(a) Assuming the ciphertext is ATTACK, we need to find a plaintext (which exists in English dictionary) that would encrypt to the ciphertext above (Don't use attack).
In order to find the plaintext that would encrypt to the ciphertext ATTACK, we need to apply the reverse monoalphabetic substitution cipher, i.e., use the inverse substitution rule. Here, we have: A → ?T → AK → T? → ?Therefore, the plaintext that would encrypt to the ciphertext ATTACK is either ETUDE or ASTIR, which are the only two English words that can be obtained using the above substitution rule.
(b) Using the plaintext ETUDE, we can find the key that produces the ciphertext ATTACK. Here, we have: E → A T → T U → K D → C Therefore, the key that produces the ciphertext ATTACK is the substitution cipher: A B C D E F G H I J K L M N O P Q R S T U V W X Y ZE D C V W X Y Z R S T U F G H I J K L M N O P Q. The inverse key is obtained by switching the positions of the plaintext alphabet and the ciphertext alphabet of the key used to encrypt the plaintext.
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