Add the first letter of the plaintext to the key and use it with next ciphertext

letter to continue decipherment. The Vigenère autokey method is not very secure. There are only 26 possible keys (the 26 letters of the alphabet). The code can be broken easily with an exhaustive search.

Is Vigenère cipher unbreakable?

At first glance the Vigenère Cipher appears to be unbreakable, due to its use of up to 26 different cipher alphabets. Ciphers like this, which use more than one cipher alphabet are known as Polyalphabetic Ciphers. These can be incredibly difficult to decipher, because of their resistance to letter frequency analysis.

Why Vigenère cipher is not secure?

The Vigenère cipher is a polyalphabetic cipher from the 16th century which uses several Caesar ciphers to encrypt the same message that renders more complexity to the whole encryption scheme/process, thereby making the cipher hard to crack.