1. A computer program product for determining if a user of a computer system is a human, the computer program product comprising: one or more computer readable storage media and program instructions stored on the one or more computer readable storage media, the program instructions comprising: program instructions to receive an indication that a computer security program is needed, and in response, 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; wherein each character of the first set of one or more characters is an alphanumeric character; and wherein each character of the second set of one or more characters is illegible; program instructions to assign a first substitute character to be used as input for each of the second set of one or more characters, wherein the first substitute character is a different character than any of the second set of one or more characters; program instructions to present 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 presenting 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 comprises: program instructions to present the at least one image, a description of the first substitute character, and an indication of when to use the first substitute character to the user; and program instructions to receive a second string of characters from the user; program instructions to determine 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, wherein the determination further comprises: program instructions to compare each character of the second string of characters to a respective character within the first string of characters, according to character locations within each string of characters; and program instructions to associate a weighting factor to each character of the second string of characters based on the comparison and the set of one or more characters within which the respective character of the first string of characters is a member; and program instructions to determine that the second string of characters substantially matches, within a second predetermined threshold, the first string of characters, based on the weighting factor associated with each character within the second string of characters, and responsive to determining that the second string of characters substantially matches, within a second predetermined threshold, the first string of characters, based on the weighting factor associated with each character within the second string of characters, program instructions to determine that the user of the computer system is a human.