function signature($id = null) {
		if ( ! $this->unsigned) {
			if ($this->get('id', null) === null) {
				$formsignature = 'freiaform'.$this->formindex;
			}
			else { # custom signature
				$formsignature = $this->get('id');
			}

			if ($id !== null) {
				return "{$formsignature}_field$id";
			}
			else { # form signature
				return $formsignature;
			}
		}
		else { # unsigned
			return null;
		}
	}