[問題] 可以解釋一下public key 跟 private key 的觀念嗎?

[wakke (衝阿 去掉C)]

列舉兩題

43.When using public key encryption scheme, how to let A securely transmit a message to
  B, and let B believe that the message is indeed sent by A？
  (A)use A’s public key to encrypt the message, and then use B’s private key to encrypt
     the previous encrypted message
  (B)use A’s private key to encrypt the message, and then use B’s public key to encrypt
     the previous encrypted message
  (C)use B’s public key to encrypt the message, and then use A’s private key to encrypt
     the previous encrypted message
  (D))use B’s private key to encrypt the message, and then use A’s public key to encrypt
     the previous encrypted message
  (E)none of the above

31.Which of the following is wrong？
  (A)public key encryption is also called asymmetric key encryption because the encryption and
     decryption use different keys
  (B)private key encryption is also called symmetric key encryption because the encryption and
     decryption use the same keys
  (C)public key encryption scheme can be used for digital signature
  (D)PGP is used to secure email messages
  (E)all of the above are correct

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