As you may have noticed, up to now, we have talked of systems where by knowing the algorithm and the key, you can both encrypt and decrypt a message. This kind of encryption is known as symmetric encryption. The big advantage of this kind of encryption is that it is easy because it does not require complex math and much calculation to execute. On the other hand, it makes critical the key exchange moment and key management. In fact, the key has to be exchanged before the transmission can start between the parties, and it has to be done securely. As for the key management problem, since both parties know the same secret (in fact, this kind of cryptography is also called shared secret), if you have multiple people that have to communicate with each other, you will need n(n-1)/2 keys—this means that in a group of 20 people, you'll need 190 keys.

The following is the schema of a communication using a symmetric encryption.

Today, the types of symmetric encryption that are used...