Siemens SIMATIC S7-1500 Function Manual page 38

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
②
Figure 3-7
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.
Communication
Function Manual, 11/2019, A5E03735815-AH
Bob encrypts his message with the symmetric key
Alice decrypts the encrypted message with the symmetric key
Symmetric encryption
Communications services
3.6 Secure Communication
37