CRYPTOGRAPHY

Cryptography



Cryptography

Question 1

Consider the one-time pad encryption scheme to encrypt a 1-bit message m, and assume m is chosen with uniform distribution from message space M={0,1}. Let E1 be the event "message m is = 1" and let E2 be the event "ciphertext c is = 0". What is the probability that both event E1 and event E2 happen?

Answer

b.0.5

Question 2

Consider the one-time pad encryption scheme to encrypt a 1-bit message m, and assume m is chosen with uniform distribution from message space M={0,1}. For b=0,1, let E[b] be the event "message m is = b" and let F be the event "ciphertext c is = 1". What is the probability that event F happens?

Answer

d.1

Question 3

Consider the one-time pad encryption scheme to encrypt a 1-bit message m. For b=0,1, let E[b] be the event "message m is = b", assume prob(E[0])=p and prob(E[1])=1-p, for some p in [0,1], and let F be the event "ciphertext C is = 1". What is the probability of event E[0] given that event F happens? Use the Bayes theorem to find your answer.

Answer

d. p

Question 4

Assume a meaningful plaintext is encrypted using the shift cipher. How many encryption attempts are sufficient for an exhaustive (or brute-force) search attack to find the plaintext with probability at least 1/2?

Answer

b.2

Question 5

Assume a meaningful plaintext is encrypted using the mono-alphabetic substitution cipher. How many encryption attempts are sufficient for an exhaustive (or brute-force) search attack to find the plaintext (with probability 1)?

Answer

a.1

Question 6

Assume a meaningful plaintext is encrypted using the poly-alphabetic substitution cipher (with t random numbers in [0,26], for a known t). How many encryption attempts are sufficient for an exhaustive (or brute- orce) search attack to find the plaintext (with probability 1)?

Answer

c. 26

Question 7 ...