Backend server for A.Chat - encrypted web chat application. Powered by socket.io and Node.js.

The A.Chat backend server is a Node.js application located in the /server directory.

Copy .env.dist file in server/ directory without the .dist extension and adapt it to your needs.

You can use nvm to install the right version of node using this command:

nvm install # If the right node version is not already installed
nvm use
npm install yarn -g # To install yarn

Install dependencies and start the server

$ yarn setup
$ yarn dev

Just run the following:

$ docker-compose up

This will automatically create the default .env file for you.

Start server

$ yarn start

Build it.

$ docker build --tag a.chat-backend:latest .

Then run it. Example:

$ docker run --init --name a.chat-backend --rm -p 3001:3001 a.chat-backend

You are able to use any of the environment variables available in server/.env.dist. The defaults are available in Dockerfile

Please report any security issues to [email protected].

A.Chat uses a combination of asymmetric encryption (RSA-OAEP), symmetric session keys (AES-CBC) and signing keys (HMAC) for security.

Here's an overview of a chat between Alice and Bob (also applies to group chats):

1. Bob creates a room and immediately creates a public/private key pair (RSA-OAEP).
2. Alice joins the room and also creates a public/private key pair. She is sent Bob's public key and she sends Bob her public key.
3. When Bob goes to send a message, three things are created: a session key (AES-CBC), a signing key (HMAC SHA-256) and an initialization vector (used in the encryption process).
4. Bob's message is encrypted with the session key and initialization vector, and a signature is created using the signing key.
5. The session key and signing key are encrypted with each recipient's public key (in this case only Alice, but in a group chat multiple).
6. The encrypted message, initialization vector, signature, encrypted session key and encrypted signing key are sent to all recipients (in this case just Alice) as a package.
7. Alice receives the package and decrypts the session key and signing key using her private key. She decrypts the message with the decrypted session key and vector, and verifies the signature with the decrypted signing key.

Group chats work the same way because in step 5 we encrypt keys with everyone's public key. When a message is sent out, it includes encrypted keys for everyone in the room, and the recipients then pick out the ones for them based on their user ID.

A.Chat does not provide any guarantee that the person you're communicating with is who you think they are. Authentication functionality may be incorporated in future versions.

A.Chat encodes documents into base64 using btoa and is encrypted the same way chat messages are.

1. When a file is "uploaded", the document is encoded on the client and the server recieves the encrypted base64 string.
2. The server sends the encrypted base64 string to clients in the same chat room.
3. Clients recieving the encrypted base64 string then decrypts and decodes the base64 string using atob.

The default transferable file size limit is 4MB, but can be changed in .env file with the REACT_APP_MAX_FILE_SIZE variable.

A.Chat uses socket.io to transmit encrypted information using secure WebSockets (WSS).

Rooms are stored in memory on the server until all participants have left, at which point the room is destroyed. Only public keys are stored in server memory for the duration of the room's life.

Chat history is stored in each participant's browser, so it is effectively erased (for that user) when their window is closed.