Systems and methods for user authentication

The present disclosure relates to a computer-implemented method for operating a server computer, a user computer and a system to authenticate a user. At a server computer a symbol arrangement and a series of authentication instructions comprising human intelligible text and computer-executable code are generated, the computer-executable code is executed in order to obtain a non-ambiguous instruction series result and the symbol arrangement and the human intelligible text representing the series of instructions are transmitted to a user computer. At the user computer, the symbol arrangement and the human intelligible text are displayed, a user input is received and the user input is transmitted to the user computer. At the server computer, the received user input is compared with the instruction series result and the user is authenticated if the received user input corresponds to the instruction series result.