Answered Essay: 20. Suppose that we use a block cipher to encrypt according to the rule

20. Suppose that we use a block cipher to encrypt according to the rule

C0 = IV xor (P0 ,K), C1 = C0 xor E(P1, K), C2= C1 xor E(P2,K),…

a. What is the corresponding decryption rule?

b. Give two security disadvantages of this mode as compared to CBC


Expert Answer


a.) The corresponding decryption rule is as follows:-

P0 = D(IV xor C0 , K), P1=D(P0 xor C1 , K), P2=D(P1 xor C2 , K)

XORing the Cipher text with the random vector IV and then decrypting it with key.

Passing the obtained plain text in place of the vector to get the next plain character.

b.) Security Disadavantages:-

CBC mode encrypts the XORed Plain text with the key which makes it difficult to crack. The encryption method shown above is actually how decryption in done in CBC. There is not much security disadvantage until the key is known to attacker, since he can easily xor and decipher by brute forcing to get the vector.

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