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What is the difference between a protocol and an algorithm?


In Prot 1.1 Alice can decide HEADS or TAILS. This may be an unfair advantage for some applications. Modify the protocol so that Alice can no longer have this advantage.

Hint: let a correct guess decide the side.


Let function f map from the space of 200-bit integers to that of 100-bit ones with the following mapping rule:


here "⊕" denotes bit-by-bit XOR operation, i.e.,


  1. Is f efficient?

  2. Does f have the "Magic Property I"?

  3. Does f have the "Magic Property II"?

  4. Can this function be used in Prot 1.1?


Is an unbroken cryptographic algorithm more secure than a known broken one? If not, why?


Complex systems are error-prone. Give an additional reason for a complex security system to be even more error-prone.

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