【71-75】Perhaps the most obvious difference between private-key and public-key encryption is that the former assumes complete secrecyof all cryptographic keys， whereas the latter requires secrecy foronly the private key.Although this may seem like a minor distinction ，the ramifications are huge： in the private-key setting thecommunicating parties must somehow be able to share the（71）keywithout allowing any third party to learn it， whereas in the public-key setting the（72）key can be sent from one party to the other overa public channel without compromising security.For partiesshouting across a room or， more realistically ， communicating overa public network like a phone line or the nternet， public-keyencryption is the only option. Another important distinction is that private-key encryption schemes use the（73）key for both encryption and decryption，whereas public-key encryption schemes use（74）keys for each operation.That is public-key encryption is inherently asymmetri C．This asymmetry in thepublic-key setting means that the roles of sender and receiver are notinterchangeable as they are in the private-key setting； a single key-pair allows communication in one direction only.（Bidirectionalcommunication can be achieved in a number of ways； the point is thata single invocation of a public-key encryption scheme forces adistinction between one user who acts as a receiver and other userswho act as senders.）ln addition， a single instance of a（75）encryptionscheme enables multiple senders to communicate privately with asingle receiver，in contrast to the private-key case where a secret keyshared between two parties enables private communication onlybetween those two parties.
