下面算法中，属于非对称密钥加密算法的是______。 A: DES(Digital Encryption Standard) B: RC2(Rivest Ciphers 2) C: RC5(Rivest Ciphers 5) D: DSA(Digital Signature Algorithm)

In block ciphers, the original key is expanded so that a different key is used for each round.

Which of the following statements are true？ （） A:  RC4 is a stream cipher B:  Stream ciphers require padding C:  AES is a block cipher D:  DES and 3DES are stream ciphers E:  AES is a stream cipher F:  AES， DES， and 3DES can be used as HMAC algorithms

The index of coincidence: A: can tell you the likelihood that a key is the same length as the ciphertext B: can tell you the likelihood that some ciphertext has the same frequency counts as English C: can distinguish between mono and polyalphabetic substitution ciphers D: a and b only.