Tetration Modulo

No.11470007 ViewReplyOriginalReport
Anyone here willing to help a retard out?
I need a way to calculate a^^x = b mod p where a, b and p are all around 80-90 bits. I found a stack overflow about it https://stackoverflow.com/questions/30713648/how-to-compute-ab-mod-m
but i dont get it, too much mathspeak.