The term kruptei/a is mentioned in ancient sources on Sparta (e.g. Plato, Laws 633B; Aristotle fr.538 Rose; Plutarch, Lycurgus 28 and Kleomenes 28): see LSJ s.v. It has conventionally been translated "secret service" (so LSJ) or the like. But it appears actually to have been part of the character-building tests of endurance, a training in stealth and (perhaps) murder, which all Spartan youths had to undertake. See generally OCD(4) s.v. (p.786).