secretbox.Overhead
// Overhead is the number of bytes of overhead when boxing a message.
const Overhead = poly1305.TagSize
secretbox.Open
// Open authenticates and decrypts a box produced by Seal and appends the
// message to out, which must not overlap box. The output will be Overhead
// bytes smaller than box.
func Open(out []byte, box []byte, nonce *[24]byte, key *[32]byte) ([]byte, bool)
secretbox.Seal
// Seal appends an encrypted and authenticated copy of message to out, which
// must not overlap message. The key and nonce pair must be unique for each
// distinct message and the output will be Overhead bytes longer than message.
func Seal(out []byte, message []byte, nonce *[24]byte, key *[32]byte) []byte