Part B

Each question carries $5$ marks $-4$x$5=20$ Marks

Theory answers must be atleast $10$ lines

$13$ A$.$ Explain the four basic tasks in designing a particular security service.

OR

B$.$ Explain details of cryptography

$14$ A$.$ Consider a computer system with three users: Alice, Bob, and Cyndy$.$ Alice owns the file alicerc, and Bob and Cyndy can read it$.$ Cyndy can read and write the file bobrc, which Bob owns, but Alice can only read it$.$ Only Cyndy can read and write the file cyndyrc$,$ which she owns. Assume that the owner of each of these files can execute it$.$

i$.$ Create the corresponding access control matrix.

ii$.$ Cyndy gives Alice permission to write cyndyrc$,$ and Alice removes Bob's ability to read alicerc. Show the new access control matrix.

OR

B$.$ Consider a registration system with three users: User $1,$ User$2,$ and User$3.$ User$3$ owns the file File$3,$ and User$1$ and User$2$ can read it$.$ User$2$ can read and write the file File $1,$ which User$1$ owns, but User$3$ can only read it$.$ Only User $1$ can read and write the file File$2,$ which the user $2$ owns. Assume that the owner of each of these files can execute it$.$ Create the corresponding access control matrix.

$5$ A$.$ With your own example perform Caesar Cipher algorithm