The Vignere cipher

The Vignere cipher is a method of encryption which uses caesar cipher on each letter based on the letters of keyword.In caeser cipher each letter of the plaintext is shifted number of tomes depending upon the key value for examlple if the key=2 then the letter A is shifted two times and encrypted as C.Vignere cipher uses this technique on each of the letters but with different keys each time.That is the reason it appears unbreakable at first.But it is quite easy to understand and implement.

For encrypting the plaintext vignere cipher uses vignere square or tabula recta which is like 26 X 26 table consisting of alphabets A-Z on ech row and columns.The columns in a table are such that each next column first alphabet is obtained by shifting previous column alphabet by one.

Here is the figure of vignere square or tabular recta

For the Encryption process the cipher uses different alphabets from the rows each time depending upon the alphabet key is having.So the process of encryption is quite simple.

Lets take the plaintext be ILOVETHISBLOG

Now for encryption process one has to take certain keyword or key of any length.Lets assume that you are encrypting the plaintext with the keyword AGREED.Now you can see that the length of keyword and plaintext is not same.So in vignere cipher if the length of key and plaintext are not same then we repeat the keyword until its length become same as plaintext.

To do that we are going to repeat the keyword again from start after the last alphabet of key.So now it will be like AGREEDAGREEDA

Now comes the part of encryption the plaintext.For encryption one has to see the vignere square and take the first alphabet of plaintext and keyword and use it as a column and row respectively.For example I is taken from plaintext and A is taken from keyword.Now see the entry corresponding to column I and row A.The value in that entry will be substituted in the first place of cipher.So the value for column I and Row A will be J.Similarly repeating the above process for every alphabet will yeild encrypted text.

Plaintext    : I L O V E T H I S B L O G

Keyword : A G R E E D A G R E E D A

Cipher     : J S G A J X I P K G Q S H

The recipient of the message should know the key before hand for decrypting the code.

There is another method used in vignere cipher which is even more complex to decrypt.In this technique the keyword is taken as same as plaintext to encrypt but include one special alphabet at the beginning of keyword which is called as the priming letter.

So for the plaintext ILOVETHISBLOG.The keyword will be KILOVETHISBLO where "K" is the priming letter.The priming letter should be known to both the sender and recipient , only then he will be able to decrypt the code.

This method is generally not used as it is little bit difficult to crack and time consuming.