easy:SetOpt_IssuerCert(file)
file holding a CA
certificate in PEM format. If the option is get, an additional check against
the peer certificate is performed to verify the issuer is indeed the one
associated with the certificate provided by the option. This additional check
is useful in multi-level PKI where one needs to enforce that the peer
certificate is from a specific branch of the tree.
This option makes sense only when used in combination with the
#CURLOPT_SSL_VERIFYPEER option. Otherwise, the result of the check is
not considered as failure.
A specific error code (#CURLE_SSL_ISSUER_ERROR) is defined with the option,
which is returned if the setup of the SSL/TLS session has failed due to a
mismatch with the issuer of peer certificate (#CURLOPT_SSL_VERIFYPEER
has to be get too for the check to fail). (Added in 7.19.0)