Question

3. We have the following encryption algorithm: XOR with the HEX key AA, rotate 4 places to the right, and XOR with the HEX key BC. Show your work for full marks.

a. If we start with the bit pattern represented by the HEX number ACE1, what would the HEX number be that represents the encrypted bit pattern?

b. Assume that we have intercepted a message that was encoded using the same algorithm. The encoded message as a HEX number was D007. What was the original message as a HEX number?

Answer 1

Given:
   Encryption:
       1) XOR with HEX Key AA
2) rotate 4 places to right
       3) XOR with HEX Key BC.

a) plain text given = ACE1
We need to twice the key to match the length of the plain text that is AAAA
1) We should XOR with key AAAA
ACE1 XOR AAAA
= 1010 1100 1110 0001
XOR 1010 1010 1010 1010
--------------------------------------
= 0000 0110 0100 1011
----------------------------------------

2) we should rotate 4 places right

0000 0110 0100 1011 -> 1011 0000 0110 0100 = B064

We need to twice the key to match the length of the text that is BCBC
3) We should XOR with key BCBC.
B064 XOR BCBC
= 1011 0000 0110 0100
XOR 1011 1100 1011 1100
----------------------------------------
= 0000 1100 1101 1000
------------------------------------------------

That is "0CD8".

The cipher text is "0CD8".

   Decryption:
       1) XOR with key BC
2) rotate 4 places left
       3) XOR with key AA

b) Given encoded message is D007.
We need to twice the key to match length of encrypted message that is BCBC.

1) We should XOR with BCBC
D007 XOR BCBC
= 1101 0000 0000 0111
XOR 1011 1100 1011 1100
-------------------------------------------
= 0110 1100 1011 1011
-------------------------------------------

2) We rotate 4 places left.

0110 1100 1011 1011 -> 1100 1011 1011 0110 = CBB6

We need to twice the key to match length of message that is AAAA
3) We shold XOR with AAAA
CBB6 XOR AAAA
= 1100 1011 1011 0110
XOR 1010 1010 1010 1010
-------------------------------------------
= 0110 0001 0001 1100
-------------------------------------------

That is "611C".
The plaintext is "611C".

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