Catalyzes the hydrolysis of inorganic orthophosphate from phytate. 1D-myo-inositol hexakisphosphate + H2O = 1D-myo-inositol 1,2,4,5,6-pentakisphosphate + phosphate Optimum pH is 5.5-6.0. Active from 3.5 to 8.5 with phytic acid as substrate. The optimum pH is shifted to more acidic values with 4-nitrophenyl phosphate as substrate. Belongs to the histidine acid phosphatase family.