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# Several reasons for this interest are pointed out in

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Several reasons for this interest are pointed out in Encryption software is quite slow. Even though the amount of data to be encrypted per message is small, there may be a steady stream of messages into and out of a system. Encryption hardware cost are nonnegligible. Encryption hardware is optimized toward large data sizes. Encryption algorithms may be covered by patents. Encryption algorithms may be subject to export control. Two communicating parties, A and B , share a common secret value SAB. When A has a message to send to B ,it calculates the hash function over the concatenation of the secret value and the message: MDM, H(SAB||M) and verify MDM. Because the secret value itself is not sent, it is not possible for an attacker to modify an intercepted message. The secret value remains secret; it is not possible for an attacker to generate a false message. This third technique, using a shared secret value, is the one adopted for IP security , it has been specified for SNMPv3. Figure 18.8 Message Authentication Using a One Way Hash Function 17. Write down the Hash function requirement. (10 marks) Solution Hash function Requirement The purpose of a hash function is to produce a “fingerprint” of a file, message, or other block of data. 1. H can be applied to a block of data of any size. 2. H produces a fixed-length output. 3. H(x) is easy to compute for any given x. making both hardward and software implementations practical. 4. For any given code h, it is computationally infeasible to find x such that H(x) 5. For any given block x, it is computationally infeasible to find y x with H(y)=H(x) 6. It is computationally infeasible to find any pair (x,y) such that H(x)=H(y) 18. Compare the public key encryption and digital signatures. (20 marks) Solution

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