3-DES, which uses three (single) DES encryption operations in series and three independent keys C = Enc(K3, Enc(K2, Enc(K1, P))) has an effective keyspace size of 2168 under the known-plaintext attack? Justify your answer in details. 4. [5pts] The RSA cryptosystem serves two main purposes in PGP when sending a signed and encrypted message -- what are they? 5. [5pts] Name an algorithm commonly used to encrypt PGP email messages: 6. [5pts] PGP is characterized by three main steps. What are they, and in what order are they applied (if used)?