Context objects have the following methods:
callback should take three arguments: a Connection object and two integers. The first integer specifies where in the SSL handshake the function was called, and the other the return code from a (possibly failed) internal function call.
callback should take a boolean argument repeat and an arbitrary argument data and return the passphrase entered by the user. If repeat is true then callback should ask for the passphrase twice and make sure that the two entries are equal. The data argument is the userdata variable passed to the set_passwd_cb method. If an error occurs, callback should return a false value (e.g. an empty string).
callback should take five arguments: A Connection object, an X509 object, and three integer variables, which are in turn potential error number, error depth and return code. callback should return true if verification passes and false otherwise.