Computer science
3. [7 marks] Let’s look at an old Roman age encryption scheme. Let’s say we intercepted a message from a known Celtic hacker group. We know from experience that this group uses the characters from A to Z, then a space, and then the numerals from 0 to 9 and employs a wrap around (moving left from A gives us 9). The first leading pairs of letters tell us what the substitution code is using the
code phrase “Trudy Jones”. For example if the first two pairs are uy du the code is 24 and 32. This would mean that we move the first 2 letters 4 places to the right (eg. A becomes E), and then the next 3 letters 2 places to the left (eg. A becomes 8), then 4 places to the right for the next 2 characters and so on to encrypt the message.
a. What is the exact message hidden in: yo ud OZGRF7T5YZ7IROUT? Remember that
you’ll have to reverse the algorithm (i.e. shift left first then right then left etc.) to decrypt the message.
b. The message is actually describing a fictional character. Give the first name in upper case. What would be the encrypted version of this name using the same encryption scheme as the original message in Part (a) of the question?

4. [9 marks] Now let’s look at a more modern symmetric encryption. Assume that the algorithm for this system is to rotate the bits in the message right 6 positions, XOR the bits with the key (repeat the key so as all the bits are covered), and rotate the bits 4 positions to the left.
a. I will email you the key which is encoded within a spam email message. Please be on the lookout for this message. Once you receive the email go to http://www.spammimic.com/ and click on Decode. Cut and paste the body of your email into the Decode window and get the key. The key will be 2 hex numerals. If you don’t get 2 hex numerals when you decode the message, you haven’t copied the entire email message. What is the key?
b. Using the key and the above algorithm, decrypt the following name (given in hex): 77 B3 F1 B6 36 71. Remember that you’ll have to reverse the order and direction of operations in order to decrypt the name (i.e. Start by rotating 4 positions right). What is the name exactly as it is decrypted?
c. The answer to Part (b) is the name of a Roman god. Find (and state in upper case) the name of his father and, using the key and the above algorithm, encrypt the name (in upper case) and put in hex format

5. [5 marks] One failing of an encryption system occurs if we never let the encrypted value of a letter be the same as the original, especially if a letter always encodes to the same value. Let’s see how we can exploit these weaknesses. Let’s assume that we know that the word STAIRWAY is in an intercepted message. If we received the following transmission, what are the encrypted values (give the mapping) of the letters S, T, A, I, R, W, Y (i.e S = ?, T = ?, etc.)?
YZGOX1G3ZUNKG0KT
To test your detective skills, what is the exact original message? (It’s a simple cipher – words are not separated by spaces.)