Consider the following key exchange protocol:

(a) Alice chooses uniform k,r E {0,1}n , and sends s = k?r to Bob.

(b) Bob chooses uniform t E {0,1}n , and sends u = s?t to Alice.

(c) Alice computes w = u?r and sends it to Bob.

(d) Alice outputs k and Bob outputs w?t.

Show that Alice and Bob output the same key. Analyze the security of this scheme: Either prove its security or show a concrete attack.