An access server communicates with a video conference device and a user device. The access server and the user device may access known mappings, including a default mapping. Each mapping is configured to map between tokens and symbols according to a distinct mapping relationship between the multi-bit tokens and the symbols. The access server instructs the video conference device to map an initial token to an initial symbol based on the default mapping and to transmit the initial symbol. The access server receives from the user device the initial token and a challenge mapping selected by the user device. The access server determines a challenge token that the challenge mapping maps to a challenge symbol, and instructs the video conference device to transmit the challenge symbol. The access server grants the user device access to an information carrying channel only if the challenge token is received from the user device.