On the manner in which messages most private may be better protected from those who have wish of their contents but no rights to the same

by Bartolomeo Gagliardi

In writing this treatise I once again hope to be of aid to our brave merchant venturers, as they ply their trade across the globe facing all manner of dangers and trials. It is my most humble desire that, just as my previous works have sought to assist in the navigation of nature's tribulations, this submission shall assist with the more human perils that may be encountered.

In the carrying out of commercial enterprises, oft there is a need for discretion and secrecy. Books of account, messages with proposed alliances and partnerships, records of dealings and purchases, all must be kept safe from the eyes of busy fools with no rightful business to peruse them.

To this end, in the Century past, the most learned and august Leone Battista Alberti devised his most ingenious disc within a disc that serves to render unreadable by any but its intended recipient any text the sender wishes to encode.

The device, for those unfamiliar with its use, consists of two disks, one inside the other. On the outside of each disk is printed the letters of the alphabet, aligned so that as one disk rotates the letters on it will face different letters on the other disk. This device is known by many as the Alberti disk, and a picture of it is set out below for the edification of the reader.

Where a message is to be made occult to all those save the intended eye, one must first take the source text hereafter called the plain text and write it out in full. One then takes the Alberti disk and by moving the inner disk a certain amount can change the correlation between the outer and the inner letters, so that whereas once outer A sat along inner A after the rotation outer A may sit along inner F or K or some other inner letter. So by reading accross, one may change each letter of the plaintext to another letter and so create a message that cannot be read by any who knows not by how much the wheel was turned. This rendered text we hereafter call the cryptogram.

So then, to pass the message one must encrypt it using this technique and communicate to the recipient, who must have an Alberti disk of his own, how many rotations were made. This will allow him to reverse the process and reveal the plaintext from the cryptogram. Variants on the disk include the incorporation of numbers to assist with the deliberation of the number of rotations.

This method, while difficult for the determined investigator to break through, is not invulnerable to patient enquiry as within our tongue some letters are used more in words than others and so by seeing which letters in the cryptogram are more used it may be possible to determine the identity of one transposed letter and so from that the number of revolutions used.

Such a flaw in the technique has become well known now to those working on such matters, and so I here propose a method by which this analysis of the frequency of letters may be defeated. This is achieved by use of a written key, which determines the number of rotations by which each letter is used.

Let us take Gagliardi as the key, and assign a number to each letter of my name, for this example let us say that G is equal to seven, A equal to four, L equal to nine, R equal to two and D equal to six. We now take the message to be concealed, let us say it is "They wish eighteen Ducats per item". We place the letters of my name under the letters of the message, restarting my name if the message has more letters than my name contains. Each letter of the message is then subjected to a number of rotations equal to the number alloted to the letter of my name to which that letter of the message corresponds. So, the first letter of the message is T, the first letter of my name is G which is equal to seven. Thus we rotate the letter T seven times on the disk.

This technique destroys the possibility of analysing the code by frequency of use, for the first letter T in the message is rotated seven times but the letter T in the word "eighteen" is rotated nine times as will corresponds to the L in my name. Thus secrecy is assured, at the cost of a mere simple code word which may be easily remembered by both parties if chosen wisely.

I pray that this humble exercise has shown how the mathematic arts and the exercise of logic may confound the enemies of trade and diplomacy, so bringing the greater glory to those most noble merchant families upon which all our fortunes are based.