A visual tag having an imprinted image, a processor for calculating a response string corresponding to the content of the imprinted image and a challenge string, and a changeable screen to display the response string in machine-readable symbols. An authentication device reads the displayed symbols, and authenticates the displayed contents corresponding to the challenge string. The challenge string is generated either according to a random generator within the authentication device, or according to a real-time clock. A plurality of tags or objects can be authenticated and spotted within an image. The tag can also authenticate inputted messages and credentials.