"Men could not live with one another if there were not mutual confidence that they were being truthful to one another." The virtue of truth gives another his just due. Truthfulness keeps to the just mean between what ought to be expressed and what ought to be kept secret: it entails honesty and discretion. In justice, "as a matter of honor, one man owes it to another to manifest the truth."
Source: Catechism of the Catholic Church (Vatican.va)