The means traditionally taken to realize this objective is to attempt to create a trusted and secure computing platform, designed so that agents (users or programs) can only perform actions that have been allowed. This involves specifying and implementing a security policy. The actions in question can be reduced to operations of access, modification and deletion. Computer security can be seen as a subfield of security engineering, which looks at broader security issues in addition to computer security.
In a secure system the authorised users of that system are still able to do what they should be able to do. One might be able to secure a computer beyond misuse using extreme measures:
The only truly secure system is one that is powered off, cast in a block of concrete and sealed in a lead-lined room with armed guards – and even then I have my doubts.