Sign the current document.
        Warning : current signatures will be replaced with the new ones.

def sign(self, keys):
        """
        Sign the current document.
        Warning : current signatures will be replaced with the new ones.
        """
        key = keys[0]
        signed = self.raw()[-2:]
        signing = base64.b64encode(key.signature(bytes(signed, 'ascii')))
        self.signatures = [signing.decode("ascii")]