'Generate a pem_password_cb function pointer that copied the password to
OpenSSL as required and returns the number of bytes copied.
typedef int pem_password_cb(char *buf, int size,
int rwflag, void *userdata);
Useful for decrypting PKCS8 files and so on.
Returns a tuple of (cdata function pointer, userdata).'

def _pem_password_cb(self, password):

    userdata = _PasswordUserdata(password=password)
    return (_pem_password_cb, userdata)
