Question

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?

Answer 1

Encryption algorithm

• XOR with the HEX key BC
• rotate 4 places to the left
• XOR with the HEX key AA

Decryption algorithm

• XOR with the HEX key AA
• rotate 4 places to the right
• XOR with the HEX key BC

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