THE CORE RULES OF LEGALITY IN CUSTOMARY INTERNATIONAL LAW
The history since World War II, set out in previous chapters, shows that the law has changed since Nuremberg. The central aspects of the principle of legality in criminal law, especially the non-retroactivity of crimes and punishments, are now rules of customary international law.
The statement of the principle of non-retroactivity of crimes and punishments from the Universal Declaration of Human Rights (UDHR) has become customary international law:
No one shall be held guilty of any penal offense on account of any act or omission which did not constitute a penal offense, under national or international law, at the time when it was committed. Nor shall a heavier penalty be imposed than the one that was applicable at the time the penal offense was committed.
The prohibition of the act and the maximum penalty must not only have been in existence at the time of the act. They must also have been applicable to the actor and the action at the time. An act cannot constitute an offense carrying a penalty except pursuant to some national or international law applicable to the actor and the act at the time committed.
This summary of the customary international human rights rules of legality is based on the practice and opinio juris of states, the organic documents, other documents, practice and opinio juris of international organizations (especially but not only international criminal courts and tribunals), writings of publicists, and other evidence.