We'll say there's a limit of 160 characters per single chat message. But obviously being a real-time chat program over the internet you can type up more than one message. Chat person #1 (Bob) generates a large list of secret random keys (or pads). Each one time pad is 160 characters long. So we end up with a long list of pads. These get put in a SQLite database or something with primary keys numbered from say 1 to 5000.