Question

Suppose the attacker has been logging the traffic between Alice and Bob. It is assumed that the attacker can reliably distinguish between messages and their signatures (i.e., tags). In the process he found two messages x and y signed by Alice, i.e., he has pairs <x, tag(x)> and <y, tag(y)>. Suppose the signing function is

tag(m) = mk mod n

where k is the shared key between Alice and Bob.Show how he can fool Bob into believing that the message m was signed by Alice.

(Important hint 1: using the mathematical properties of RSA)

(Important hint 2: in other words, I let you prove whether the above MAC is secure or not, like our homework. Could you find a different message m’, and you can find t’)

Answer 1

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In this problem attacker was Trudy in which Trudy is attenticated to Bob as Alice and break the authentication using reflection attack and Bob thought that meassage was signed by Alice.

RSA mathematical function used g^xmod n but we used this function in diffie Hellman.

Mutual attenticatation can be achieved by public authority using digital signature.

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