6. John Smith works at a company that makes network storage systems. He is given a task to design a method to allow encryption of files stored in the system. In other words, all files are stored in an encrypted form. If a host using the storage system requests a block of a file, the system should retrieve the block, decrypt it and return the plaintext to the host. Similarly, if a host writes a block to the storage system, the system should retrieve the right keying material, encrypt the block, and only save the ciphertext on disk. Consider the block cipher modes. Which one should be used? Why