To prevent spoofed chat messages and prove a message was sent by a specific user, what should you do client‑side?

To prevent spoofing and prove message origin, you need a mechanism that definitively links the message to the sender. Using Public Key Infrastructure (PKI), the originating user can sign the message with their private key. This creates a digital signature that can be verified by anyone with the user's public key. If the signature is valid, it proves the message was sent by that specific user and hasn't been tampered with. Tagging messages with user identifiers is easily spoofed. Encrypting with a shared key ensures confidentiality but doesn't prove origin, as anyone with the shared key could send the message. SSL/TLS secures the communication channel between the client and server but doesn't authenticate the individual user's message content itself.