Online Certificate Status Protocol
The Online Certificate Status Protocol (OCSP) was created as an alternative to certificate revocation lists (CRLs). Similar to CRLs, OCSP enables a requesting party (eg, a web browser) to determine the revocation state of a certificate.
When a CA signs a certificate, they will typically include an OCSP server address (eg, <span class="pre">http://ocsp.example.com</span>
) in the certificate. This is similar in function to <span class="pre">crlDistributionPoints</span>
used for CRLs.
As an example, when a web browser is presented with a server certificate, it will send a query to the OCSP server address specified in the certificate. At this address, an OCSP responder listens to queries and responds with the revocation status of the certificate.
Note
It’s recommended to use OCSP instead where possible, though realistically you will tend to only need OCSP for website certificates. Some web browsers have deprecated or removed support for CRLs.
Prepare the configuration file
To use OCSP, the CA must encode the OCSP server location into the certificates that it signs. Use the <span class="pre">authorityInfoAccess</span>
option in the appropriate sections, which in our case means the <span class="pre">[</span><span> </span><span class="pre">server_cert</span><span> </span><span class="pre">]</span>
section.
[ server_cert ]
# ... snipped ...
authorityInfoAccess = OCSP;URI:http://ocsp.example.com
Create the OCSP pair
The OCSP responder requires a cryptographic pair for signing the response that it sends to the requesting party. The OCSP cryptographic pair must be signed by the same CA that signed the certificate being checked.
Create a private key and encrypt it with AES-256 encryption.
# cd /root/ca
# openssl genrsa -aes256 \
-out intermediate/private/ocsp.example.com.key.pem 4096
Create a certificate signing request (CSR). The details should generally match those of the signing CA. The Common Name, however, must be a fully qualified domain name.
# cd /root/ca
# openssl req -config intermediate/openssl.cnf -new -sha256 \
-key intermediate/private/ocsp.example.com.key.pem \
-out intermediate/csr/ocsp.example.com.csr.pem
Enter pass phrase for intermediate.key.pem: secretpassword
You are about to be asked to enter information that will be incorporated
into your certificate request.
-----
Country Name (2 letter code) [XX]:GB
State or Province Name []:England
Locality Name []:
Organization Name []:Alice Ltd
Organizational Unit Name []:Alice Ltd Certificate Authority
Common Name []:ocsp.example.com
Email Address []:
Sign the CSR with the intermediate CA.
# openssl ca -config intermediate/openssl.cnf \
-extensions ocsp -days 375 -notext -md sha256 \
-in intermediate/csr/ocsp.example.com.csr.pem \
-out intermediate/certs/ocsp.example.com.cert.pem
Verify that the certificate has the correct X509v3 extensions.
# openssl x509 -noout -text \
-in intermediate/certs/ocsp.example.com.cert.pem
X509v3 Key Usage: critical
Digital Signature
X509v3 Extended Key Usage: critical
OCSP Signing
Revoke a certificate
The OpenSSL <span class="pre">ocsp</span>
tool can act as an OCSP responder, but it’s only intended for testing. Production ready OCSP responders exist, but those are beyond the scope of this guide.
Create a server certificate to test.
# cd /root/ca
# openssl genrsa -out intermediate/private/test.example.com.key.pem 2048
# openssl req -config intermediate/openssl.cnf \
-key intermediate/private/test.example.com.key.pem \
-new -sha256 -out intermediate/csr/test.example.com.csr.pem
# openssl ca -config intermediate/openssl.cnf \
-extensions server_cert -days 375 -notext -md sha256 \
-in intermediate/csr/test.example.com.csr.pem \
-out intermediate/certs/test.example.com.cert.pem
Run the OCSP responder on <span class="pre">localhost</span>
. Rather than storing revocation status in a separate CRL file, the OCSP responder reads <span class="pre">index.txt</span>
directly. The response is signed with the OCSP cryptographic pair (using the <span class="pre">-rkey</span>
and <span class="pre">-rsigner</span>
options).
# openssl ocsp -port 127.0.0.1:2560 -text -sha256 \
-index intermediate/index.txt \
-CA intermediate/certs/ca-chain.cert.pem \
-rkey intermediate/private/ocsp.example.com.key.pem \
-rsigner intermediate/certs/ocsp.example.com.cert.pem \
-nrequest 1
Enter pass phrase for ocsp.example.com.key.pem: secretpassword
In another terminal, send a query to the OCSP responder. The <span class="pre">-cert</span>
option specifies the certificate to query.
# openssl ocsp -CAfile intermediate/certs/ca-chain.cert.pem \
-url http://127.0.0.1:2560 -resp_text \
-issuer intermediate/certs/intermediate.cert.pem \
-cert intermediate/certs/test.example.com.cert.pem
The start of the output shows:
- whether a successful response was received (
<span class="pre">OCSP</span><span> </span><span class="pre">Response</span><span> </span><span class="pre">Status</span>
) - the identity of the responder (
<span class="pre">Responder</span><span> </span><span class="pre">Id</span>
) - the revocation status of the certificate (
<span class="pre">Cert</span><span> </span><span class="pre">Status</span>
)
OCSP Response Data:
OCSP Response Status: successful (0x0)
Response Type: Basic OCSP Response
Version: 1 (0x0)
Responder Id: ... CN = ocsp.example.com
Produced At: Apr 11 12:59:51 2015 GMT
Responses:
Certificate ID:
Hash Algorithm: sha1
Issuer Name Hash: E35979B6D0A973EBE8AEDED75D8C27D67D2A0334
Issuer Key Hash: 69E8EC547F252360E5B6E77261F1D4B921D445E9
Serial Number: 1003
Cert Status: good
This Update: Apr 11 12:59:51 2015 GMT
Revoke the certificate.
# openssl ca -config intermediate/openssl.cnf \
-revoke intermediate/certs/test.example.com.cert.pem
Enter pass phrase for intermediate.key.pem: secretpassword
Revoking Certificate 1003.
Data Base Updated
As before, run the OCSP responder and on another terminal send a query. This time, the output shows <span class="pre">Cert</span><span> </span><span class="pre">Status:</span><span> </span><span class="pre">revoked</span>
and a <span class="pre">Revocation</span><span> </span><span class="pre">Time</span>
.
OCSP Response Data:
OCSP Response Status: successful (0x0)
Response Type: Basic OCSP Response
Version: 1 (0x0)
Responder Id: ... CN = ocsp.example.com
Produced At: Apr 11 13:03:00 2015 GMT
Responses:
Certificate ID:
Hash Algorithm: sha1
Issuer Name Hash: E35979B6D0A973EBE8AEDED75D8C27D67D2A0334
Issuer Key Hash: 69E8EC547F252360E5B6E77261F1D4B921D445E9
Serial Number: 1003
Cert Status: revoked
Revocation Time: Apr 11 13:01:09 2015 GMT
This Update: Apr 11 13:03:00 2015 GMT
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