EverCrypt.Chacha20_Poly1305SourceMultiplexing interface for Chacha20-Poly1305
encrypt key iv ad pt takes a key, an initial value iv, additional data ad, and plaintext pt and returns a tuple containing the encrypted pt and the authentication tag for the plaintext and the associated data.
decrypt key iv ad ct tag takes a key, the initial value iv, additional data ad, ciphertext ct, and authentication tag tag, and, if successful, returns the decrypted ct.