Basics of Cryptography: Vigenére Cipher

The Vigenère cipher is a method of encrypting alphabetic text by using a series of different Caesar ciphers based on the letters of a keyword. It is a simple form of polyalphabetic substitution. Vigenére Cipher has been reinvented many times. The method was originally described by Giovan Battista Bellaso in his 1553 book La cifra del. Sig. Giovan Battista Bellaso; however, the scheme was later misattributed to Blaise de Vigenère in the 19th century, and is now widely known as the "Vigenère cipher".

Though the cipher is easy to understand and implement, for three centuries it resisted all attempts to break it; this earned it the description le chiffre indéchiffrable (French for 'the indecipherable cipher'). Many people have tried to implement encryption schemes that are essentially Vigenère ciphers. Friedrich Kasiski was the first to publish a general method of deciphering a Vigenère cipher, in 1863.

Step 1: Creating the Cipher

*First write down the "Tabula Recta"(The Vigenére Square).

Now we shall determine a key for a cipher and a text to encrypt, in this case; our key word is "CAT" and text is "Instructables".

(There are several ways of creating the cipher. They are all in the same logic but in slightly different ways.

Now I will continue with Vigenére Square, than show the rest of them.)

*Key will repeat it self as long a the plaintext.

Now if we apply our chart to Vigenére Square we will be able to encrypt it. By following the lines we can see that "K" is where our key and plain text crosses and it is the first letter of our cipher. After we repeat this to all letters we get the cipher text.

Step 2: Creating a Custom Chart for Encrypting

*Instead of writing all of the alphabet, write down the key letter vertically (and repeat it according to your letter lenght) and the alphabet horizontally. After you create the table, you can encrypt your letter same as before but you shall go one line down for the next word as shown in the picture.

*You can see how you can encrypt the "Instructables" letter with "CAT" key on the second chart.(I didn't show all of the words, because the chart would be too complicated.)

*If you are going to cipher a long text, instead of rewriting the key over and over again as shown as on the first chart, you can write the key only one time and and repeat it by your self as the encryption goes.

Step 3: Creating Numeric Charts for Encrypting

*Write down the alphabet and number them from 1 to 26.

Give numbers to your key and text according to the chart above.

*After you have given numbers to all words, add the number together which are on the same column and subtract 1.

If the result is bigger than 26 than subtract 26 and use the result a your number.

When you convert the numbers to letters with the chart, you get the cipher.

Step 4: Decrypting

Decrypting is the same is encrypting but you will do the process backwards.

Derypting with the Chart:

*Write down the chart, mark the cipher word in order and going one line down after each word.

The word above the marked word is decrypted word. Repeat this to all and you will get the decrypted message.

You will be doing the same on the first chart (Vigenére Square) I have shown on step 1.

Decrypting using Numeric method:

*Write the numeric equivalent of the cipher, and the key underneath.

Subtract the value of the key from the cipher text.

If it is less than 0, add 26.

Convert numbers to their alphabetical equivalents.

Step 5: The End

This is the end of this instructables. There might be some spelling mistakes, anacoluthon and mixed sentences.

I would be glad if you notice me if there is any wrong information or any error on grammar.

Thank you for reading.

Sources: https://en.wikipedia.org/wiki/Vigenère_cipher