Randomly reversing a substring

I was thinking about how to make this with the StringReverse, but I failed:

I have a sequence of (for example):

str = "FDSRTYNHFNKHLIUHG";

I wish to reverse a randomly long contiguous sequence of characters starting at a random location, for example FDSRTYNHFNKHLIHUG (UH-HU), but it can also be FDSFHNYTRNKHLIUHG (RTYNHF-FHNYTR). Do you have some good ideas? Thank you!!!