第67題： （  ）攻擊是指借助于客戶機(jī)/服務(wù)器技術(shù),將多個(gè)計(jì)算機(jī)聯(lián)合起來(lái)作為攻擊平臺(tái)，對(duì)一個(gè)或多個(gè)目標(biāo)發(fā)動(dòng)DoS攻擊，從而成倍地提高拒絕服務(wù)攻擊的威力。
A.緩沖區(qū)溢出
B.分布式拒絕服務(wù)
C.拒絕服務(wù)
D. 口令
答案解析與討論：m.xiexiliangjiufa.com/st/4114823197.html

第68題： 如果對(duì)一個(gè)密碼體制的破譯依賴于對(duì)某一個(gè)經(jīng)過(guò)深入研究的數(shù)學(xué)難題的解決，就認(rèn)為相應(yīng)的密碼體制是（  ）的。
A.計(jì)算安全
B.可證明安全
C.無(wú)條件安全
D.絕對(duì)安全
答案解析與討論：m.xiexiliangjiufa.com/st/411494857.html

第69題： 移位密碼的加密對(duì)象為英文字母，移位密碼采用對(duì)明文消息的每一個(gè)英文字母向前推移固定Key位的方式實(shí)現(xiàn)加密。設(shè)key=3，則對(duì)應(yīng)明文MATH的密文為(  )。
A. OCVJ
B. QEXL
C. PDWK
D. RFYM
答案解析與討論：m.xiexiliangjiufa.com/st/4115021966.html

第71題： The modern study of symmetric-key ciphers relates mainly to the study of block ciphers and stream ciphers and to their applications. A block cipher is, in a sense, a modern embodiment of Alberti's polyalphabetic cipher: block ciphers take as input a block of （71 ）and a key, and output a block of ciphertext of the same size. Since messages are almost always longer than a single block, some method of knitting together successive blocks is required. Several have been developed, some with better security in one aspect or another than others. They are the mode of operations and must be carefully considered when using a block cipher in a cryptosystem.
The Data Encryption Standard (DES) and the Advanced Encryption Standard (AES) are( 72 )designs which have been designated cryptography standards by the US government (though DES's designation was finally withdrawn after the AES was adopted). Despite its deprecation as an official standard, DES (especially its still-approved and much more secure triple-DES variant) remains quite popular; it is used across a wide range of applications, from ATM encryption to e-mail privacy and secure remote access. Many other block ciphers have been designed and released, with considerable variation in quality. Many have been thoroughly broken. See Category: Block ciphers.
Stream ciphers, in contrast to the ‘block’type, create an arbitrarily long stream of key material, which is combined ( 73 )the plaintext bit-by-bit or character-by-character, somewhat like the one-time pad. In a stream cipher, the output( 74 )is created based on an internal state which changes as the cipher operates. That state change is controlled by the key, and, in some stream ciphers, by the plaintext stream as well. RC4 is an example of a well-known, and widely used, stream cipher; see Category: Stream ciphers.
Cryptographic hash functions (often called message digest functions) do not necessarily use keys, but are a related and important class of cryptographic algorithms. They take input data (often an entire message), and output a short fixed length hash, and do so as a one-way function. For good ones, ( 75 ) (two plaintexts which produce the same hash) are extremely difficult to find.
Message authentication codes (MACs) are much like cryptographic hash functions, except that a secret key is used to authenticate the hash value on receipt. These block an attack against plain hash functions.
（71）
A.plaintext
B.ciphertext
C.data
D.hash
（72）
A.stream cipher
B.hash function
C.Message authentication code
D.Block cipher
（73）
A.of
B.for
C.with
D.in
（74）
A.hash
B.stream
C.ciphertext
D.plaintext
（75）
A.collisions
B.image
C.preimage
D.solution
答案解析與討論：m.xiexiliangjiufa.com/st/4115223167.html