/**
 * The Authenticator interface provides methods for authenticating a user over an established protocol connection.
 */
interface Authenticator {
    /**
     * Authenticate a user identified by the given [username] and whose authenticity is confirmed by the given [key].
     * The authentication protocol can read and write data within the underlying connection using [readFun] and
     * [writeFun].
     *
     * @param username The name the user goes by.
     * @param key The confirmation of the user's identity and authenticity. It could be any public, private, or derived
     * (e.g. mixed with something else or hashed) datum, possibly shared between the server and the client, such as a
     * UTF-8-encoded password or a raw public key.
     * @param readFun A function to read incoming data from the underlying connection.
     * @param writeFun A function to write outgoing data into the underlying connection.
     * @throws except.FailedAuthenticationException if the authentication could not be performed. A human-readable
     * reason for the failure can be provided if necessary.
     */
    fun authenticate(username: String, key: List<UByte>, readFun: () -> List<UByte>, writeFun: (b: List<UByte>) -> Unit)
}