The "Keys Changed" notification in Facebook Messenger signifies a change in encryption keys used to secure your conversations. Such changes are typically normal and occur due to app reinstallation, device switching, updates, or security upgrades.
Key transparency provides messaging users with a verifiable and auditable record of public keys. It allows them to confirm that their conversations are indeed encrypted with the correct keys for their contacts, and that these keys haven’t been maliciously swapped by a compromised server.
Everything I can find on FB docs showcases a single key per side scenario. After I inspect usage of the secret chat, I can only speculate it is build on top of the existing message exchange. The key exchange is just an additional key or index so FB can remove those message from the server.
When end-to-end encrypted Messenger calling is turned on, you and whoever you’re talking to in an end-to-end encrypted call are the only people who have unique and matching device keys and can access the call.
Facebook Messenger’s Secret Conversations use end-to-end encryption with unique device keys. The 'first seen by this device' date and key indicate when a device first accessed the encrypted chat, even if you didn’t initiate it.
Encryption keys are essential components of secure messaging. They serve as digital "locks" and "keys" that safeguard your conversations from unauthorized access. When you start a secret conversation on Messenger, a set of cryptographic keys is generated specifically for that chat.
When you send a message on Messenger, the text is encrypted on your device using a unique encryption key. This key is then sent along with the message to the recipient’s device, where it is decrypted using the same key.