Answer the following short questions a. Caesar Cipher: Julius Caesar is about to invade Britain (again) and he sends an encrypted message to one of his top commanders in the area that is now known as Cardiff, Wales. Unfortunately (or fortunately depending on how you look at it) his message is intercepted by an intrepid band of Britons and they come to you asking for help in decrypting the message. Here is the encrypted text: (15 points) DWWDFNDWGDZQ I. Using your knowledge that this message was encrypted using a Caesar cipher with a shift of 3 help these poor Britons decipher Caesars message.

II. After deciphering the message the Britons decide to send back an impudent message to the despot Caesar. They ask you to encrypt the message GOBACKTOROMEYOUJERK . Using the same cipher that Caesar used, what cipher text should be output to match this plaintext?

b. What are the next three numbers in the pseudo-random number generator 3x_i +2 mod 11, starting from 5? (5 points)

c. Show the result of encrypting M = 4 using the public key (e; n) = (3; 77) in the RSA cryptosystem. (5 points)

d. Benny is a thief who tried to break into an Automated Teller Machine (ATM) using a screwdriver, but was only able to break five different keys on the numeric keypad and jam the card reader, at which point he heard Alice coming, so he hid. Alice walked up, put in her ATM card, successfully entered her 4-digit PIN, and took some cash. But she was not able to get her card back, so she drove off to find help. Benny then went back to the ATM, and started entering numbers to try to discover Alice's PIN and steal money from her account. What is the worst-case number of PINs that Benny has to enter before correctly discovering Alice's PIN? (10 points)

e. Give an example of the false sense of security that can come from using the security by obscurity" approach. (5 points)

Cyber Security

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