Perhaps the most obvious difference between private-key andpublic-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 schemesuse the (73) key for both encryption and decryption, whereas publickey encryption schemes use (74) keys for each operation.That is public-key encryption is inherently asymmetric.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.
