(e) Consider the following encryption scheme to provide confidentiality. A and B

Question

(e) Consider the following encryption scheme to provide confidentiality. A and B have previously exchanged a secret key K for an encryption scheme E which works with 4-bit blocks. A wants to send an 8-bit M to B, where M = b_7, b_6, b_5, b_4, b_3, b_2, b_1, b_0. A is going to calculate F where F is the first 4 bits of M XORed with the last 4 bits of M i.e. F = b_7, b_6, b_5, b_4 XOR b_3, b_2, b_1, b_0. A then calculates C = E_K(F), and transmits C to B, and B recovers M from C. Is this a good scheme? i. Give a YES/NO answer. ii. If you said NO explain what you think is the single biggest weakness of the scheme.

Explanation / Answer

From the data given in question c we can infer the following. I shall be answering the question considering that the data given in the question is the only information required to answer the question.

If incase you have any doubt or contradiction or even different idea that you think is more valid reason, please comment them. I shall be glad to discuss them with you and explain if they are the possible reason or not.

Now looking at the problem from a different aspect we can say that recovery of M from F is not possible.

Consider a value of F, let it be 1011

we can obtain same F from different M
e.g. 10010010 or 00001011 or 11000111 (16 possibilities) all will generate F = 1011

So the compression of data (from 8bit M to 4bit F) is a lossy compression and cannot be recovered.

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