1.How protection is different from security?

Modern computer systems allows multiple users to excite their multiple process concurrently in the system, these multiple processes may access data simultaneously. This access has to be regulated so that only valid users are given access to the data. It is the job of the operating system to apply protection and security mechanism to the system.
    Protection is a mechanism of controlling access of computer resources by users or processes. The mechanism should provide tools so that the administration can define the restriction on various users. It should also provide tools to enforce those restrictions and differences between an authorized and unauthorized access and usage and can take measures to defend the system against misuse. IF protection is not employed. Then errors may also occur among sub components of system, this happens usually when one defective subsystem. This happens usually when one defective subsystem interacts with other healthy subsystem through its interface then the healthy subsystem may start malfunctioning.
    Security refers to the process of preventing the system from attacks. Even after employing a good protection mechanism, hackers may breach the protection to access restricted data. There are several kinds of attacks like internal, external, which includes viruses, worms, denial of service, theft of user identity and authentication information, etc..
    Operating systems maintain several data structures like access list, access matrix, to store user names and their respective privileges. Each user is given a username and unique user identifier. This information is stored in a list so that during authentication, the username entered by the user is verified with an entry present in this list. The user ID is prefixed with all the processes and threads belong to that particular user. Various operating systems have several notations for user id, for operating systems have several notations for user ID, for example in Windows NT, user ID nothing but security ID.
    In some situations we need to apply some security mechanism to a set of users. All the users belonging to a particular group share common security mechanism. In UNIX, group names and group identifiers are used to apply such group policy.
    Sometimes a user also need escalate privileges in addition to user id and group id probably when accessing restricted device or data. It has to take extra permissions. For example, in unix for changing user id attribute of a file. The command segued is executed as effectiveuid the uid of the file owner but not the current user is used.

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