Computer science

1. [5 marks] Let’s get some practice with the binary and hexadecimal number systems
a. Convert the hex value A1B2C3 into binary
b. Convert the binary value 1100 0000 0001 1101 1100 1010 1011 0101 into hex.
c. Convert the binary value 1000 0100 0010 0001 0011 0110 1100 1111 into hex.
d. What is ABCFFE + 2 equal to (assuming hex values)?
e. What is 133AC + AC567 equal to (assuming hex values)?
2. [4 marks] Now let’s get some practice converting text to its hex equivalent value.
a. What is the ASCII representation in hex for the word Crime?
b. What is the ASCII representation in hex for the word HACKER?
c. What is the ASCII representation in hex for the phrase Good Luck?
d. What is the text message represented by the following hex digits?
4F 6E 63 65 20 55 70 6F 6E 20 41 20 54 69 6D 65
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 message hidden in du yd JQD8FQXFTCDLK? 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. What would the encrypted name
of this character be using the same encryption scheme as the original message?
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 4 positions, XOR the bits with the key, and rotate
the bits 8 positions to the left.
a.  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 that 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): 22 E2
93 43. Remember that you’ll have to reverse the order and direction of operations in order
to decrypt the name (i.e. Start by rotating 8 positions right)
c. The answer to part b is the name of a Norse god. Find 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 name MITNICK is in an
intercepted message. If we received the following transmission, what are the encrypted values of
the letters M, I, T, N, C, K?
HTGGMGXKPOKVPKEM

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