Questions$/$Tasks:

a$.$ The following cipher text is obtained using a rail$-$fence method with $4$ rails.

What is the plaintext? What does it mean in English?

$(5$ marks$)$

FNIRIUFDOTLAENLS

b$.$ Consider a language consisting of the alphabets $\{A,B,$dots,$Z\}.$ These are used in the affine

cipher of the form $y=ax+b,$ where $a=7,b=15.$

i$.$ Encrypt the plaintext message "HOWDAREYOU"

ii$.$ Decrypt the ciphertext "QRELRGREPCDR"

iii. What is the keyspace for this cipher?

iv$.$ How can you modify the cipher to increase the number of possible keys?

$(5$ marks$)$