We have the following encryption algorithm: XOR with the HEX key BC, rotate 4 places to the left, and XOR with the HEX key AA. 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?
Encryption algorithm
Decryption algorithm
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?
Plaintext = ACE1
Encryption
Step 1: ACE1 XOR BCBC (Key is repeated twice to match the length)
= 1010 1100 1110 0001 XOR 1011 1011 1100 1100
= 0001 0111 0010 1101 => 172D
Step 2: Rotate 0001 0111 0010 1101 4 places left
= 0111 0010 1101 0001 => 72D1
Step 3: 72D1 XOR AAAA (Key is repeated twice to match the length)
= 0111 0010 1101 0001 XOR 1010 1010 1010 1010
= 1101 1000 0111 1011 => D87B
Ciphertext = D87B
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?
Ciphertext = D007
Decryption
Step 1: D007 XOR AAAA (Key is repeated twice to match the length)
= 1101 0000 0000 0111 XOR 1010 1010 1010 1010
= 0111 1010 1010 1101 => 7AAD
Step 2: Rotate 0111 1010 1010 1101 4 places right
= 1101 0111 1010 1010 => D7AA
Step 3: D7AA XOR BCBC (Key is repeated twice to match the length)
= 1101 0111 1010 1010 XOR 1011 1011 1100 1100
= 0110 1100 0110 0110 => 6C66
Plaintext = 6C66
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