The One-Time Pad

 This is the curse of all intelligence agencies because unless a copy has been compromised and is in their hands it is not dechiperable. They usually come in the form of a document with about sixty pages of which there are only two copies. When the sender wishes to get a message to the recipient, which should be short so as to avoid a technique for checking the most used letters in the alphabet, he or she transposes a letter of the alphabet which they wish to send for a substitute letter on the one time pad. An example follows:-

 ABCDEFGHIJKLMNOPQRSTUVWXYZ
 LFCGKLDXPVNEHLOIZTGLVRUIOPC

 For each word that I wish to send I use the above only once. So if I want to send THE, I would send VXK.

If the next word that I want to send is TIME I would go to the next row in the one time pad, for example:-

ABCDEFGHIJKLMNOPQRSTUVWXYZ
VBZZFGRDXNHWQHHLNBTNZQKHG

and send VXKTXWF (THE TIME). Note the use of no spaces. Once the message has been sent both the sender and receiver should destroy their copies. Simples, if not the following from Wiki, that most helpful organisation ever to embrace the Internet, is more comprehensive and definitive than the above:-

http://en.wikipedia.org/wiki/One-time_pad