Perhaps the most obvious difference between private-key and public-key encryption is that the former assumes complete secrecy of all cryptographic keys, whereas the latter requires secrecy for only the private key.Although this may seem like aminor distinction,the ramifications are huge: inthe private-key setting the communicating parties must somehow be able to share the （ ） key without allowing any third party to learn it,whereasin the public-key setting the （ ） key can be sentfrom one party to the other over a public channelwithout compromising security.For parties shouting across a room or,more realistically,communicating over a public network like a phone line or the lnternet, public-key encryption is the onlyoption.
Another important distinction is that private-keyencryption schemes use the （ ） key for both encryption and decryption,whereas public-key encryption schemes use （ ） keys for each operation.That is， public-key encryption is inherently asym metric.This asymmetry in the pubIC-key seltmrgmeans that the roles of sender and receiver are not interchangeable as they are in the private-key setting; a single key-pair allows communication in one direction only.(Bidirectional communicatio n can be achieved in a number of ways; the point is that a single invocation of a public-key encryption scheme forces a distinction between one user who acts as a receiver and other users who actas senders.)In addition, a single instance of a （ ） encryption scheme enables multiple senders to communicate privately with a single receiver，in contrast to the private-key case where a secretkey shared between two parties enables private communication only between those two parties.