One-time Pad – The Principle and History of Origin

Today we will focus on one type of cipher, namely a cipher that cannot be decrypted without the knowledge of key, as proven by exact mathematics. By no means.  And since you are, along with all the other technical experts who have ever lived on this planet, skeptical about any absolutely uncrackable ciphers, as well as absolutely unsinkable ships, we will examine its principles, properties, and the history of its origin in more detail than we have done with other ciphers in previous chapters.

The basic principle of this cipher is using a key extensive enough to encrypt each character in the original message with another part of the key, i.e. to avoid reusing any part of the key (for encryption of more than one character in the original message).

One-time pad had apparently been invented repeatedly and independently in various places and during various times. It was not until the WWII when the need to send a classified information had become significant enough to motivate both the experts to try and join the home defense in their fields and the governments to provide a sufficient space and means for the steady development of their ideas.

Back in 1882, Frank Miller described the one-time pad principle for telegraph security. In 1917, Gilbert Vernam (of AT&T) invented a teleprinter security principle based on one-time pad which was patented in 1919 as the U.S. Patent 1,310,719 and subsequently refined by Joseph Mauborgne by using a teleprinter tape with random punched hole combinations as the key.

The inventor’s original intention was to use the device commercially and sell it to trading companies. However, the market did not show enough interest in the device due to its high operating costs. A second attempt to sell it to the armed forces in some countries during the WWI was equally unsuccessful.

It was during the WWII when Claude Elwood Shannon (who had worked at Bell Laboratories since 1942 and who is called the “father of the theory of information”) noticed the older patent and tried to apply it to the U.S. Army needs. His 1945 work was classified and remained unpublished until 1949. His article in ‘Communication Theory of Secrecy Systems’ is considered the beginning of the modern mathematical cryptography. In this article, Shannon, among other things, proved mathematically that the cipher principle itself made it impossible to decrypt the delivered message without the knowledge of key.

The proof is, by the way, surprisingly simple; it can be found after spending a few minutes on the Internet, and its principle is intelligible and verifiable by common sense.

Instead of the usual anecdote, let me conclude today with a provocative question: why has this sole uncrackable cipher not been used for a long time already? You will find the answer (unless you figure it out or research it yourself) at the beginning of the next chapter in our series.