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About two hundred years before, the word "computer" started

to appear in the dictionary. Some people even didn't know what is

a computer. However, most of the people today not just knowing

what is a computer, but understand how to use a computer.

Therefore, computer become more and more popular and

important to our society. We can use computer everywhere and

they are very useful and helpful to our life. The speed and

accuracy of computer made people felt confident and reliable.

Therefore, many important information or data are saved in the

computer. Such as your diary, the financial situation of a oil

company or some secret intelligence of the military department. A

lot of important information can be found in the memory of

computer. So, people may ask a question: Can we make sure that

the information in the computer is safe and nobody can steal it

from the memory of the computer?

Physical hazard is one of the causes of destroying the data

in the computer. For example, send a flood of coffee toward a

personal computer. The hard disk of the computer could be

endangered by the flood of coffee. Besides, human caretaker of

computer system can cause as much as harm as any physical hazard.

For example, a cashier in a bank can transfer some money from one

of his customer's account to his own account. Nonetheless, the

most dangerous thief are not those who work with computer every

day, but youthful amateurs who experiment at night --- the


The term "hacker "may have originated at M.I.T. as students'

jargon for classmates who labored nights in the computer lab. In

the beginning, hackers are not so dangerous at all. They just

stole computer time from the university. However, in the early

1980s, hackers became a group of criminals who steal information

from other peoples' computer.

For preventing the hackers and other criminals, people need

to set up a good security system to protect the data in the

computer. The most important thing is that we cannot allow those

hackers and criminals entering our computers. It means that we

need to design a lock to lock up all our data or using

identification to verify the identity of someone seeking access

to our computers.

The most common method to lock up the data is using a

password system. Passwords are a multi-user computer system's

usual first line of defense against hackers. We can use a

combination of alphabetic and number characters to form our own

password. The longer the password, the more possibilities a

hacker's password-guessing program must work through. However it

is difficult to remember a very long passwords. So people will

try to write the password down and it may immediately make it a

security risk. Furthermore, a high speed password-guessing

program can find out a password easily. Therefore, it is not

enough for a computer that just have a password system to protect

its data and memory.

Besides password system, a computer company may consider

about the security of its information centre. In the past, people

used locks and keys to limit access to secure areas. However,

keys can be stolen or copied easily. Therefore, card-key are

designed to prevent the situation above. Three types of card-keys

are commonly used by banks, computer centers and government

departments. Each of this card-keys can employ an identifying

number or password that is encoded in the card itself, and all

are produced by techniques beyond the reach of the average

computer criminals. One of the three card-key is called watermark

magnetic. It was inspired by the watermarks on paper currency.

The card's magnetic strip have a 12-digit number code and it

cannot be copied. It can store about two thousand bits in the

magnetic strip. The other two cards have the capability of

storing thousands of times of data in the magnetic strip. They

are optical memory cards (OMCs) and Smart cards. Both of them are

always used in the security system of computers.

However, it is not enough for just using password system and

card-keys to protect the memory in the computer. A computer

system also need to have a restricting program to verify the

identity of the users. Generally, identity can be established by

something a person knows, such



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