Computer viruses.

Note The lecture on computer viruses (Gresham College, London, 6pm 21 February 2002) will include discussion of real viruses, details of how they work, and discussion of how they may be tackled. These notes provide some background, history, and further information.  Computers can do anything: from running spreadsheets, word processors, power stations to music synthesisers and missile control systems. And because computers can do anything, they can in particular run viruses and any other nasty software. Of course viruses aren't the only way computer systems can go wrong: you can have your computer stolen; you might simply have a disc crash or some other hardware fault. None of these problems are predictable. However, viruses are unique in their abilities. They can stop many computers at once: this would be much more serious for a small company than 'normal' faults that affect only one PC at a time. Thus, viruses rank with hazards like power cuts and fire in their effect and speed of action. Worse than fire, you may find that you cannot take your work elsewhere, for if you did you might simply take the virus infection with you and bring more systems down too! Secondly, viruses can spread disinformation and bring disrepute to individuals or organisations: viruses may send malicious email apparently on behalf of the person whose computer has been infected. Normal information management precautions, such as taking backups, can be of considerable help in avoiding and recovering after virus attacks. (But you may also have backed up your viruses.) Definition A computer virus is a piece of program code that attaches copies of itself to other programs, incorporating itself into them so that the modified programs, while still possibly performing their intended function, surreptitiously do other things. Programs so corrupted seek others to which to attach the virus, and so the " infection " spreads. Successful viruses lie low until they have thoroughly infiltrated the system, and only reveal their presence when they cause damage. The effect of a virus is rarely traceable back to its originator, so viruses make attractive weapons for vandals. Computer viruses generally work by altering files that contain otherwise harmless programs. This is infection. When an infected program is invoked, it seeks other programs stored in files to which it has write permission, and infects them by modifying the files to include a copy of itself (replication) and inserting an …