正解:A
Entropy is a measure of disorder. A plaintext will usually exhibit low entropy as it represents a message in a human language or programming language or data structure. The plaintext must be ordered for it to be intelligible to a person, computer processor, or database. One of the requirements of a strong cryptographic algorithm is to produce a disordered ciphertext. Put another way, the ciphertext must exhibit a high level of entropy. If any elements of order from the plaintext persist, it will make the ciphertext vulnerable to cryptanalysis, and the algorithm can be shown to be weak.