The block cipher is defined as follows. The alphabet is {0,1}, block length is 3 and the key space is . Given a key and a plaintext , let be written as a decimal number. We define to be written as a 3-digit binary number (possibly with initial zeros), and we define in exactly the same way. The functions and are the encryption and decryption functions respectively of the cryptosystem.

(a) Find

(b) Given an excryption key , find a corresponding decryption key

Any help would be greatly appreciated, thanks=)