Encryption

One Time Cypher Pad

The best solution then is to use a one time cypher pad, which is a long keyword (even better still made up randomly) which is only ever used once. These would be printed on a pad of paper and once each one was used, that sheet would be torn off and destroyed. The person receiving the message would need the same sheets in the same order to decode the message. This is theoretically unbreakable, but represents a massive hassle to use correctly. You would have to have a different set of one time pads for all the people you correspond with, and make sure that everybody always correctly uses the next one in the sequence.

All this is completely useless of course, if you don't actually know the person you want to exchange a message with - for example an online shop such as Amazon that you want to give your credit card number to.....