Cookie encryption

I'm trying to mix the CI3 framework with a node.js server (socket.io) to make real-time chats rooms depending on CI's users groups.

I'm using the redis driver and i need to pass datas from the php server to the node server. 

As i'm using Redis for the session management, i suppose that i could just pass the session_id of the logged user to node (reading the cookie genberated by CI) , and from this id, retrieve redis session datas with node. ( basically i need to know the group of the logged user)


I dont understand how works the cookie encryption in CI. I just get an hash. I dont know how to decrypt it , and i dont know how CI can retrieve its own datas inside.