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Security Key
If we use a certain key to lock (or encrypt) an information and use the same key to unlock (decrypt) the information, we call this key system as "Symmetric Key".
The concept is very simple and intuitive since it is almost same as the lock-and-key that we use for your door or luggage back etc. If you use this kind of key to send an information to another person, it can be illustrated as follows.
So far so good ? Now we have some problems. To use this method, both Bob and Alice should have the same key. It means Alice should send the same key to Bob somehow. Then the problem is how they can share the key without any copies of the key being stollen ?
Isn't there any way to exchange the symmetric key (the same secret key on both side) without the risk of being stolen by a bad person ? One very famous and widely used method as an answer to this question is called 'D-H Algorithm' and this algorithm will be explained in a separate page : Diffie-Hellman Algorithm
If we use a certain key to lock (or encrypt) an information and use the different key to unlock (decrypt) the information, we call this key system as "Asymmetric Key". If you use this kind of key to send an information to another person, it can be illustrated as follows.
The biggest advantage of this system is that you don't have to share a single key to the other person. So the possibility of the key being stollen in the process of sharing the key would be removed.
However, the concept is not so intuitive because it is different from those ordinary key that you use to lock your door. How this can be possible ? How we can make a lock which we use different key when we lock and unlock. It would be not easy to make this kind of key physically (I don't know if it is even possible at all), but it turned out to be possible in computer system using a special mathematical technique. I will try to explain this mathematical technique later.
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