24. Suppose Bob and Alice want to flip a coin over a network. Alice proposes the

Question

24. Suppose Bob and Alice want to flip a coin over a network. Alice proposes the following protocol.

(i) Alice randomly selects a value X e {0,1}.

(ii) Alice generates a 256-bit random symmetric key K.

(iii) Using the AES cipher, Alice computes Y = E(X,R,K), where R consists of 255 randomly selected bits.

(iv) Alice sends Y to Bob.

(v) Bob guesses a value Z £ {0,1} and tells Alice.

(vi) Alice gives the key K to Bob who computes (X, R) = D(Y, K).

(vii) If X = Z then Bob wins, otherwise Alice wins.

This protocol is insecure.

a. Explain how Alice can cheat.

b. Using a cryptographic hash function h, modify this protocol so that Alice cannot cheat.

Explanation / Answer

Answer:

So in the above scenario, Alice and Bob are flip coins over a message and Alice uses the AES cipher.

There are many things by which Alice can cheat. So let us suppose that Alice can use the man in the middle attack in which Alice send him the original key but in between Alice places an attracter who without knowing Bob changes the secret message and by which bob cannot guess the right keyword and hence bob will loose.

As we have seen above how Alice can cheat so let us use the hash function to secure the message and by this Alice won’t be able to cheat. So cryptography hash function is a function on which it takes input/message and then returns a fixed-size alphanumeric string, The string which is being generated is said to be a hash function.

There are basically three properties of has function which are as follows:

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