4. Suppose a system of 3 users (Ahmed, Fares, Salman) and 5 files (F1, F2, F3, F4,F5) Ahmed can read and write F1, F4, F5 Ahmed can read F3 and has no access to F2 Fares can read and write all the files Salman can write F2, F3, read F1,F4 and no access to F5 a- Create an access control matrix for the system? b- Write the ACL for each files c- Write the capabilities for each user 5. Assume a network with 150 users and they have to use symmetric cryptography to exchange messages. How many keys are required, to ensure a secure communication? 6. Suppose that we use a block cipher to encrypt according to the rule C0=IVE(P0,K),C1=C0E(P1,K),C2=C1E(P2,K), a. What is the corresponding decryption rule? b. Give two security disadvantages of this mode as compared to CBC mode. 7. Suppose that you know a MAC value X and the key K that was used to compute the MAC, but you do not know the original message. Show that you can construct a message M that also has its MAC equal to X. Note that we are assuming that you know the key K and the same key is used for both MAC computations