XORs message with ChaCha20 keyed by extendKey(key), the nonce being the last twelve bytes of the extended key reversed. Applying it twice restores the message; the empty message stays empty.
message
extendKey(key)
XORs
messagewith ChaCha20 keyed byextendKey(key), the nonce being the last twelve bytes of the extended key reversed. Applying it twice restores the message; the empty message stays empty.