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  • Lara-Johanna Pirk

The Creation of the Vigenère Cipher

After hundreds of years of only using substitution ciphers, cryptanalysts had gotten the upper hand in the war between cryptographers and cryptanalysts. All substitution ciphers were easily cracked by using frequency analysis no matter what symbols you used. Cryptographers needed to find a new cipher if they wanted to communicate in secret. The beginning of the Vigenère cipher can be traced back to the fifteenth-century Florentine Polymath Battista Alberti.

Alberti was born in 1404 and was one of the leading figures of the Renaissance. He was a painter, composer, poet and philosopher. He is also well known as an architect as he designed Rome’s first Trevi fountain. After meeting his friend Leonardo Dato, he became interested in cryptography and wrote an essay on the subject. He believed his essay introduced a new type of cipher. His idea was that instead of using one substitution cipher, you could use two. You would then encipher your text by using the first substitution cipher on the first letter and the second on the second letter. You would then repeat the process and use the first substitution cipher for the third letter and the second for the fourth letter. Repeating the process like this would confuse the cryptanalysts and no form of frequency analysis would work. Although this was one of the biggest breakthroughs in hundreds of years, Alberti failed to develop it into a used cipher.

Other people kept on developing this idea, including Johannes Trithemius and Giovanni Porta. Lastly, it fell into the hands of Blaise de Vigenère. He brought together all the research from Alberti, Trithemius and Porta and wove it into a new and more powerful cipher. Although many others contributed to the cipher it was known as the Vigenère cipher because of the man who made it into its final form.

The strength of the Vigenère cipher is that it not only uses one but 26 different substitution ciphers. To encrypt a message, the easiest way is to first draw a Vigenère square.

You then choose a key, for example, hi, and encrypt your plaintext according to the key. If your plaintext is Book, then you encrypt B according to the H column in the Vigenère square which would be I. Next, you would encrypt the letter O according to the I column which would be W. You then repeat with the key from the beginning so the second O would be encrypted with I. You then continue until you have encrypted the whole message.

The Vigenère cipher was a major breakthrough for cryptographers and would stay uncracked for many years to come as well as help messages stay secret for military and private purposes.

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