A hypothetical encryption/decryption scheme has a key length of 16 bits.

What is the size of the keyspace? Briefly explain the effect on the size of the keyspace if the key length is increased to 20 bits? (Do not state the size of the keyspace for the longer key, just describe the effect of increasing the key length.)

Q4. Briefly explain why the final operation in a Feistel cipher is an additional swap operation.

Q5. Encrypting with TDEA and a single key is pointless, because it is equivalent to encrypting with the original "single" DES, which is no longer considered strong enough to resist feasible attacks. Briefly explain why decrypting with TDEA and a single key was a useful operation when TDEA was developed.

Q6. In your own words, briefly explain how Cipher Block Chaining mode produces non-repetitive ciphertext from repetitive plaintext.