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.
问题1
正确答案BCDAB有两种常用的方法可以攻击对称密钥加密方案。第一种攻击叫做密码分析学。密码分析攻击依赖于算法的特性，也许还要加上某些有关明文的一般性特征的知识，甚至需 要某些明文-密文对的样品作为辅助。这种类型的攻击利用了算法的特点，企图推导出特 殊的明文或者推导出当前使用的密钥。如果这种攻击成功地导出了密钥，其效果将是灾难性的：所有将来和过去用这个密钥加密的报文都会被突破。第二种方法叫做蛮力攻击， 就是用每一种可能的密钥在一段密文上进行试验，直到将其转换为可理解的明文。平均来说，要达到成功需要试验的密钥数量为各种可能的密钥数量的一半。
