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 （） 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.
A．stream  B．symmetric  C．asymmetric D．advanced
A．operation B．publication  C．plaintext  D．ciphertext
A．message  B．knowledge  C．algorithm  D．attack
A．brute  B．perfect   C．atribue   D．re search
A．plaintext  B．ciphertext  C．sample  D．code