PART II
INTERNATIONAL CRIMES AND OFFENCES AGAINST THE ADMINISTRATION OF JUSTICE
6. Crimes against humanity -
(2) For the purposes of this section, “crime against humanity” is an act specified in article 7 of the Statute and includes any other act which, at the time and in the place of its commission, constitutes a crime against humanity according to customary international law or conventional international law or by virtue of it being criminal according to the general principles of law recognized by the community of nations, whether or not it constitutes a contravention of the law in force at the time and in the place of its commission.
Non-retroactivity
Nullum crimen sine lege
EDIT.