Idea

An alernate mode of the encryption system which would work like a private chat room;
the messages sent using this system (which would act as a ghost chat hosted by the mod, not minecraft) would only show to people that have set their 'chat room key' set as the sender's one, you would be able to start a room using something like ".ncr create (key)" and join it using something like ".ncr join (key)", once you are connected, only you & the other people that have connected would be able to see the messages, which you would send using something like ".ncr (message)", or just a toggle so that you can talk in the server chat normally and it'd still send

Completely undetectable by the server or the game, only usable by people using NCR.
This would allow you to communicate with multiple people at a time using the mod without having the server interfere and would allow you to communicate with 100% safety on realms & servers.

Reasoning

Safer, & will not interfere with the other people on the server that have no idea what you're saying other than random letters & symbols

Other Information

No response

Related: #14

I think you shouldn't use client command (.ncr) to send external messages. Just add a button like Encryption.
An additional idea is make a external chatroom setting GUI, and you can set whether showing external messages in chatbox.