Security of Encryption Schemes Quiz
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Questions and Answers

What is the probability distribution of message space for the shift cipher instance described?

  • Pr[M=a]=0.25, Pr[M=b]=0.5, Pr[M=c]=0.25 (correct)
  • Pr[M=a]=0.5, Pr[M=b]=0.25, Pr[M=c]=0.25
  • Pr[M=a]=0.14, Pr[M=b]=0.36, Pr[M=c]=0.5
  • Pr[M=a]=0.3, Pr[M=b]=0.3, Pr[M=c]=0.4
  • What is the correct statement regarding the probability of the ciphertext 'D' and 'E' in the given shift cipher instance?

  • Probability that the ciphertext is “D” is 126
  • Probability that the ciphertext is “A” is 14th the probability that the ciphertext is “D”
  • Probability that the ciphertext is “D” is half the probability that the ciphertext is “E”
  • Probability that the ciphertext is “D” is the same as the probability that the ciphertext is “E” (correct)
  • What is the incorrect statement about the probabilities in the given shift cipher instance?

  • The probability that the ciphertext is 'D' is 126 (correct)
  • The probability that the ciphertext is 'E' is double the probability of 'A'
  • The probability that the ciphertext is 'E' is higher than 'A'
  • The probability that the ciphertext is 'B' is 5 times higher than 'A'
  • In the given shift cipher with the provided probability distribution, what is the likelihood of generating a message 'A'?

    <p>Lower than that of 'B'</p> Signup and view all the answers

    Which statement correctly compares the probabilities of 'D' and 'E' in the shift cipher instance?

    <p>'D' has half the probability of 'E'</p> Signup and view all the answers

    Which of the following encryption schemes is not perfectly secure?

    <p>Vigenère Cipher</p> Signup and view all the answers

    In a perfectly-secure encryption scheme, which condition is valid?

    <p>Pr[M=m|C=c] = Pr[M=m]</p> Signup and view all the answers

    In a Vigenère cryptosystem, if the word 'CRYPTOGRAPHY' occurs twice in a 500-character encrypted message, what is the length of the encryption key?

    <p>6</p> Signup and view all the answers

    Which statement is correct about encryption schemes?

    <p>(I) Any encryption scheme with key length less than message length cannot be perfectly secure.</p> Signup and view all the answers

    Which of the following conditions is necessary for perfect security in encryption?

    <p>Changing keys after every few messages</p> Signup and view all the answers

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