I was watching a Stanford lecture on Vigenère cipher and in it the professor said that – to break the cipher – we assume the length of the key is known. We then break the cipher into groups of this known length and then pick up (successively) the first, second, etc. members of each group and then break each of these using Caesar cipher.

The problem is: how would one break the cipher if the message length is short? Because then, we can not use the method which utilizes the frequency of letters to break the code.

For example:
Given the ciphertext ZZZJUCLUDTUNWGCQS and key is of size 6, can you crack this?
(Assuming you don’t know that the plaintext is WHATANICEDAYTODAY and the key is crypto.)