SecWebSocketKey

The Sec-WebSocket-Key header is used in the WebSocket handshake. It is sent from the client to the server to provide part of the information used by the server to prove that it received a valid WebSocket handshake. This helps ensure that the server does not accept connections from non-WebSocket clients (e.g. HTTP clients) that are being abused to send data to unsuspecting WebSocket servers.

See: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Sec-WebSocket-Key