The De Justo, or Peri Dikaiou, is occasionally found in manuscripts of Plato's dialogues; it has, however, never been generally regarded as other than spurious. It is not especially readable -- it reads like an abridgment and clumsy adaptation of some other work, and is usually taken to be such -- but it does have some interest. It has close affinities with a passage in Xenophon's Memorabilia (IV.2.12-20), which suggests two possibilities -- either the author is adapting Xenophon or, what is usually taken to be more likely, both Xenophon and the author have a common text that they are adapting -- Hutchinson's introduction in the Complete Works suggests as a possibility the (nonextant) dialogue On Law by Socrates' student Antisthenes.
I can find no online translation of this short dialogue, but you are not missing much if you don't read it; it is somewhat unclear and abrupt, and of more historical than substantive interest.
One of the characters is explicitly called Socrates; the other is not named.
Socrates opens by asking what the just is; his interlocutor suggests that it whatever is established as just by custom. Socrates doesn't like this answer; he notes that if you asked what an eye was, the natural way to respond would be to say it is what you see with. So what does justice do? The interlocutor doesn't know, so Socrates goes through a number of other analogies, and manages to get the interlocutor to recognize that justice has something to do with what judges do by speaking. Since the interlocutor is still unable to say what justice is, Socrates asks whether people do injustice willingly or unwillingly. The interlocutor says willingly, but Socrates quotes Epicharmus of Syracuse: "No one is willingly wicked, nor unwillingly blessed" (374b).
The interlocutor responds that singers tell many lies, so this turns the discussion to lying. Socrates gets the interlocutor to admit that "lying, harming, and deceiving are unjust" (374c), but when he asks if this is the case even with enemies, the interlocutor balks, and then on further pressing says that it is not unjust to help a friend by lying. Socrates gets the interlocutor to characterize justice by saying that an act is just if it is done if and when one should do it. But in general, Socrates notes with a few examples, people who do what they should do it because of their knowledge; so people would then do justice because of their knowledge: "justice is what our ancestors handed down to us as wisdom and injustice is what they handed down to us as ignorance" (375c).
But if it comes down to knowledge, people are ignorant unwillingly, so they would be unjust unwillingly. Socrates concludes that Epicharmus did not lie, and his interlocutor concedes.
Quotations from Andrew S. Becker's translation in Plato, Complete Works, Cooper and Hutchinson, eds., pp. 1687-1693.