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.