Describe the concept of a digital signature and its role in message authentication.
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Akash M.
Given are two protocols in which the sender's party performs the following operation: Protocol A: y = Ek1(Ekpub,r(x)||H(k2||x)) where x is the message, H is a hash function, E is a symmetric-key encryption algorithm, "||" denotes simple concatenation, k1 and k2 are secret keys which are only known to the sender and the receiver, and kpub,r is a public key of the receiver. Protocol B: y = Ek(Ekpub,r(x)||sigkpr,s(H(x))) where k is a shared secret key only known to the sender and the receiver, kpr,s is a private key of the sender (not shared with the receiver), and kpub,r is a public key of the receiver. (a) For each protocol, provide a step-by-step description (e.g., with an itemized list) of what the receiver does upon reception of y. [kpub,s is a public key of the sender, kpub,r is a public key of the receiver] (b) State whether the following security services are given for each of the two prescribed protocols: - confidentiality - integrity - non-repudiation (preventing an entity from denying previous commitments or actions)
2. We explore what happens when the basic CBC-MAC construction is used with messages of different lengths. (a) Say the sender and receiver do not agree on the message length in advance (and so Vrfy_k(m, t) = 1 iff t = Mac_k(m), regardless of the length of m), but the sender is careful to only authenticate messages of length 2n. Show that an adversary can forge a valid tag on a message of length 4n. (b) Say the receiver only accepts 3-block messages (so Vrfy_k(m, t) = 1 only if m has length 3n and t = Mac_k(m)), but the sender authenticates messages of any length a multiple of n. Show that an adversary can forge a valid tag on a new message.
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