procedure PowMod(var R:TBigInt; A,B,C:TBigInt); { R = A^B mod C }
var E, T: TBigInt;
       I: Integer;
begin
   Init(R,1);
   for I := 0 to Bites(B)-1 do
      begin
         if IsBit(B,I) then
            begin
               Mul(E, R, A);
               DivMod(E, C, T);
               R := E;
            end;
         Mul(E, A, A);
         DivMod(E, C, T);
         A := E;
       end;
end;