ExtraChat is a Dalamud plugin and an associated server that function to add extra chat channels to FFXIV. Basically, this adds cross-data-centre linkshells that don't have a member limit.
Security and privacy
For your privacy and to ensure a lack of liability for server hosts, messages and linkshell names are encrypted between the members of each linkshell. The server is unable to decrypt the content of messages or linkshell names.
The server does know which characters are in which linkshells (an operational requirement).
Due to this design decision, it is impossible for a server to moderate these extra linkshells, and linkshells will need to self-moderate instead. As such, there is no ability to report users.
When a user initiates the process to create a linkshell, their client generates a random shared secret. The secret is saved locally by the client. When the user invites another user to the linkshell, a Diffie-Hellman key exchange is mediated by the server between the two users, and then the inviter transmits the shared secret to the invitee, encrypting it with their ephemeral shared secret created by the key exchange. Due to the nature of the Diffie-Hellman exchange, the server is unable to read the shared secret when it is sent.
After this, the newly-invited user receives information about the linkshell they have been invited to, and can decrypt the name, as well as see any members. If the invitee decides to join, their client will save this shared secret.
Any messages sent to the linkshell are encrypted with the shared secret, making their contents opaque to the server. The only way to read these messages is to know the shared secret, which the server is never able to discern.