All arguments must be options.
Responses are typically signed/issued by designated certificates or certificate authorities and thus this tool requires on verification the certificate of the issuer or the full certificate chain in order to determine the appropriate signing authority. The specified certificate of the issuer is assumed trusted.
To parse an OCSP request and print information about the content, the -i or --request-info parameter may be used as follows. The -Q parameter specify the name of the file containing the OCSP request, and it should contain the OCSP request in binary DER format.
$ ocsptool -i -Q ocsp-request.der
The input file may also be sent to standard input like this:
$ cat ocsp-request.der | ocsptool --request-info
Print information about an OCSP response
Similar to parsing OCSP requests, OCSP responses can be parsed using the -j or --response-info as follows.
$ ocsptool -j -Q ocsp-response.der $ cat ocsp-response.der | ocsptool --response-info
Generate an OCSP request
The -q or --generate-request parameters are used to generate an OCSP request. By default the OCSP request is written to standard output in binary DER format, but can be stored in a file using --outfile. To generate an OCSP request the issuer of the certificate to check needs to be specified with --load-issuer and the certificate to check with --load-cert. By default PEM format is used for these files, although --inder can be used to specify that the input files are in DER format.
$ ocsptool -q --load-issuer issuer.pem --load-cert client.pem --outfile ocsp-request.der
When generating OCSP requests, the tool will add an OCSP extension containing a nonce. This behaviour can be disabled by specifying --no-nonce.
Verify signature in OCSP response
To verify the signature in an OCSP response the -e or --verify-response parameter is used. The tool will read an OCSP response in DER format from standard input, or from the file specified by --load-response. The OCSP response is verified against a set of trust anchors, which are specified using --load-trust. The trust anchors are concatenated certificates in PEM format. The certificate that signed the OCSP response needs to be in the set of trust anchors, or the issuer of the signer certificate needs to be in the set of trust anchors and the OCSP Extended Key Usage bit has to be asserted in the signer certificate.
$ ocsptool -e --load-trust issuer.pem --load-response ocsp-response.der
The tool will print status of verification.
Verify signature in OCSP response against given certificate
It is possible to override the normal trust logic if you know that a certain certificate is supposed to have signed the OCSP response, and you want to use it to check the signature. This is achieved using --load-signer instead of --load-trust. This will load one certificate and it will be used to verify the signature in the OCSP response. It will not check the Extended Key Usage bit.
$ ocsptool -e --load-signer ocsp-signer.pem --load-response ocsp-response.der
This approach is normally only relevant in two situations. The first is when the OCSP response does not contain a copy of the signer certificate, so the --load-trust code would fail. The second is if you want to avoid the indirect mode where the OCSP response signer certificate is signed by a trust anchor.
Here is an example of how to generate an OCSP request for a certificate and to verify the response. For illustration we'll use the blog.josefsson.org host, which (as of writing) uses a certificate from CACert. First we'll use gnutls-cli to get a copy of the server certificate chain. The server is not required to send this information, but this particular one is configured to do so.
$ echo | gnutls-cli -p 443 blog.josefsson.org --save-cert chain.pem
The saved certificates normally contain a pointer to where the OCSP responder is located, in the Authority Information Access Information extension. For example, from certtool -i < chain.pem there is this information:
Authority Information Access Information (not critical): Access Method: 184.108.40.206.220.127.116.11.1 (id-ad-ocsp) Access Location URI: https://ocsp.CAcert.org/
This means that ocsptool can discover the servers to contact over HTTP. We can now request information on the chain certificates.
$ ocsptool --ask --load-chain chain.pem
The request is sent via HTTP to the OCSP server address found in the certificates. It is possible to override the address of the OCSP server as well as ask information on a particular certificate using --load-cert and --load-issuer.
$ ocsptool --ask https://ocsp.CAcert.org/ --load-chain chain.pem