What are ex post facto laws designed to do?

Ex post facto laws are specifically designed to criminalize actions that were not considered illegal at the time they were performed. These laws apply retroactively, which means they allow the government to impose punishment for behavior that was legal when it occurred. The use of ex post facto laws is prohibited by the U.S. Constitution because they violate fundamental principles of fairness and justice, such as the notion that individuals should have knowledge of what constitutes criminal behavior before they can be prosecuted.

In contrast to this, making future acts illegal pertains to legislation that outlines what will be considered unlawful going forward, which has no retrospective effect. Enhancing criminal punishment refers to increasing penalties for offenses but does not pertain to the retroactive application of the law. Protecting civil liberties would involve legislation that safeguards individual rights and freedoms, rather than imposing restrictions or retroactive punishments. Thus, the primary function of ex post facto laws is to criminalize actions that have already occurred, which is why the selected answer correctly identifies their purpose.