Monoalphabetic Cipher Key I am not sure how to get the key for the following Monoalphabetic Cipher question. This is a textbook question and I know the answer, but I juts dont know how they got the key.
Question:

Decrypt the following Cipher texts, and give the key.  (Note that the correct key is the Encryption key, so if F changes to H and F is the plaintext, then the key is “C”):
(a) YRRYAIYRMLAC (b) DOBXQPZELLI

Answer: a) Text:ATTACKATONCE Key = X  b) Text: GREATSCHOOL Key = A
Can somone please explain to me how to determine the key?
You try all 26 possibilities, and check which of them results in an English-looking test. In principle you can do the latter statistically, but in this case there are only 26 possibilities, so you might as well just look at all of them manually.