The last step is repealed until all of the message blocks have been encrypted. D

Question

The last step is repealed until all of the message blocks have been encrypted. Decrypt the ciphertext 10010110 10101101 using key 01100110. (3.2) Describe at least two security problems of this protocol. Consider a cryptosystem based on Feistel network and consisting of n such rounds, where the function f, is defined by f_1(X) = for each round .Here we assume that the key K is 32-bits long and that the same key is used at each round. This system is used to encry pt a 64-hit message M as follows: L_0 is the leftmost 32-b.ts of M and R_0 is the rightmost 32-bits of M. The ciphertext is L_a || R_a Describe how to decrypt messages encrypted by the system. Express L_1, R_1, L_2, R_2 in terms of L_0, R_0, and K. Show that increasing the number of rounds n would not improve security.

Explanation / Answer

3)

a)

decrypt using the same way as encryption

int other words, encrypt the cypher text using the same key to get message text

b)

L1 = R0

R1 = L0 ^ R0 ^ K (^ means xor operator and K means Key)

L2 = R1 = L0 ^ R0 ^ K

R2 = L1 ^ R1 ^ K = R0 ^ L0 ^ R0 ^ K ^ K = L0

c)

from above we can say that

with each increase in level is not increasing any security nothing but swapping the outputs

3.1)

C= 1001011010101101

K = 01100110

R0 = L1 = 10010110

L0 = R0 ^ L1 ^ K

3.2)

1) no increase in security level with increase of no. of levels

2) If we have a message and its corresponding cipher text , then we can find the Key (breaking the system)

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