Public Key encryption is the answer to a prayer for remote communication.

Briefly, an implementation would work as follows:-

Every subscriber has a Public and Private Key. The keys work as follows:-

whatever one of the keys locks, only the other key can unlock. The locking key is a one-way key - knowing the locking key does not help find the unlocking key. The Keys themselves are two large (maybe 100 digit) numbers. They are stored in the Radio Telex.

The subscriber makes the Public Key known to anyone and everyone who wishes to communicate. It would be a part of your address. The Sender encrypts the message with the receiver's Public Key. The Sender then 'signs' the encrypted message with his/her Private Key. I call this a 'wrapper'. This is not a lock, just a tag that is difficult to generate for anyone except the sender, that performs the same function as a conventional signature. The message is passed to the receiver, possibly through many systems.

An intermediate system can find out:-

It was indeed sent by the Sender (verified by decrypting the 'wrapper' with the Senders Public Key) Though the addressee's address is on the packet, it is encrypted and unreadable (without the addressee's Private Key). The message cannot be forged - because the 'wrapper' would also have to be forged, which is not possible without the Sender's Private Key. The Receiver gets the packet, which can be verified as being from the Sender and not forged, and can be decrypted with the Receivers Private Key.