A canary goes something like "This website has not received or acted on any government orders to disclose or modify or remove material." When they ever do, then they remove that notice. The government enforcement usually includes a gag order prohibiting the target from saying that they're under orders, so the intent is that you can infer government gag pressure by the canary having been removed. Wikileaks used to have such a notice and no longer does, so we assume government enforcement is why.

I'm not sure what the cryptographic part has to do with anything. I'd guess it was signed in a way that you can verify the government itself didn't tamper with the notice.