{ my @shiftby = (0, 8, 16, 24, 0, 8, 16, 24, 8, 16, 24); 
  my $l;
  foreach $l (reverse 1 .. $len) {
     if    ($l>8) {   $c += $k[$l-1]<<$shiftby[$l]; }
     elsif ($l>4) {   $b += $k[$l-1]<<$shiftby[$l]; }
     else         {   $a += $k[$l-1]<<$shiftby[$l]; }
  }
}