10. Bob is assigned a task to design a way to allow encryption of files stored in the system: all files are stored in an encrypted form. If a block of a file is requested, 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 modes of operations discussed in class (i.e., ECB, CBC. CFB, CTR). Which one should be used in terms of read/write efficiency? Why? (You do NOT need to consider the key storage problem.)