2. 计算机
  3. 试题(71)～(75) The modem study of symmetric-key ciphers relates mainly t0 the study of block ciphersand stream ciphers and to t

试题(71)～(75) The modem study of symmetric-key ciphers relates mainly t0 the study of block ciphersand stream ciphers and to t

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问题 试题(71)～(75)
    The modem study of symmetric-key ciphers relates mainly t0 the study of block ciphersand stream ciphers and to their applications．A block cipher is，in a sense，a modernembodiment 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 alwayslonger 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 cipherin a cryptosystem．
    The Data Encryption Standard(DES)and the Advanced Encryption Standard(AES)are (72)________ designs which have been designated cryptography standards by the USgovernment(though DES’s designation was finally withdrawn after the AES was adopted)．Despite its deprecation as an official standard，DES(especially its still-approved and muchmore secure triple-DES variant)remains quite popular；it is used across a wide range ofapplications，from ATM encryption to e-mail privacy and Secure remote access．Many otherblock ciphers have been designed and released，with considerable variation in quality．Manyhave been thoroughly broken．See Category：Block ciphers．
    Stream ciphers，in contrast to the‘block’type，create an arbitrarily long stream of keymaterial，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 allinternal 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 awell-known，and widely used，stream cipher；see Category：Stream ciphers．
    Cryptographic hash functions(often called message digest functions)do not necessarilyuse keys，but are a related and important class of cryptographic algorithms．They take inputdata(often an entire message)，and output a short fixed length hash，and do so as a one-wayfunction．For good ones，(75)________ (two plaintexts which produce the same hash)are extremelydifficult 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 attackagainst 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
选项
答案(71)A (72)D (73)C (74)B (75)A
解析 对称密钥密码的现代研究主要涉及分组密码和流密码的研宄及其应用。在某种意义上，分组密码是阿尔贝蒂多字母密码的现代体现：分组密码以明文和密钥作为输入，并输出相同大小的密文块。由于消息几乎总是比单个块长，因此需要一些将连续块编织在一起的方法。已经开发了一些，有些在某个方面比其他方面具有更好的安全性。它们是操作模式，在密码系统中使用分组密码时必须仔细考虑。
数据加密标准(DES)和高级加密标准(AES)是美国政府指定的分组密码设计(尽管DES的指定在AES被采用后最终被撤销)。尽管DES作为一种官方标准受到了抨击，但它(特别是它仍然被认可的、更安全的三重DES变体)仍然非常流行；它被广泛应用，从ATM加密到电子邮件隐私和安全的远程访问。许多其他的块密码已经被设计和发布，在质量上有相当大的变化。很多已经被彻底破坏了。(参见类别：分组密码)
与“块”类型不同，流密码创建任意长的密钥材料流，密钥材料流与明文逐位或逐字符组合，有点像一次一密密码本。在流密码中，输出流是基于内部状态创建的，内部状态随着密码的操作而变化。这种状态变化由密钥控制，在某些流密码中，也由明文流控制。RC4是一个著名的、广泛使用的流密码的例子(参见类别：流密码)。
加密哈希函数(通常称为消息摘要函数)不一定使用密钥，但却是一类相关的重要加密算法。它们接受输入数据(通常是整个消息)，并输出一个固定长度的短散列，作为单向函数执行此操作。对于好的冲突(产生相同散列的两个明文)是很难找到的。
消息身份验证码(MACs)与加密散列函数非常相似，只是在接收时使用密钥对散列值进行身份验证。它们阻止了对纯哈希函数的攻击。
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