Unbreakable Ciphers

If we use ciphers and these ciphers leave behind clues they will be broken. Do we have ciphers that do not leave behind clues? Do we, in other words, have unbreakable ciphers?

Cryptologists have understood for a long time where clues come from, so yes there are unbreakable ciphers. These are ciphers that are not only practically unbreakable but theoretically unbreakable. When cryptologists talk about unbreakable ciphers without qualification they usually mean theoretically unbreakable.

A quick way to understand how we can construct an unbreakable cipher is the following. Encrypt a random string of alphabets using a simple substitution cipher (that is, anything resembling the Dancing Men Cipher). Examine the ciphertext thus obtained for clues. You will find that however hard you look, you will not discover any. Why? Because the ciphertext is just another random string. Clues are the characteristics of structures. A random string has no structure. A string without structure to begin with cannot acquire the structure of English or any other language after encipherment.

But we do not send each other random strings to communicate. When we send messages the cleartext is not a random string. However, if we can do something to the cipher so that it will always generate a random string as the ciphertext whether the cleartext is random or not, this cipher will be unbreakable--because there will be no clues.

How can we construct a cipher that will always generate random strings?

Simple! Just do the following. In encrypting your message change your cipher after every letter in a random way. If you proceed in this fashion your opponents, even if they know how you encrypt the first letter in your message, will have no way of knowing how you encrypt the second, and the third, and the fourth and so on. Needless to say, a cipher like this is very clumsy. For one thing, the intended receiver of your messages will have to know in advance the actual sequence of ciphers you use (which cipher for the first letter, which for the second, which for the third, and so on). Because they are clumsy unbreakable ciphers were not used all that often in the old days. But nowadays we have computers. What is clumsy for human beings is not clumsy for computers. Nowadays, theoretically unbreakable ciphers are more common than before.