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; presenting, by the one or more processors, in association with the at least one image, an indication of the first substitute character and an indication of when to use the first substitute character to the user, wherein the first substitute character is a different character than any of the second set of one or more characters; 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.