, it requires continuous availability of the OCSP responder for seamless functionality. 

Certificate Revocation Lists (CRLs) 

CRLs are lists of revoked certificates published by the CA at regular intervals. These lists are periodically downloaded and stored by clients for offline certificate status verification. CRLs are useful for environments where real-time access to an OCSP responder is not feasible. 

However, CRLs have their limitations, including increased bandwidth consumption and potential delays in listing recently revoked certificates. 

OCSP Stapling 

OCSP Stapling improves the efficiency of OCSP by allowing the server to query the OCSP responder and “staple” the legitimacy response to the certificate during the TLS handshake. 

This reduces the reliance on the CA for frequent status queries and enhances the speed of secure communication. It also addresses privacy concerns, as the client no longer needs to directly query the CA. 

Key Features and Components 

Certificate Status Checking includes the following essential features and components:

• Real-time Revocation Information (OCSP): OCSP enables real-time responses to certificate status checks, ensuring timely verification.
• Periodic Revocation Information (CRLs): Updates at preset intervals, allowing for offline verification in environments with limited online access.
• Reduced Reliance on CA for Every Check (OCSP Stapling): Minimizes dependency on the CA for each status check, reducing query loads and improving efficiency.
• Enhances Trust and Security: Certificate Status Checking ensures that revoked or expired certificates are not mistakenly trusted, maintaining the integrity of PKI-based systems.

Use Cases and Applications 

Certificate Status Checking is critical for numerous security-sensitive applications. It ensures the validity of digital certificates in the following scenarios:

HTTPS (Web Browsing) 

The validity of SSL/TLS certificates is routinely verified to ensure secure communication between web browsers and servers. By checking the status of these certificates, users are safeguarded against potential man-in-the-middle (MITM) attacks. 

VPN Connections 

Secure remote access relies on digital certificates in VPN environments. Certificate Status Checking ensures that only valid certificates are used during authentication processes, protecting private network resources. 

Code Signing 

Code signing certificates verify the authenticity and integrity of software or code. Certificate status verification ensures that signed applications haven’t been compromised and originate from a trusted source. 

Email Security (S/MIME) 

S/MIME employs certificates to encrypt and sign emails. Verifying the validity of these certificates is essential to prevent phishing and ensure message confidentiality. 

EAP-TLS Authentication 

Extensible Authentication Protocol-Transport Layer Security (EAP-TLS) is commonly used for secure network access. Certificate Status Checking ensures that only valid certificates are presented during authentication, reducing the risk of unauthorized access. 

Key Terms Appendix 

• Certificate Status Checking: Methods used to verify whether a digital certificate is valid and trustworthy.
• Digital Certificate: A credential indicating the authenticity of an entity in a PKI framework.
• Public Key Infrastructure (PKI): A system managing digital certificates and public-private key pairs for secure communication.
• Certificate Authority (CA): The entity responsible for issuing and validating digital certificates.
• Certificate Revocation List (CRL): A list of revoked certificates, used for offline verification.
• Online Certificate Status Protocol (OCSP): A protocol for real-time certificate status checks.
• OCSP Stapling: TLS extension that enhances performance by inflating the revocation response in the handshake.
• Revocation: The process of marking a certificate as invalid before its expiration date.