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
http://ocsp.example.com) in the certificate. This is similar in
crlDistributionPoints 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.
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
authorityInfoAccess option in the appropriate
sections, which in our case means the
[ server_cert ] 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¶
ocsp 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
localhost. Rather than storing revocation status
in a separate CRL file, the OCSP responder reads
index.txt directly. The
response is signed with the OCSP cryptographic pair (using the
# 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
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 (
OCSP Response Status)
- the identity of the responder (
- the revocation status of the certificate (
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
Cert Status: revoked and a
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