CHAPTER IV
Fundamental Rights and Freedoms of the Individual
39. Provisions to secure protection of law.
(8) No person shall be held to be guilty of a criminal offence on account of any act or omission that did not, at the time it took place, constitute such an offence, and no penalty shall be imposed for any criminal offence which is severer in degree or description than the maximum penalty which might have been imposed for that offence at the time when it was committed.
Non-retroactivity
Nulla poena sine lege
EDIT.