Consider the following key-exchange protocol: (a) Alice chooses k, r leftarrow {0, 1}^n at random, and sends s:= k r to Bob. (b) Bob chooses t leftarrow {0, 1}^n at random and sends u:= s t to Alice. (c) Alice computes w:= u r and sends w to Bob. (d) Alice outputs k and Bob outputs w t. Show that Alice and Bob output the same key. Analyze the security of the scheme (i.e., either prove its security or show a concrete attack)