a$.$ For the given plaintext: "THIS IS A SECRET MESSAGE", what will be the ciphertext if you shift forward seven $(7)$ times?

$(10$ marks$)$

b$.$ With a key of $4213,$ get the ciphertext using columnar transposition cipher for the following plaintext: "ATTACKATDUSK"

$(6$ marks$)$

c$.$ Explain three $(3)$ limitations of classical ciphers in modern security.

$(6$ marks$)$

d$.$ Explain three $(3)$ cryptanalysis techniques