[ciphernomad-org]

This is a classic Feistel network round: L_new = L_old ^ F(key, R_old). It's a deliberate design.

The security of the Labyrinth relies on the composition of many rounds, not any single one. The unchanged right half from a Hash round is fully encrypted if the next round is a Stream round. Since the Labyrinth is a deep, aperiodic mix of both round types, the entire block is guaranteed to be diffused and encrypted.