Computer Crimes

There are a variety of different crimes that can be committed in computing, including:

Spreading viruses – distributing programs that can reproduce themselves and are written with the purpose of causing damage or causing a computer to behave in an unusual way;

Hacking – gaining unauthorized access to a network system;

Salami shaving – manipulating programs or data so that small amounts of accumulated money are deducted from a large number of transactions or accounts and elsewhere. The victims are often unaware of the crime because the amount taken from any individual is so small.

Denial of service attack – swamping a server with large numbers of requests;

Trojan horse – a technique that involves adding concealed instructions to a computer program so that it will still work but will also perform prohibited duties. In other words, it appears to do something useful but actually does something destructive in the background.

Trapdoors – a technique that involves leaving, within a completed program, an illicit program that allows unauthorized – and unknown – entry;

Mail bombing – inundating an email address with thousands of messages, slowing or even crashing the server;

Software piracy – unauthorized copying of a program for sale or distributing to other users;

Piggybacking – using another person’s identification code or using that person’s files before he or she has logged off (disconnected from a network account);

Spoofing – tricking a user into revealing confidential information such as an access code or a credit card number;

Defacing – changing the information shown on another person’s website;

Hijacking – redirecting anyone trying to visit a certain site elsewhere.

A biological virus is a very small, simple organism that infects living cells, known as the host, by attaching itself to them and using them to reproduce itself. This often causes harm to the host cells.

Similarly, a computer virus is a very small program routine that infects a computer system and uses its resources to reproduce itself. It often does this by patching the operating system to enable it to detect program files, such as COM or EXE files. It then copies itself into those files. This sometimes causes harm to the host computer system.

When the user runs an infected program, it is loaded into memory carrying the virus. The virus uses a common programming technique to stay resident in memory. It can then use a reproduction routine to infect other programs. This process continues until the computer is switched off.

The virus may also contain a payload that remains dormant until a trigger event activates it, such as the user pressing a particular key. The payload can have a variety of forms. It might do something relatively harmless such as displaying a message on the monitor screen or it might do something more destructive such as deleting files on the hard disk.

When it infects a file, the virus replaces the first instruction in the host program with a command that changes the normal execution sequence. This type of command is known as a JUMP command and causes the virus instructions to be executed before the host program. The virus then returns control to the host program which then continues with its normal sequence of instructions and is executed in the normal way.

To be a virus, a program only needs to have a reproduction routine that enables it to effect other programs. Viruses can, however, have four main parts. A misdirection routine that allows it to copy itself to other programs; a trigger that causes the payload to be activated at a particular time or when a particular event takes place; and a payload that may be a fairly harmless joke or may be very destructive. A program that has a payload but does not have a reproduction routine is known as a Trojan.