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@subheading Print information about an OCSP request
To parse an OCSP request and print information about the content, the
@code{-i} or @code{--request-info} parameter may be used as follows.
The @code{-Q} parameter specify the name of the file containing the
OCSP request, and it should contain the OCSP request in binary DER
format.
@example
$ ocsptool -i -Q ocsp-request.der
@end example
The input file may also be sent to standard input like this:
@example
$ cat ocsp-request.der | ocsptool --request-info
@end example
@subheading Print information about an OCSP response
Similar to parsing OCSP requests, OCSP responses can be parsed using
the @code{-j} or @code{--response-info} as follows.
@example
$ ocsptool -j -Q ocsp-response.der
$ cat ocsp-response.der | ocsptool --response-info
@end example
@subheading Generate an OCSP request
The @code{-q} or @code{--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 @code{--outfile}. To generate an OCSP request the issuer of the
certificate to check needs to be specified with @code{--load-issuer}
and the certificate to check with @code{--load-cert}. By default PEM
format is used for these files, although @code{--inder} can be used to
specify that the input files are in DER format.
@example
$ ocsptool -q --load-issuer issuer.pem --load-cert client.pem \
--outfile ocsp-request.der
@end example
When generating OCSP requests, the tool will add an OCSP extension
containing a nonce. This behaviour can be disabled by specifying
@code{--no-nonce}.
@subheading Verify signature in OCSP response
To verify the signature in an OCSP response the @code{-e} or
@code{--verify-response} parameter is used. The tool will read an
OCSP response in DER format from standard input, or from the file
specified by @code{--load-response}. The OCSP response is verified
against a set of trust anchors, which are specified using
@code{--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.
@example
$ ocsptool -e --load-trust issuer.pem \
--load-response ocsp-response.der
@end example
The tool will print status of verification.
@subheading 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
@code{--load-signer} instead of @code{--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.
@example
$ ocsptool -e --load-signer ocsp-signer.pem \
--load-response ocsp-response.der
@end example
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 @code{--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.
@subheading 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 @code{blog.josefsson.org} host, which (as of writing) uses a
certificate from CACert. First we'll use @code{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.
@example
$ echo | gnutls-cli -p 443 blog.josefsson.org --save-cert chain.pem
@end example
The saved certificates normally contain a pointer to where the OCSP
responder is located, in the Authority Information Access Information
extension. For example, from @code{certtool -i < chain.pem} there is
this information:
@example
Authority Information Access Information (not critical):
Access Method: 1.3.6.1.5.5.7.48.1 (id-ad-ocsp)
Access Location URI: https://ocsp.CAcert.org/
@end example
This means that ocsptool can discover the servers to contact over HTTP.
We can now request information on the chain certificates.
@example
$ ocsptool --ask --load-chain chain.pem
@end example
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.
@example
$ ocsptool --ask https://ocsp.CAcert.org/ --load-chain chain.pem
@end example
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