European Convention on Human Rights. Article 7, Clause 1 defined the prohibition of retroactive justice thus:
1. No one shall be held guilty of any criminal offence on account of any act or omission which did not constitute a criminal offence 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 criminal offence was committed.
They did, however, make a reservation with Clause 2, which stated:
2. This article shall not prejudice the trial and punishment of any person for any act or omission which, at the time when it was committed, was criminal according the general principles of law recognized by civilized nations.
Until the late 1940s, West Germany had followed the precedent set at the Nuremberg Trials, whereby defendants were found guilty in spite of the state policies at the time their crimes were committed. Arguments that they were following orders from superiors, or acting in accordance with the laws of the time, were generally disregarded. From the early fifties onwards, the legal process shifted somewhat to reflect the prohibition of retroactive justice; some prisoners were released early or received lesser sentences than they would have done previously. This does not mean that the issue of prosecuting citizens for war crimes was not taken seriously; the Frankfurt Auschwitz Trials of 1963-1965 are testament to this.
There is a detailed article about the impact and influence of the War Crimes Trials on Germany at this link.