Encryption for criminal or terror purposes is so easy to produce that it is childs play. Public communications are too easily intercepted and read by the real nasties, those who are "protecting us". Secure encryption easy? I hear you say. Too right mate.
Write out an alphabet and underneath it write the numbers 1-0. Repeat the numbers until the alphabet is used up. On the 3rd time through with Roman letters there will be some numbers left over, so add some useful punctuation [space] [comma] [full stop] perhaps ["]. Depending on how you start and the placement of the punctuation we abtain about 100 distinct cyphers which are difficult for simple frequency analysis to break. By itself suitable for casual protection but easily broken by the experts. To make it very hard to break take a book, any book any language and even another alphabet. Use the system outlined above to number the characters in the book starting at a random or carefully defined place. A defined place could be given by date and/or time as prearanged with the recipient. Add these numbers to your initial encryted message rolling the numbers over so that 18 becomes 8 etc. Either send your now secure message as is or repeat using another book. Write the message onto the back of a letter or postcard using invisible ink (lemon juice or milk work very well). Possibly use the outside of the envelope to write your message on.
As long as the startng place in the book is varied for each message it becomes very difficult to decrypt without the key, and even then needs to be done manually because each code number can have 3 different answers or even 4 if digits were included in the original coding format.
Oooops, I've just described a secure coding system for small cells of crims. or t'rists. Do I hear a black chopper?