Imagine you are a secret agent and intercepted the following encrypted message (ciphertext): 

ZNK GMKTZ ZNKXK OY ZNK UTK 

Unfortunately, you do not have the key for the encryption algorithm. Decrypt the above ciphertext knowing that the message was encrypted using a substitution cipher in which each plaintext character (an alphabet letter) is replaced by a ciphertext character using the key.

Write your answers on white paper, showing all your workings, scan or take pictures of them, and upload the resulting file. (If you have more than one page, combine them into one file.)