Torture is, and has long been, illegal in every case, without exception. It is banned by our Bill of Rights, the Universal Declaration of Human Rights, the Geneva Convention relative to the Treatment of Prisoners of War, the International Covenant on Civil and Political Rights, the Convention Against Torture and Other Cruel Inhuman or Degrading Treatment or Punishment, and Title 18, U.S. Code, Section 2340A. Nothing any president can do can change this or unchange it, weaken it or strengthen it in any way.
Manfred Nowak, the UN Special Rapporteur on Torture, says treaty obligations require the US to prosecute those responsible for torture, up to and including George Bush. If the US fails to prosecute its own officials, then the UN can step in.