Basic functions used in encryption algorithms

  2 What are the two basic functions used in encryption algorithms? 3 How many keys are required for two people to communicate via a symmetric cipher? 4 What is the difference between a block cipher and a stream cipher? 5 What are the two general approaches to attacking a cipher? 6 Why do some block cipher modes of operation only use encryption while others use both encryption and decryption? 7 What is triple encryption? 8 Why is the middle portion of 3DES a decryption rather than an encryption?"