There are two general approaches to attacking a ( )encryption scheme.The first attack is known as cryptanalysis.Cryptanalytic attacks rely on the nature of the algorithm plus perhaps some knowledge of the general characteristics of the ( ) or even some sample plaintext- ciphertext pairs. This type of ( 3 ) exploits the characteristics of the algorithm to attempt to deduce a specific plaintext or to deduce the key being used. If the attack succeeds in deducing the key, the effect is catastrophic: All future and past messages encrypted with that key are compromised. The second method, known as the ( )-force attack, is to try every possible key on a piece of ( ) until an intelligible translation into plaintext is obtained. On average, half of all possible keys must be tried to achieve success.
（3）应选__________.
