Derive private key from the brain key and the current sequence
            number

def get_private(self):
        """ Derive private key from the brain key and the current sequence
            number
        """
        a = compat_bytes(self.account + self.role + self.password, 'utf8')
        s = hashlib.sha256(a).digest()
        return PrivateKey(hexlify(s).decode('ascii'))