Charles Babbage and deciphering codes

Charles Babbage wrote an autobiography Passages from the Life of a Philosopher which was published in London in 1864. In our biography of Babbage we have quoted several passages from the book which tell us about his life and the analytical engine. Here we quote from Chapter XVIII of the book where Babbage writes about deciphering. What we present here is only an extract. In fact we omit a more technical part which describes the considerable effort that he had put in constructing dictionaries with particular properties to assist in deciphering:-

Deciphering is, in my opinion, one of the most fascinating of arts, and I fear I have wasted upon it more time than it deserves. I practised it in its simplest form when I was at school. The bigger boys made ciphers, but if I got hold of a few words, I usually found out the key. The consequence of this ingenuity was occasionally painful: the owners of the detected ciphers sometimes thrashed me, though the fault really lay in their own stupidity.

There is a kind of maxim amongst the craft of decipherers ..., that every cipher can be deciphered.

I am myself inclined to think that deciphering is an affair of time, ingenuity, and patience; and that very few ciphers are worth the trouble of unravelling them.

One of the most singular characteristics of the art of deciphering is the strong conviction possessed by every person, even moderately acquainted with it, that he is able to construct a cipher which nobody else can decipher. I have also observed that the cleverer the person, the more intimate is his conviction. In my earliest study of the subject I shared in this belief, and maintained it for many years.

In a conversation on that subject which I had with the late Mr Davies Gilbert, President of the Royal Society, each maintained that he possessed a cipher which was absolutely inscrutable. On comparison, it appeared that we had both imagined the same law, and we were thus confirmed in our conviction of the security of our cipher.

Many years after, the late Dr Fitton, having asked my opinion of the possibility of making an inscrutable cipher, I mentioned the conversation I had had with Davies Gilbert, and explained the law of the cipher, which we both thought would baffle the greatest adept in that science. Dr Fitton fully agreed in my view of the subject; but even whilst I was explaining the law, an indistinct glimpse of defeating it presented itself vaguely to my imagination. Having mentioned my newly-conceived doubt, it was entirely rejected by my friend. I then proposed that Dr Fitton should write a few sentences in a cipher constructed according to this law, and that I should make some attempts to unravel it. I offered to give a few hours to the subject; and if I could see my way to a solution, to continue my researches; but if not on the road to success, to tell him I had given up the task.

Late in the evening of that day I commenced a preparatory inquiry into the means of unravelling this new cipher, and I soon arrived at a tolerable certainty that I should succeed. The next night, on my return from a party, I found Dr Fitton's cipher on my table. I immediately commenced my attempt. After some time I found that it would not yield to my means of treating it; and on further examination I succeeded in proving that it was not written according to the law agreed upon. At first my friend was very positive that I was mistaken; and having taken it to his sister, by whose aid it was composed, he returned and told me that it was constructed upon the very law I had proposed. I then assured him that they must have made some mistake, and that my evidence was so irresistible, that if my life depended upon the result I should have no hesitation in making my election.

Dr Fitton again retired to consult his sister; and after the lapse of a considerable interval of time again returned, and informed me that I was right - that his sister had inadvertently mistaken the enunciation of the law. I now remarked that I possessed an absolute demonstration of the fact I had communicated to him; and added that, having conjectured the origin of the mistake, I would decipher the cipher with the erroneous law before he could send me the new cipher to be made according to the law originally proposed. Before the evening of the next day both ciphers had been translated.

This cipher was arranged upon the following principle: - Two concentric circles of cardboard were formed, each divided into twenty-six or more divisions.

On the outer were written in regular order the letters of the alphabet. On the inner circle were written the same twenty-six letters, but in any irregular manner.

In order to use this cipher, look for the first letter of the word to be ciphered on the outside circle. Opposite to it, on the inner circle, will be another letter, which is to be written as the cipher for the former.

Now turn round the inner circle until the cipher just written is opposite the letter a on the outer circle. Proceed in the same manner for the next, and so on for all succeeding letters.

Many varieties of this cipher may be made by inserting other characters to represent the divisions between words, the various stops, or even blanks. Although Davies Gilbert, I believe, and myself, both arrived at it from our own efforts, I have reason to think that it is of very much older date. I am not sure that it may not be found in the Steganographia of Schott, or even of Trithemius.

Last Updated March 2006