Secure time broadcast
Broadcasts using sound or radiation, from sources such as bell towers, radio towers, satellites, and pulsars, must send out the same value to every receiver. A remote beacon such as a pulsar has perfect security: the access structure is any party, and its complement, the attack structure, is the empty set. For human controlled broadcasts, the attack structure consists only of the broadcaster and the access structure is any receiver.
Natural broadcasts are thus immune to the problem, described in the discussion of the Byzantine Generals problem below, of a transmitter sending different values to different receivers. Indeed, as we will see below, distributed researchers have gone to great lengths just to recreate this simple property on the Internet with logical broadcast protocols.
Nature provides clocks that are oblivious to the malicious intentions of any outside parties. In the case of a remote high-energy system such as a pulsar, this means anybody. Pulsars are many orders of magnitude more accurate than random delays that face attackers on the Internet. If critical Internet servers were synchronized to natural clocks in a secure and timely fashion, they would be immune to attacks that relied on uncertainties in timing.
Detecting pulsars is still, alas, a difficult process, but not so hard that amateurs have had no success doing so. Amateurs have had some success tracking pulsars with software defined radio and quagi antennas combined with digital signal processing.