$1.$ Which statements are correct

A$)$ A block cipher according to Shannon should comprise two atomic operations, i$.$e$.$ confusion $($obscuring relationship between plaintext and ciphertext$)$ and diffusion $($single bits of ciphertext influenced by many bits of plaintext$).$

B$)$ In cryptography key space refers to possible keys used by a certain cryptographic protocol.

C$)$ In modular arithmetic under a mod $5$ operation the numbers $7$ and $12$ are equivalent.

D$)$ Substitution cipher is an example of a historical cypher in which every plaintext letter is replaced by another fixed letter, hence it is an example of an asymmetric cipher.