Computer science

3. [7 marks] Let’s look at an old Roman age encryption scheme. Let’s say we intercepted a messagefrom a known Celtic hacker group. We

know from experience that this group uses the charactersfrom A to Z, then a space, and then the numerals from 0 to 9 and employs a wrap

around (movingleft 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 thenext 3 letters 2 places to the left (eg. A becomes 8), then 4 places to

the right for the next 2characters and so on to encrypt the message.
a. What is the exact message hidden in: youd OZGRF7T5YZ7IROUT? Remember that
you’ll have to reverse the algorithm (i.e. shift left first then right then left etc.) to decrypt themessage.
b. The message is actually describing a fictional character. Give the first name in uppercase. What would be the encrypted version of

this name using the same encryptionscheme 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 thissystem is to rotate the bits in

the message right 6 positions, XOR the bits with the key (repeat thekey 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 thelookout 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 andget the

key. The key will be 2 hex numerals. If you don’t get 2 hex numerals when youdecode 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 B3F1 B6 36 71. Remember that you’ll have to

reverse the order and direction of operationsin order to decrypt the name (i.e. Start by rotating 4 positions right). What is the

nameexactly as it is decrypted?
c. The answer to Part (b) is the name of a Roman god. Find (and state in upper case) thename of his father and, using the key and the

above algorithm, encrypt the name (in uppercase) and put in hex format

5. [5 marks] One failing of an encryption system occurs if we never let the encrypted value of a letterbe the same as the original,

especially if a letter always encodes to the same value. Let’s see howwe can exploit these weaknesses. Let’s assume that we know that

the word STAIRWAY is in anintercepted 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 arenot separated by spaces.)
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