$4.$ Which of the statements below are correct

A$)$ A condition for the affinity cipher to work with a key $($a$,$b$)$ is that for having a greatest common divisor for a and m $($number of letters in alphabet$)$ equal to $1,$ which will mean that the cipher's encryption function is injective.

B$)$ In cryptography unencrypted message is referred to as the plaintext, while the encrypted message is referred to as the ciphertext.

C$)$ An exhaustive key search attack $($brute$-$force type of attack$)$ is considered effective against substitution cipher.

D$)$ Unconditional security of a cipher can be referred to as information$-$theoretic security $($meaning it cannot be broken even with infinite computing resources$)$ and is only true for the OTP cipher, however under a condition that the truly random key $($of length such as of the plaintext message$)$ can be distributed between the parties, which can only be done unconditionally securely using the quantum key distribution.