If I had to specify the protection in UNIX that had to support 5,000 users, but only wanted to give 4,990 users access, I would add each of the users in a group, and then allow access to the file by the group. Alternately, on some UNIX systems, you can create an access control list (ACL) and assign according to that list. It is known that UNIX files were created by an owner. Being though an owner may be belong to a certain group of people the file might be associated with people working on one experiment, for example, and there are also users. UNIX protects its files based on these classifications, I just mentioned. I specify my protection based on the UNIX system, which is the permission the read, permission to write(change) and the permission to execute(to run the file). Each three permissions are applied to each one of the groups that I mentioned before: owner, group, and the users. Every user on UNIX has a login id and password associated with it. The effectiveness of this protection scheme depends on how well protected a user’s password is. I like encryption protection, being that this software is a tool that converts files (scramble it) to a form that is completely different from its original version. The new file is called encrypted file and the process is called encryption. The same tool is used to perform the reverse process which called decryption. You can tell your editor to decrypt a file when it loads it, and encrypt it again when you write the file out to disk. You will be asked to enter a key or a password for the editor to decrypt the file. This type is provided by a mechanism that prevents users from accessing each other’s files when they are not logged on as a file owner. As a file owner, you can attach certain access right to your files that dictate who can and cannot access them for various type of file operation (read, write, execute). I’m a fan of file of file protection...