7. 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 acquire 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; program instructions to assign a first substitute character to be used as input for each of the second set of one or more characters; program instructions to present, 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; program instructions to present the at least one image to a user; 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; and program instructions to determine 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.
7. 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 acquire 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; program instructions to assign a first substitute character to be used as input for each of the second set of one or more characters; program instructions to present, 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; program instructions to present the at least one image to a user; 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; and program instructions to determine 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. 12. The computer program product of claim 7 , further comprising the step of: program instructions, stored on at least one of the one or more computer readable storage media, to associate the first string of characters to one or more audio clips, wherein within the one or more audio clips the second set of one or more characters are each represented by a sound specified in the indication of when to use the first substitute character; and program instructions, stored on at least one of the one or more computer readable storage media, to present the one or more audio clips to the user.