Siemens SIMATIC ET 200AL System Manual page 742

Communications services
3.6 Secure Communication
Course of the secure communication
The figure below shows, in simplified terms, how communication is established
("handshake") focusing on the negotiation of keys used for data exchange (here with HTTP
over TLS).
However, the course can be applied to all communication options that are based on the
usage of TLS, i.e. also for Secure Open User Communication (see Basics for secure
communication).
Figure 3-14
The figure does not show the measures taken at Alice's end (browser) to verify the certificate
sent by the Web server. Whether Alice can trust the Web server certificate received and
therefore the identity of the Web server, and can accept the exchange of data, depends on
positive verification.
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Handshake with https
Function Manual, 12/2017, A5E03735815-AF
Communication