I'm trying to break some encryption to see if I can duplicate the algorithm. I am able to feed it strings and get the resulting output. Anyone know how to attack this problem?

Some Examples:
Input___________Output
abcd ------------> FZi7yOhygv8-
bcde ------------> F0gxYetzhf4-
cdef ------------> rIkpoup0hP0-
123 ------------> 10hz5rgi0g--
1234 ------------> drhRX7gi0q8-

That should help, but I'm not so sure where to begin.