2. 计算机
  3. Symmetric-key cryptosystems use the ________ key for encryption and decryption of a message，though a message or group of message

Symmetric-key cryptosystems use the ________ key for encryption and decryption of a message，though a message or group of message

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问题 Symmetric-key cryptosystems use the ________ key for encryption and decryption of a message，though a message or group of messages may have a different key than others．A significant  disadvantage of symmetric ciphers is the key management necessary to use them securely．Each  distinct pair of communicating parties must，ideally,share a different key,and perhaps each ciphertext  exchanged as well．The number of keys required increases as the square of the number of network  members，which very quickly requires complex key management schemes to keep them all straight  and secret．The difficulty of securely establishing a secret  ________between two communicating  parties，when a secure channel doesn’t already exist between them，also presents a chicken—and—egg  problem which is a considerable practical obstacle for cryptography users in the real world．
    Whitfield Diffie and Martin Hellman，authors of the first paper on public—key cryptography．
    In a groundbreaking 1 976 paper,Whitfield Diffie and Martin Hellman proposed the notion of  public—key(also，more generally,called asymmetric key)cryptography in which two different but  mathematically related keys are used——a public key and a private key．A public key system is so  constructed that calculation of one key(the private key)is computationally infeasible  ________ the  other(the public key)，even though they are necessarily related．Instead，both keys are generated  secretly,as an interrelated pair．The historian David Kahn described public—key cryptography as  “the most revolutionary new concept in the field since poly—alphabetic substitution emerged in the  Renaissance”．
  In public-key cryptosystems，the ________  key may be freely distributed，while its paired  private key must remain secret．The public key is typically used for encryption，while the private or  secret key is used for decryption．Diffie and Hellman showed that public-key cryptography was  possible by presenting the Diffie—Hellman key exchange protocol．
    In 1978，Ronald Rivest，Adi Shamir,and Len Adleman invented ________，another public—key  system．
    In 1997，it finally became publicly known that asymmetric key cryptography had been  invented by James H．Ellis at GCHQ，a British intelligence organization，and that，in the early  1970s，both the Diffie—Hellman and RSA algorithms had been previously developed(by Malcolm J．Williamson and Clifford Cocks，respectively)．
⑤
选项 A、DES
B、AES
C、RSA
D、IDEA
答案C
解析  对称密钥密码系统使用相同(same)的密钥对消息进行加密和解密，尽管一条消息或一
组消息可能使用与其他消息不同的密钥。对称密码的一个显著缺点是为了安全使用必须进行
密钥管理。理想情况下，每对不同的通信双方必须共享不同的密钥，或许每个密文也需要交
换。随着网络成员的增加，需要的密钥数量以网络成员的平方倍增加，这很快就需要复杂的密钥管理方案来保持密钥的透明性和保密性。当通信双方之间不存在安全信道时，很难在它
们之间安全地建立密钥(key)，这是一个先有鸡还是先有蛋的问题，对于现实世界中的密码
学用户来说是一个相当大的实际困难。
    Whitfield Diffie和Martin Hellman是公钥密码学方面第一篇论文的作者，在1976年的
一篇开创性论文中，Whitfield Diffie和Martin Hellman提出了公钥(也更普遍地称为非对称
密钥)密码学的概念，其中使用了两个不同但数学上相关的密钥—公钥和私钥。在公钥系
统中，虽然两个密钥是必须相关的，但从(from)公钥却无法计算出私钥。相反，这两个密
钥都是秘密生成的，它们是相互关联的一对。历史学家David Kahn将公钥密码学描述为“自
文艺复兴时期多表代换出现以来，该领域最具有革命性的新概念”。
    在公钥密码系统中，公钥(public)可自由分发，但与其对应的私钥必须保密。公钥常
用于加密，而私钥或秘密密钥用于解密。Diffie和Hellman通过提出Diffie-Hellman密钥交换
协议证明了公钥密码学的可能性。
    1978年，  Ronald Rivest、Adi Shamir和Len Adleman创建了另一种公钥系统(RSA)。
    英国情报机构GCHQ的．James H．Ellis早已发明非对称密钥密码学，并且在20世纪70
年代初，Diffie-Hellman和RSA算法也已被发明(分别由Malcolm J．Williamson和Clifford
Cocks发明)，但这些事件直到1997年才被大众所知。
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