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3.6 Secure Communication

Communication

Function Manual, 11/2019, A5E03735815-AH

Symmetric encryption

The central aspect of symmetric encryption is that both communication partners use the

same key for message encryption and decryption, as shown in the figure below. Bob uses

the same key for encryption as Alice uses for decryption. In general, we also say that the two

sides share the secret key with which they encrypt or decrypt a message as a secret.

①

Bob encrypts his message with the symmetric key

②

Alice decrypts the encrypted message with the symmetric key

Figure 3-7 Symmetric encryption

The process can be compared to a briefcase to which the sender and recipient have the

same key, which both locks and opens the case.

● Advantage: Symmetric encryption algorithms (such as AES, Advanced Encryption

Algorithm) are fast.

● Disadvantages: How can the key be sent to a recipient without getting into the wrong

hands? This is a key distribution problem. If enough messages are intercepted, the key

can also be worked out and must therefore be changed regularly.

If there are a large number of communication partners, there is also a large number of keys

to distribute.

Siemens SIMATIC ET 200SP Function Manual

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