The &#x27;no encryption&#x27; rule is there to make sure amateur radio can not be (easily) used for nefarious purposes. Of course two people with Yagi&#x27;s, very low power transmitters and a couple of very low bandwidth modems could still use the amateur bands (or any other frequency, really) to exchange information without anybody being the wiser. If they manage to be on the hoof while transmitting and are on the air for a very short time only then they could probably keep such a scheme alive for a really long time before being detected.