Suppose that someone suggests the following way to confirm that the two of you a

Question

Suppose that someone suggests the following way to confirm that the two of you are both in possession of the same secret key. You create a random bit stream the length of the key, XOR it with the key, and send it over the channel. Your partner XORs the incoming block with the key, (which should be the same as your key) and sends it back. You check and if what you receive is your original random string, you have verified that your partner has the same secret key, yet neither of you has ever transmitted the key. Is there a flaw in this scheme? Why or Why not?

Explanation / Answer

Yes, an attacker can listen the communication and find the key in following way,- Let A and B are the partners, and K is the key of size N. 1> A generates random string R 2> Calculates S : = K XOR R 3> send S to B 4> B calculates S1:= S XOR K 5> sends S1 to A 6> A validates whether R is equal to S1 or not? So an attacker listening the conversation will have both S and S1. So, he will compute the key as follows, K:= S1 XOR S

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