How to decrypt WebRTC with Diffie-Hellman?

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I see no problem with the decryption:

$ tshark -o tls.keylog_file:ssl-keys.log -r wireshark.pcapng -Y "tls.record.content_type in {20..22}" -T fields -e tcp.stream -e frame.number -e _ws.col.Info | sort -n
11  63  Client Hello
11  65  Server Hello, Change Cipher Spec, Encrypted Extensions, Finished
11  66  Change Cipher Spec, Finished
11  70  SETTINGS[0], WINDOW_UPDATE[0]
18  111 Client Hello
18  172 Server Hello, Certificate, Server Hello Done
18  182 Alert (Level: Fatal, Description: Certificate Unknown)
19  112 Client Hello
19  151 Server Hello, Certificate, Server Hello Done
19  162 Alert (Level: Fatal, Description: Certificate Unknown)
20  116 Client Hello
20  194 Server Hello, Certificate, Server Hello Done
20  209 Alert (Level: Fatal, Description: Certificate Unknown)
21  210 Client Hello
21  273 Server Hello, Certificate, Server Hello Done
21  276 Client Key Exchange, Change Cipher Spec, Finished
21  298 Change Cipher Spec
21  374 Finished
22  219 Client Hello
22  259 Server Hello, Certificate, Server Hello Done
22  260 Client Key Exchange, Change Cipher Spec, Finished
22  313 Change Cipher Spec
22  373 Finished
23  224 Client Hello
23  292 Server Hello, Certificate, Server Hello Done
23  294 Client Key Exchange, Change Cipher Spec, Finished
23  349 Change Cipher Spec
23  380 Finished
24  1923    Client Hello
24  1925    Server Hello
24  1926    Alert (Level: Fatal, Description: Certificate Unknown)
24  1930    Change Cipher Spec, Finished
25  1942    Client Hello
25  1947    Server Hello
25  2059    Change Cipher Spec, Finished
25  2060    Change Cipher Spec, Finished
25  4955    Alert (Level: Warning, Description: Close Notify)
34  2378    Client Hello
34  2381    Server Hello
34  2382    Alert (Level: Fatal, Description: Certificate Unknown)
34  2384    Change Cipher Spec, Finished
35  2389    Client Hello
35  2392    Server Hello
35  2398    Change Cipher Spec, Finished
35  2399    Change Cipher Spec, Finished
35  4936    Alert (Level: Warning, Description: Close Notify)
36  3053    Encrypted Alert
$

All TLS sessions that have a TLS handshake are decrypted, as can be seen by the "finished" handshake message at the end of each TLS handshake.

However, traffic on port 8443 is not automatically dissected as HTTP once it is decrypted. You need to add 8443 to the "SSL/TLS Ports" list in the HTTP protocol preferences.