The symmetric encryption is a cryptographic procedure, in which the encryption and decryption of a message is done with the same key (see picture). This implies that the participants have already exchanged keys before they start communicate with each other .
Bob has important information that wants to send to Alice, using an insecure channel (e.g. internet). In order for Bob to be sure that the information, which he will send, will not be read by a third party (Eve), he should first encrypt the message. From the beginning though, should Bob and Alice agree on which will be the common key K1 that they will use. With the key K1 Bob encrypt now the information and send them via the insecure channel to Alice. Even if Eve receives this piece of information, it is useless to her, because after the encryption it has no meaning at all. For the decryption of the information the same key K1, which was used for the encryption of the information, is required again. Since only Bob and Alice have previously exchanged the key, only they can read the original text.
- Very fast encryption and decryption of a message
- The bigger the code of the key, the greater the safety
- Exchange of the key: It must be ensured that the key to encryption, will be exchanged via a secure channel (e.g. in person). However, in practice this can be really difficult
- The number of the keys that are required: For each pair of participants that wish to exchange encrypted messages, a new key is required 
|||A. Menezes, “Handbook of Applied Cryptography “, CRC Press, 1996, ISBN 978-0849385230.|
|||www.it-administrator.de: symmetrische Verschlüsselung.
www.it-administrator.de/lexikon/symmetrische_verschluesselung.html – Day 29.11.2016