Ecila and Selrahc are exchanging messages over an insecure line. Yrollam is listening in between and has the ability to modify, delete, or insert messages. How can Ecila and Serahc ensure each of the following? Explain the process and the technique(s) for each of the problems below: 

a. If Selrahc receives a message from Ecila, the contents have not been modified by Yrollam. 

b. If Selrahc receives a message from Ecila, it is not a replay of an older message previously sent by Ecila, 

c. If Ecila sends three messages to Selrahc, Yrollam cannot delete the second message without getting detected by Ecila. 

d. Yrollam cannot insert a fake message from Ecila to Selrahc (i.e., Yrollam sends the fake message to Selrahc and pretend that this is actually fro Ecila).