Diving into the History of Cryptography II

Last time we discussed about how cipher techniques used to be back in Romulan Era.. Now fast forwarding into time, we will see how cipher techniques developed by the Renaissance Era. There used to be a Roman Cryptologer called Vigenere and he came up with an idea that might be used in way ahead of his time. Although the way he implemented it, was not so secure but the idea was really nice. Read ahead to find out more. 

2.)  Vigener Cipher ( 16th Century Rome) :

So basically what Vigener did was he designed a method for encrypting the letters by providing a key. And he assigned a variable to the plain text and then he will just add the key with plain text with modulo 26. So the new text received will be a cipher text.

                For e.g. :

                K(key) = CRYPTO

                W(plain Text) = WHATANICEDAY

               

                Therefore the key will become of the same length to that of the plain text and the new key will be K = CRYPTOCRYPTO and now we will add all the letters with modulo 26. That is the first letter of the plain text and the first letter of the key W and C will add to form Z and likewise every letter from the Key will be added to the plaintext and the new text received will be our Cipher text. The Decrypting process is also as easy as encryption process all you need to do is subtract letters of the cipher text from the key to get the original plain text. So the encryption will look like this:

K = CRYPTOCRYPTO

W = WHATANICEDAY

C = ZZZJUCLUDTUN

So breaking the vigener cipher is actually quite easy. What we need to do at first is we need to assume the length of the key. In the above case we will assume it to be 6 and then we will divide the cipher text into the groups of six letters. And then we’re gonna look into the first letter in each group. Now what we will actually do by looking at the first letters from each groups is that we know that each of them are encrypted with the same letter using the letter c. SO now we know that the most common letter from the english text is the letter “E”. So the most common letter in the group of six letters is most likely to be the encryption of the letter “E”. So lets say that the most common  letter in the group is H and by subtracting the letter E we get the encryption key’s first letter. That is H – E = C. SO this way we get the first letter of the key and likewise we ca get every other letter of the encryption key. So the only problem in this solution is that we had to assume the length of the key and that was in this case 6. But what if we don’t know the length of the key ahead of time? WE just start off by assuming the length of the key as 1 and then we check for the length of key as 2 and so on and keep guessing it until the decryption message makes some sense. So this way the viginer cipher can be broken very easily and this was also the cipher text only attack. The good thing about this cipher was the method of using modulo 26. It was just poorly used in this case but can be potentially be used as a very secure cipher method.