Whenever a X509_STORE_CTX object is created for the verification of the
peers certificate during a handshake, a pointer to the SSL object is
stored into the X509_STORE_CTX object to identify the connection affected.
To retrieve this pointer the X509_STORE_CTX_get_ex_data() function can
be used with the correct index. This index is globally the same for all
X509_STORE_CTX objects and can be retrieved using
SSL_get_ex_data_X509_STORE_CTX_idx(). The index value is set when
SSL_get_ex_data_X509_STORE_CTX_idx() is first called either by the application
program directly or indirectly during other SSL setup functions or during
The value depends on other index values defined for X509_STORE_CTX objects
before the SSL index is created.
The index returned from SSL_get_ex_data_X509_STORE_CTX_idx() allows to
access the SSL object for the connection to be accessed during the
verify_callback() when checking the peers certificate. Please check
the example in SSL_CTX_set_verify(3),