Aeneas Tacticus 31 (extracts): Sending secret messages; encrypted messages

31. Of Secret Messages

1. As regards secret messages, there are all sorts of ways of sending them: a private arrangement should be made beforehand between the sender and the recipient. I will give some of the most successful methods.
2. A message was once sent in the following manner. A book or some other document, of any size and age, was packed in a bundle or other baggage. In this book the message was written by the process of marking certain letters of the first line, or the second, or the third, with tiny dots, practically invisible to all but the man to whom it was sent: then, when the book reached its destination, the recipient transcribed the dotted letters, and placing together in order those in the first line, and so on with the second line and the rest, was able to read the message.
3. Another, similar way of sending just a short message is this. Write an ordinary letter at some length on any subject, and employ the same device of marking letters, indicating by these whatever you wish, The marking should be made as inconspicuous as possible, either by placing dots at long intervals, or by strokes of unusual length: in this way the message will be intelligible to the recipient, without arousing the suspicions of anybody else.
4. Again, a man may be sent with a message or even a letter on some other subject, not anything private, while a letter is secretly inserted between the sole and the lining of the messenger’s shoes before he starts, and sewn up. In case the road is wet and muddy, the message should be written on a thin sheet of tin to prevent the letters from being obliterated by the water. 4a. When the messenger has reached his destination and is asleep at night, the person for whom the letter is intended must undo the stitches in his shoes, take out the letter, read it, write a reply unobserved while the man is still asleep, sew it up in the sole, and send him off, after giving him the answer to be delivered openly. 5. In this way, neither the messenger nor anyone else will know the secret: only take care to make the stitches in his shoes as inconspicuous as possible.

…

10. Another way of conveying letters is to get a bladder to fit an oil-flask, the bladder being of whatever size you please, according to the length of the letter you wish to send: inflate this, tie it up and dry it thoroughly, then write your message on it in ink mixed with glue. 11. When the writing has dried, let the air out of the bladder, squeeze it and push it into the flask; but let its mouth project beyond the lid of the flask. 12. Then blow up the bladder to its fullest extent inside the flask, fill it with oil, cut of its projecting end and fit it to the mouth of the flask so that no-one will notice it; put a put a bung in the flask, and carry it about openly. The oil will now be plainly seen in the flask, and there will not appear to be anything else in it. 13. When the flask reaches the man for whom it was intended, he will empty out the oil, blow up the bladder and read the message; and after sponging off the writing he may write his reply on the same bladder and send it back.
14. Again, a man has before now poured wax on a writing tablet, after writing on the wooden part, and has written another letter on the wax: when it has come to its destination, the recipient has scratched off the wax, read the letter, written the reply in the same way, and sent it off.
Another device recorded is to write on a boxwood tablet with the very best ink, let it dry, then whiten it over to conceal the writing. When the tablet reaches the man to whom it was sent, he must take it and put it in water: and in the water every word will come out clearly.
15. Again, you may write any message you wish on a votive tablet: then whiten it thoroughly, dry it, and draw on it a picture, say, of a horseman with a torch, or anything else you like; his dress and horse should be white, or, if not white, any colour but black. Then give it to someone to set it up in some temple near the city, as if you were paying a vow. 16. The man who is to read the message must come into the temple, identify the tablet by some prearranged mark, carry it home, and dip it in oil: then all the writing will become visible.

Encrypted Messages
16. ... The hardest method of all to detect, but the most troublesome, that without writing, I will now explain. It is as follows. 17. Take a good sized die [an astragalos or knuckle bone] and bore in it twenty-four holes, six on each side. These holes are to represent the twenty-four letters of the alphabet; 18. and be careful, too, to remember, counting from one side, whichever it is, on which the A comes first, the letters which follow on each side in turn. Afterwards, when you wish to place a message on this contrivance, pass a thread through. Suppose, for instance, that you wish to signify AINEIAS by the way in which the thread is passed through. Begin from the side of the die where the A is, and pass over the succeeding letters till you come to I; when you reach the side where the I is, pull the thread through again; then leave out the next letters, and do the same where N happens to be; then again leave out the next letters and pull the thread through at E; and in the same way copy the rest of the message on the die by passing the thread through the holes, as in the case of the letters AINE, which we have just placed on the die. 19. In this way, there will be a ball of thread wound round the die when it is dispatched, and the recipient must read the message by writing on a tablet the letters signified by the different holes, the thread being unwound from the holes in the reverse order to that in which was wound on. It does not make any difference that the letters are written on the tablet in the reverse order: they will be intelligible just the same. But the task of reading the message is really harder than the composition of it.
20 A handier method would be to get a piece of wood seven or eight inches long, and bore as many holes in it as there are letters in the alphabet; then pass the thread through the holes in the same way as before. When it happens that the thread has to go through the same hole twice, that is when the same letter occurs twice in succession, twist the thread once around the wood before passing it through the hole again. 21. Another plan would be this: instead of the die or the piece of wood, make a wooden disk and polish it; next bore twenty-four holes in a line round the circumference for the letters of the alphabet, and to disarm suspicion, bore holes in the middle as well. After this the thread must be passed through the different letters in the line. 22. When you have to repeat a letter, pass the thread through one of the holes in the middle before returning to the same letter – by ‘letter’ I mean of course hole.

…

30 Again, you may use the following cipher. Arrange beforehand to represent the vowels by dots, a different number of dots according to the order in which each vowels stands in the alphabet. For example:

And the messages in some place known by the recipient, to whom arrival of the man in the city to buy or sell something should be a signal that a letter has come for him, and has been deposited in the place agreed upon. In this way the messenger does not know for whom the letter was brought, nor will it be known that the recipient has it.

…

35. When Glous the Persian admiral went up to see the king, and found it impossible to carry his memoranda into the presence chamber (the matters of which he had to speak being numerous and important), he noted down in the spaces of his fingers the subjects he had to discuss.
The sentry at the gates must keep a sharp lookout for such things as I have described, to see that nothing, whether arms or letters, enters the city unobserved.