2. Rab_bits is a computer system with three users: Allen, Bill, and Charlie. Allen owns the file alfa, and Bill and Charlie can read it. Charlie can read and write the file bfile, which Bill owns, but Allen can only read it. Only Charlie can read and write the file cfile, which he owns. Assume that the owner of each file can execute it. Hint: use r for read; w for write; o for own; x for execute.

a. Draw the corresponding access control matrix 15 pts

b. Charlie gives Allen permissions to read cfile, and Allen removes Bills ability to read alfa. Show the new access control matrix. 5 pts