@Carl Love got it. thanks

@Carl Love 

restart:
InverseCheck:=proc(a,b)
 local x, inverse;
 inverse:=true:
   for x from 0 to 255 do
  if (f(a)@f(b))(x)<> x or (f(b)@f(a))(x)<> x then inverse:=false; break; end if;
 end do:

 inverse;

end proc:

SS:={}:
for a from 0 to 255 do
 for b from 0 to 255 do
  if InverseCheck(a,b)=true then SS:=SS union {{a,b}};
  end if;
 end do;
end do:
SS;