The Vigenère cipher was thought to be completely unbreakable for hundreds of years, and indeed, if very long keys are used the Vigenère cipher can be unbreakable. But if short keys are us

The Vigenère cipher was thought to be completely unbreakable for hundreds of years, and indeed, if very long keys are used the Vigenère cipher can be unbreakable. But if short keys are used, or if we have a lot of ciphertext compared to the key length, the vigenere cipher is quite solvable. 

In this  you need to study how a cryptanalyst thinks and breaks the code. Applying the Cipher Text Only attack, your job is to break the Vigenère cipher using cryptanalytic techniques and find the key used for the encryption. For this exercise, assume the key length is less than five characters long and only English upper case letters from A-Z are used. You may also assume the plaintext contains only the upper case English letters from A-Z, ignore the space characters.