2.6. Exercises

1:

Consider a computer system with three users: Alice, Bob, and Cyndy. Alice owns the file alicerc, and Bob and Cyndy can read it. Cyndy can read and write the file bobrc, which Bob owns, but Alice can only read it. Only Cyndy can read and write the file cyndyrc, which she owns. Assume that the owner of each of these files can execute it. Create the corresponding access control matrix. Cyndy gives Alice permission to read cyndyrc, and Alice removes Bob's ability to read alicerc. Show the new access control matrix.

2:

Consider the set of rights {read, write, execute, append, list, modify, own}. Using the syntax in Section 2.3, write a command delete_all_rights (p, q, s). This command causes p to delete all rights the subject q has over an object s. Modify your command so that the deletion can occur only if p has modify rights over s. Modify your command so that the deletion can occur only if p has modify rights over s and q does not have own rights over s.