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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'?

Lower than that of 'B' (A) Signup and view all the answers

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

'D' has half the probability of 'E' (C) Signup and view all the answers

Which of the following encryption schemes is not perfectly secure?

Vigenère Cipher (C) Signup and view all the answers

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

Pr[M=m|C=c] = Pr[M=m] (A) 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?

6 (C) Signup and view all the answers

Which statement is correct about encryption schemes?

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

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

Changing keys after every few messages (D) Signup and view all the answers