This is a method known for thousands of years and is based on the fact that the sender and the recipient both have access to a message by knowing a secret shared key. This key can take on a wide variety of forms: The Romans used a stick of a certain diameter for encryption and decryption.
Today's digital communications rely in the main upon a password as the key. Using this password and an encryption algorithm, the data from the sender are changed. The recipient uses the same key and the fitting encryption algorithm so that the data become legible again. Other persons who do not know the key cannot read the data. A common symmetrical method of encryption is 3DES, for example.
Example:
- Alice wants to send a confidential message to Bob. To do this, she encrypts the message with a secret key and a suitable method, e.g. 3DES. She sends the encrypted message to Bob informing him of the encryption method she used.
- Bob has the same key as Alice. Since he knows which encryption method was used, he can decrypt the message and transform it back into plain text.
Symmetrical encryption is simple and efficient but has two serious disadvantages:
- A different key is required for every secret communications relationship. If Alice and Bob are joined by Carol, three keys are necessary for secure data communications between all parties; with four participants, the number of keys required is six; with 12 participants, 66 keys are required and with 1000 participants, almost 500,000 keys are necessary! In a worldwide network with ever increasing demands for secure communications and higher numbers of participants, the serious nature of this problem is obvious.
- While this first disadvantage could be solved with technology, the second problem that is the core problem for symmetrical encryption: The secret key must be known at both ends of the communication and must not fall into the hands of unauthorized persons. Thus it is not possible for Alice simply to send the key to Bob per e-mail before the data connection has been secured sufficiently—which is the whole point of the encryption. She has to give the key to Bob in person, or at least make use of a communications method which is safe from eavesdroppers. This is a task which is almost impossible to handle in these times of worldwide dynamic communications.