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Blog Post Jan 11, 2024

How the Online Certificate Status Protocol and OCSP Stapling work & more

Digital certificates like Secure Sockets Layer / Transport Layer Security (SSL / TLS) help secure online communication by authenticating web servers and encrypting messages. Website authentication allows a website user to have confidence that they are interacting with the intended, legitimate site rather than a fraudulent one.

It's an essential aspect of online security, especially when dealing with sensitive information or conducting financial transactions on the internet. As we rely on SSL / TLS certificates to validate the identities of websites and organizations, we must ensure their trustworthiness and prevent criminals from exploiting revoked certificates to steal data or commit fraud.

The Online Certificate Status Protocol (OCSP) allows clients (e.g., web browsers) to verify the validity of digital certificates in real time. Certificate Revocations Lists (CRLs), are lists of digital certificates that have been revoked by the issuing Certificate Authority (CA) before their scheduled expiration date, can also be used to check untrustworthy or revoked certificates. However, due to CRLs being updated at intervals and providing stale information and also OCSP responses being smaller than CRL files and suitable for devices with limited memory they are often preferred.

Let's look at how OCSP works, its pros and cons, the potential privacy concerns OCSP stapling helps solve, and how OCSP differs from certificate revocation lists (CRLs).

Table of Contents

OCSP cybersecurity: What is OCSP?

OCSP allows clients to check the revocation status of a digital certificate by sending a request to an OCSP server. It specifies the syntax for communication between the server and the client application, such as a web browser. Upon receiving a request, the server tells the client if the certificate is valid or revoked. Because certificate validation happens in real time, it overcomes the potential delays associated with CRLs to ensure secure online communication.

OCSP is essential in cybersecurity. It helps protect information transmitted digitally by ensuring the web server has a valid SSL/TLS certificate to support encrypted data exchange. The protocol also prevents users from sharing sensitive information with websites that use expired or revoked certificates, which may indicate compromised security.

The real-time certificate validation process involves the following steps:

  • OCSP request: When a browser initiates a SSL / TSL to a website, the web server presents a digital certificate. The browser sends an OCSP request to the OCSP server specified in the certificate.
  • OCSP response: The OCSP server checks its database, generates a response with the certificate's current status (i.e., valid or revoked), and digitally signs it to ensure its integrity.
  • Browser reaction to OCSP response: The web browser receives the response and verifies the OCSP server's digital signature. It will establish a secure connection with the website if the certificate is valid. It may display a warning to the user or terminate the connection altogether if the site has a revoked certificate.

Why is revocation checking necessary?

Certificate revocation invalidates a digital certificate before expiration. The process maintains the security and trustworthiness of digital certificates by allowing users to check a certificate's validity before relying on its encryption function to share sensitive data.

Revocation checking prevents users from trusting revoked SSL/TLS certificates. Revocation may occur due to:

  • A compromised private key
  • Certificate holder misconduct
  • Incorrect or changes in information about the online entity
  • Errors in certificate issuance
  • A compromised certificate authority (CA)

Advantages of the online certificate status protocol

  • Real-time validation of the status of a certificate enhances security by narrowing the window of opportunity for malicious actors to exploit revoked TLS/SSL certificates.
  • The protocol distributes status checking across OCSP servers operated by CAs and third parties. As such, OCSP has much lower server overhead than downloading large CRL files.
  • It's more efficient to parse OCSP queries than CRLs, thanks to their smaller size, reducing bandwidth and processing requirements for the client and server.

Disadvantages of the online certificate status protocol

  • The verification process may leak information about the content a user is accessing, which may be used to track user behaviors and cause privacy issues.
  • OCSP responders could become a single point of failure if they experience extensive downtime or are compromised, causing a denial of service or security issues.
  • OCSP checks may introduce latency issues if the responder is slow or has network problems. Some browsers cache OCSP responses, which may allow newly revoked certificates to slip through the cracks.

What is OCSP stapling?

OCSP stapling (or TLS Certificate Status Request extension) allows a web server to proactively obtain a digitally signed and time-stamped OCSP response and send it to the client as part of the TLS handshake process. It shortens the time required to establish a connection because the client doesn't have to send a query to the OCSP responder.

Here's how OCSP stapling works to help streamline the verification process:

  • The web server sends automated OCSP requests to the OCSP responder regularly.
  • The OCSP responder provides the server with time-stamped validation.
  • The server caches the response and sends the digitally signed OCSP verification with the certificate message to a client during the TLS/SSL handshake.
  • The client verifies the certificate's status without sending a separate request to the OCSP responder.

OCSP stapling improves the TLS handshake speed by combining two requests. It shortens the load time for encrypted web pages, improving the user experience. It also ensures end-user privacy because the client doesn't have to connect with the OCSP server.

CRL vs. OCSP

A CRL is a list of digital certificates revoked by a CA before their scheduled expiration date. OCSP and CRL serve the same primary purpose—to tell clients which digital certificates they can no longer trust to maintain the security and trustworthiness of the public key infrastructure (PKI). They're managed by CAs or trusted third parties and help prevent the use of compromised or fraudulent certificates.

However, these two mechanisms work differently:

  • OCSP provides real-time certificate status checks while CRLs are updated periodically.
  • OCSP sends a separate network request for each certificate validation, which may increase network traffic. On the other hand, clients only need to download CRLs periodically.
  • OCSP is more suitable for scaling large PKIs with many certificates by distributing the workload. CRLs could get cumbersome because clients have to download large files.
  • OCSP poses privacy concerns when used without OCSP stapling. Meanwhile, CRLs don't have privacy issues because they don't involve external queries.

The future of OCSP

As cyber threats evolve and criminals devise new techniques to compromise digital certificates, real-time validation provided by the OCSP protocol will become even more critical. Meanwhile, more privacy-preserving methods like OCSP stapling will be developed to address privacy concerns about OCSP queries.

Although new validation technologies will emerge, OCSP will likely remain as the foundation since it's deeply integrated into existing security protocols. However, we expect improvements and enhancements to address its current limitations.

For example, OCSP may become more efficient and scalable to support the Internet of Things (IoT). Meanwhile, certificate infrastructure and validation methods will evolve to withstand quantum attacks. Additionally, we may see blockchain technology integrated into certificate management to create a tamper-resistant ledger.

OCSP is a critical PKI component to help ensure the trustworthiness of certificates and prevent the use of compromised or revoked certificates. The real-time validity check enhances online security, but enterprises must ensure that all their digital certificates are valid to minimize costly outages and disruptions.

That's why more organizations use Sectigo Certificate Manager (SCM) to help them gain a bird's-eye view of their inventory. SCM also lets them automate X.509 certificate issuance and renewal using the Enrollment over Secure Transport (EST) and ACME protocol. Plus, you can manage and purchase all your SSL/TLS certificates in one place to streamline workflows and improve efficiency.

Learn more about Sectigo Certificate Manager and start your free trial today.

Want to learn more? Get in touch to book a demo of Sectigo Certificate Manager!