OCSP (Online Certificate Status Protocol)

The Online Certificate Status Protocol (OCSP) is an Internet protocol used for obtaining the revocation status of an X.509 digital certificate. It was created as an alternative to Certificate Revocation Lists (CRLs) to provide real-time certificate status information.

How OCSP Works

OCSP allows clients (such as web browsers) to query an OCSP responder (a server) to check if a certificate is valid, revoked, or unknown. The process involves the following steps:

  1. Client Request: The client sends an OCSP request to the OCSP responder, including the certificate serial number.

  2. Responder Reply: The OCSP responder returns a signed response indicating the certificate status (good, revoked, or unknown).

Setting Up OCSP with OpenSSL

Step 1: Generate the OCSP Responder Certificate

The OCSP responder requires its own certificate, signed by the CA.

  1. Create a private key for the OCSP responder:

openssl genpkey -algorithm RSA -out ocsp_responder.key -aes256

  1. Generate a certificate signing request (CSR) for the OCSP responder:

openssl req -new -key ocsp_responder.key -out ocsp_responder.csr

  1. Sign the CSR with the CA to create the OCSP responder certificate:

openssl ca -config openssl.cnf -extensions v3_OCSP -days 365 -in ocsp_responder.csr -out ocsp_responder.crt

Step 2: Configure the OCSP Responder

Add the OCSP extensions to the CA configuration file (openssl.cnf):

[ v3_OCSP ]
basicConstraints = CA:FALSE
keyUsage = nonRepudiation, digitalSignature, keyEncipherment
extendedKeyUsage = OCSPSigning

Step 3: Run the OCSP Responder

Start the OCSP responder using the following command:

openssl ocsp -index /root/ca/index.txt -port 2560 -rsigner ocsp_responder.crt -rkey ocsp_responder.key -CA cacert.pem -text

This command starts an OCSP responder on port 2560, using the provided certificates and keys.

Sending OCSP Requests

To test the OCSP responder, you can use OpenSSL to send OCSP requests:

openssl ocsp -CAfile cacert.pem -issuer cacert.pem -cert certificate_to_check.crt -url http://localhost:2560 -resp_text -noverify

This command sends an OCSP request to the responder running on http://localhost:2560 to check the status of certificate_to_check.crt.

Use Cases

Real-Time Certificate Validation: OCSP provides real-time status of certificates, making it more efficient than CRLs for checking certificate revocation.

Enhanced Security: By using OCSP, organizations can immediately invalidate compromised certificates, thus improving overall security.

Web Browsers and Servers: Many modern web browsers and servers support OCSP to validate SSL/TLS certificates in real-time, enhancing trust and security for online communications.

Conclusion

OCSP is an important protocol for maintaining the security and integrity of digital certificates. It offers a more efficient alternative to CRLs by providing real-time certificate status information. By setting up and using an OCSP responder with OpenSSL, administrators can ensure that their PKI environment remains secure and up-to-date with the latest certificate statuses.

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