Περὶ τοῦ πῶς χρὴ πολιορκουμένους ἀντέχειν (How to Survive Under Seige) written circa 360 BCE by Aeneas Tacticus, one of the earliest Greek writers on the art of war, was the first handbook on urban warfare. It also provided the first extensive guide to securing military communications. Humanist and philologist Isaac Casaubon, who issued the editio princeps of Aeneas Tacticus as an appendix to his Greek and Latin edition of Polybius, Historiarum libri qui supersunt (Paris, 1609), considered Aeneas Tacticus a contemporary of Xenophon and identical with the Arcadian general Aeneas of Stymphalos, whom Xenophon (Hellenika, VII.3.1) mentioned as fighting at the Battle of Mantineia (362 BCE).
From Aineiou Poliorketika = Aeneas On Siegecraft: A Critical Edition prepared By L.W. Hunter; Revised With Some Additions By S.A. Handford (1927) I quote chapter 31:
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.
6. Again, a message was brought to Ephesus in the following way. A man was sent with a letter written on leaves, the leaves being bound on a wound in his leg.
7. Again, writing may be conveyed in women’s ears, wrapped in thin pieces of lead worn instead of earrings.
8. Again, a letter containing an offer of betrayal was once conveyed by a traitor into the enemy’s camp near at hand in the following manner. One of a troop setting out from the city for a foray had a note sewn up under the skirt of his cuirass, with orders, if the enemy came into view, to fall from his horse as if he had been thrown, and allow himself to be made a prisoner; on arrival in the enemy’s camp he was duly to deliver the note. In this case he was assisted by a brother trooper.
9. Another man sent out a trooper with a note sewn up in his bridle rein. Here is another story about a letter. During a siege the bearer of some letters arrived within the city, but, instead of delivering them to the traitor and those for whom they were intended, went and laid information before the governor, and offered the letters to him.
9a. On hearing his story, the governor bade him deliver the letters he already had to those for whom they were intended, but to bring the traitor’s reply to him, if there was any truth to his story. His informant did so; whereupon the governor, after receiving the replies, summoned the traitors and confronted them with the seals of their own signets, which they were forced to acknowledge, and then opened the letters and discovered the plot.
9b. He certainly convicted them very cleverly by not taking the original letters from the bearer: for the traitors might have denied complicity and asserted that it was a plot against them; but by getting hold of the replies he convicted them all beyond dispute.
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. 1
6. 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. 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. 1
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.
23. Again, a note has been written on very thin papyrus, in long lines of fine characters, so as to make the packet as small as possible; it was then inserted into the shoulder of a tunic [chiton], and part of the tunic folded back on the shoulder. A good way of getting the letter through without suspicion would, I think, be for a man to put the tunic on and carry it in this way.
24. Here is the proof of the difficulty of thwarting plots for bringing things into a city. The men round Ilion, after all this time, and in spite of their efforts, are not yet able to prevent the Locrian maidens from coming into their city, for all their eager watching: a few men by studious precautions have managed to smuggle in women unobserved every year.
25. In earlier years the following trick was once played. Timoxenus wished to betray Potidaea to Artabazus: they therefore agreed upon a certain spot in the city chosen by Timoxenus, and one in the lines chosen by Artabazus, into which they used to shoot arrows carrying any information which they wished to communicate to each other, ; the following was the device they used: they wound the note round the grooved end of an arrow, which they then feathered and shot into the places agreed upon.
26. But Timoxenus’ treachery was discovered: for Artabazus shot in the usual direction, but owing to the wind and the bad feathering of the arrow missed his mark, and hit a Potidaean in the shoulder. As often happens in war, a crowd ran up to the wounded man: and they at once seized the arrow and took it to the generals, so that the plot was discovered.
27. Again, when Histiaeus wished to communicate with Aristagoras, and could find no other safe means of sending a message, as the roads were guarded and it was very difficult for a letter to get through without detection, he took his most trusty slave and shaved his head, then tattooed the message on it, and waited till the hair grew again. As soon as it had grown, he sent him to Miletus, with no other orders than to tell Aristagoras, when he reached Miletus, to shave his head and examine it. The marks told Aristagoras what to do.
28. 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:
DEAR DIONYSIOS D: . R D:. ::N:::S:. :: S
HERACLEIDAS WANTED H : R . CL : :. D . S W . NT : D
29. 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.
30. Dogs were often used in Epirus in the following way. They led them away from their homes on leashes, and fasted round their necks a strap in which a letter was sewn up. Then, either by night or by day, they let them go and find their way home, which they were sure to do. This method is used in Thessaly.
31. All letters that arrive should be opened at once. A letter was sent to Astyanax, tyrant of Lampsacus, containing information of the plot which proved fatal to him: since, however, he did not open it at once and read the contents, but took no notice and attended to other business first, he was murdered with the letter unopened in his fingers.
32. The same delay caused the capture of the citadel in Thebes, and something like it happened in Mytilene in Lesbos.
33. 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."