1. A method for determining if a user of a computer system is a human, the method comprising the steps of: acquiring, by one or more processors, at least one image depicting a first string of characters including at least a first set of one or more characters and a second set of one or more characters; assigning, by the one or more processors, a first substitute character to be used as input for each of the second set of one or more characters; presenting, by the one or more processors, the at least one image to a user; receiving, by the one or more processors, a second string of characters from the user; determining, by the one or more processors, whether the second string of characters substantially matches, within a predetermined threshold, the first string of characters based on the first substitute character assigned to each of the second set of one or more characters; and determining, by the one or more processors, whether the user is a human based on whether the second string of characters substantially matches, within a predetermined threshold, the first string of characters.