Under Known-Issues I found this paragraph:
The RLPx handshake is considered 'broken crypto' because aes-secret and mac-secret are reused for both reading and writing. The two sides of a RLPx connection generate two CTR streams from the same key, nonce and IV. If an attacker knows one plaintext, they can decrypt unknown plaintexts of the reused keystream.
Q: Can someone please Explain what this means and how this can be exploited by a third party who is able to observe the encrypted communication between two nodes.