ocsptool command examples

ocsptool command examples

ocsptool – GnuTLS OCSP tool

Print information about an OCSP request

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.

Real-world example

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 --print-cert > chain.pem

Use a text editor on chain.pem to create three files for each separate certificates, called cert.pem for the first certificate for the domain itself, secondly issuer.pem for the intermediate certificate and root.pem for the final root certificate.

The domain certificate normally contains a pointer to where the OCSP responder is located, in the Authority Information Access Information extension. For example, from certtool -i < cert.pem there is this information:

Authority Information Access Information (not critical):

Access Method: (id-ad-ocsp)
Access Location URI: http://ocsp.CAcert.org/

This means the CA support OCSP queries over HTTP. We are now ready to create a OCSP request for the certificate.

$ ocsptool --ask ocsp.CAcert.org --load-issuer issuer.pem --load-cert cert.pem --outfile ocsp-response.der

The request is sent via HTTP to the OCSP server address specified. If the address is ommited ocsptool will use the address stored in the certificate.


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