It's a secret (1)

(See post dated 4/2/15)
So A wants to send a message to B which is for B's eyes only.  The need or desire to do this has been around for ever, and many ways have been found to accomplish the task.  Back in the mists of time, we are told of a powerful ruler who, wishing to send a secret message to someone, would call for one of his slaves, have the slave's head shaved, write his message on the slave's scalp and then lock him away until his hair grew again.  The slave would then be sent as one of a party to whoever the message was meant for and who (obviously being in the know) would have the slave segregated, shave his head, read the message and then have the poor slave killed and his body burned so that no-one else would know what the message said. This is an example of what is known as steganography (two Greek words steganos = hidden, and graphos = writing).  The idea being to conceal from any outside observer the fact that a message is being sent.  A modern-day equivalent would be invisible ink - A would take a document which he might quite reasonably be sending to B and then on blank parts of the document write his message in liquid which vanishes as it dries but can be made visible again by some process (often just the appliance of heat).  Once again, the idea is that to an observer A is simply sending an innocent document to B - only B knows of the existence of the secret message,  More to come...