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)．
④
选项 A、public
B、private
C、symmetric
D、asymmetric
答案A
解析
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