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)designs which have been designated cryptography standards by the US government (though DES's designation was finally withdrawn after the AES was adopted).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

参考答案：A、D、C、B、A

参考解析：

第一段疑译文：

The modern study of symmetric-key ciphers relates mainly to the study of block ciphers and stream ciphers and to their applications. 

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究及其应用。

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. 

从某种意义上说，分组密码是阿尔贝蒂多字母密码的现代体现：分组密码以明文块和密钥作为输入，并输出相同大小的密文块。

Since messages are almost always longer than a single block, some method of knitting together successive blocks is required. 

由于消息几乎总是比单个块长，因此需要某种将连续块编织在一起的方法。

Several have been developed, some with better security in one aspect or another than others. 

已经开发了几种，其中一些在某一方面或另一方面比其他方面具有更好的安全性。

They are the mode of operations and must be carefully considered when using a block cipher in a cryptosystem.

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。

第二段疑译文：

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). 

数据加密标准（DES）和高级加密标准（AES）是美国政府制定的分组密码设计（虽然在AES被采用后DES的指定最终被撤销）。

 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.

尽管 DES 作为官方标准已被弃用，但 DES（尤其是其仍被批准且更安全的三重 DES 变体）仍然很受欢迎；它用于广泛的应用程序，从 ATM 加密到电子邮件隐私和安全远程访问。

Many other block ciphers have been designed and released, with considerable variation in quality. 

许多其他分组密码已被设计和发布，但质量差异很大。

Many have been thoroughly broken. See Category: Block ciphers.

许多已被彻底破坏。请参阅类别：分组密码。

第三段疑译文：

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. 

与“块”类型相比，流密码创建了任意长的密钥材料流，密钥材料流与明文逐位或逐字符的明文组合而成，有点像一次性密码本。

In a stream cipher, the output( 74 )is created based on an internal state which changes as the cipher operates.

在流密码中，输出流是基于随着密码操作而改变的内部状态创建的。

That state change is controlled by the key, and, in some stream ciphers, by the plaintext stream as well.

该状态更改由密钥控制，在某些流密码中，也由明文流控制。

RC4 is an example of a well-known, and widely used, stream cipher; see Category: Stream ciphers.

RC4 是众所周知且广泛使用的流密码的一个例子；请参阅类别：流密码。

第四段疑译文：

Cryptographic hash functions (often called message digest functions) do not necessarily use keys, but are a related and important class of cryptographic algorithms. 

加密散列函数（通常称为消息摘要函数）不一定使用密钥，但它是一类相关且重要的加密算法。

They take input data (often an entire message), and output a short fixed length hash, and do so as a one-way function. 

它们获取输入数据（通常是整条消息），并输出一个固定长度的短散列，并作为单向函数执行此操作。

For good ones, ( 75 ) (two plaintexts which produce the same hash) are extremely difficult to find.

对于好的冲突（产生相同散列的两个明文）非常难以找到。

第五段疑译文：

Message authentication codes (MACs) are much like cryptographic hash functions, except that a secret key is used to authenticate the hash value on receipt. 

消息验证码 (MAC) 与加密散列函数非常相似，不同之处在于使用密钥来验证收到的散列值。

These block an attack against plain hash functions.

这些阻止了对普通哈希函数的攻击。