  We show that, in order to preserve the equivalence principle until late times
in unitarily evaporating black holes, the thermodynamic entropy of a black hole
must be primarily entropy of entanglement across the event horizon. For such
black holes, we show that the information entering a black hole becomes encoded
in correlations within a tripartite quantum state, the quantum analogue of a
one-time pad, and is only decoded into the outgoing radiation very late in the
evaporation. This behavior generically describes the unitary evaporation of
highly entangled black holes and requires no specially designed evolution. Our
work suggests the existence of a matter-field sum rule for any fundamental
theory.
