What block cipher mode of operation encrypts plaintext and computes a message authentication code to ensure that the message was created by the sender and that it was not tampered with during transmission?

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Suppose there is a relation r(a, b, c) with a b+-tree index with search keys (a, b).1. what is the worst-case cost of finding records satisfying 10 < a < 50 using this index, in terms of the number of records n1, retrieved and the height h of the tree? 2. what is the worst-case cost of finding records satisfying 10 < a < 50 and 5 < b < 10 using this index, in terms of the number of records n2 that satisfy this selection, as well as n1 and h defined above? 3. under what conditions on n1 and n2, would the index be an efficient way of finding records satisfying the condition from part (2)?

A. in packet tracer, only the server-pt device can act as a server. desktop or laptop pcs cannot act as a server. based on your studies so far, explain the client-server model.