package brontide import ( "errors" "fmt" "io" "net" "time" "github.com/btcsuite/btcd/btcec" ) // defaultHandshakes is the maximum number of handshakes that can be done in // parallel. const defaultHandshakes = 1000 // Listener is an implementation of a net.Conn which executes an authenticated // key exchange and message encryption protocol dubbed "Machine" after // initial connection acceptance. See the Machine struct for additional // details w.r.t the handshake and encryption scheme used within the // connection. type Listener struct { localStatic *btcec.PrivateKey tcp *net.TCPListener handshakeSema chan struct{} conns chan maybeConn quit chan struct{} } // A compile-time assertion to ensure that Conn meets the net.Listener interface. var _ net.Listener = (*Listener)(nil) // NewListener returns a new net.Listener which enforces the Brontide scheme // during both initial connection establishment and data transfer. func NewListener(localStatic *btcec.PrivateKey, listenAddr string) (*Listener, error) { addr, err := net.ResolveTCPAddr("tcp", listenAddr) if err != nil { return nil, err } l, err := net.ListenTCP("tcp", addr) if err != nil { return nil, err } brontideListener := &Listener{ localStatic: localStatic, tcp: l, handshakeSema: make(chan struct{}, defaultHandshakes), conns: make(chan maybeConn), quit: make(chan struct{}), } for i := 0; i < defaultHandshakes; i++ { brontideListener.handshakeSema <- struct{}{} } go brontideListener.listen() return brontideListener, nil } // listen accepts connection from the underlying tcp conn, then performs // the brontinde handshake procedure asynchronously. A maximum of // defaultHandshakes will be active at any given time. // // NOTE: This method must be run as a goroutine. func (l *Listener) listen() { for { select { case <-l.handshakeSema: case <-l.quit: return } conn, err := l.tcp.Accept() if err != nil { l.rejectConn(err) l.handshakeSema <- struct{}{} continue } go l.doHandshake(conn) } } // rejectedConnErr is a helper function that prepends the remote address of the // failed connection attempt to the original error message. func rejectedConnErr(err error, remoteAddr string) error { return fmt.Errorf("unable to accept connection from %v: %v", remoteAddr, err) } // doHandshake asynchronously performs the brontide handshake, so that it does // not block the main accept loop. This prevents peers that delay writing to the // connection from block other connection attempts. func (l *Listener) doHandshake(conn net.Conn) { defer func() { l.handshakeSema <- struct{}{} }() select { case <-l.quit: return default: } remoteAddr := conn.RemoteAddr().String() brontideConn := &Conn{ conn: conn, noise: NewBrontideMachine(false, l.localStatic, nil), } // We'll ensure that we get ActOne from the remote peer in a timely // manner. If they don't respond within 1s, then we'll kill the // connection. conn.SetReadDeadline(time.Now().Add(handshakeReadTimeout)) // Attempt to carry out the first act of the handshake protocol. If the // connecting node doesn't know our long-term static public key, then // this portion will fail with a non-nil error. var actOne [ActOneSize]byte if _, err := io.ReadFull(conn, actOne[:]); err != nil { brontideConn.conn.Close() l.rejectConn(rejectedConnErr(err, remoteAddr)) return } if err := brontideConn.noise.RecvActOne(actOne); err != nil { brontideConn.conn.Close() l.rejectConn(rejectedConnErr(err, remoteAddr)) return } // Next, progress the handshake processes by sending over our ephemeral // key for the session along with an authenticating tag. actTwo, err := brontideConn.noise.GenActTwo() if err != nil { brontideConn.conn.Close() l.rejectConn(rejectedConnErr(err, remoteAddr)) return } if _, err := conn.Write(actTwo[:]); err != nil { brontideConn.conn.Close() l.rejectConn(rejectedConnErr(err, remoteAddr)) return } select { case <-l.quit: return default: } // We'll ensure that we get ActTwo from the remote peer in a timely // manner. If they don't respond within 1 second, then we'll kill the // connection. conn.SetReadDeadline(time.Now().Add(handshakeReadTimeout)) // Finally, finish the handshake processes by reading and decrypting // the connection peer's static public key. If this succeeds then both // sides have mutually authenticated each other. var actThree [ActThreeSize]byte if _, err := io.ReadFull(conn, actThree[:]); err != nil { brontideConn.conn.Close() l.rejectConn(rejectedConnErr(err, remoteAddr)) return } if err := brontideConn.noise.RecvActThree(actThree); err != nil { brontideConn.conn.Close() l.rejectConn(rejectedConnErr(err, remoteAddr)) return } // We'll reset the deadline as it's no longer critical beyond the // initial handshake. conn.SetReadDeadline(time.Time{}) l.acceptConn(brontideConn) } // maybeConn holds either a brontide connection or an error returned from the // handshake. type maybeConn struct { conn *Conn err error } // acceptConn returns a connection that successfully performed a handshake. func (l *Listener) acceptConn(conn *Conn) { select { case l.conns <- maybeConn{conn: conn}: case <-l.quit: } } // rejectConn returns any errors encountered during connection or handshake. func (l *Listener) rejectConn(err error) { select { case l.conns <- maybeConn{err: err}: case <-l.quit: } } // Accept waits for and returns the next connection to the listener. All // incoming connections are authenticated via the three act Brontide // key-exchange scheme. This function will fail with a non-nil error in the // case that either the handshake breaks down, or the remote peer doesn't know // our static public key. // // Part of the net.Listener interface. func (l *Listener) Accept() (net.Conn, error) { select { case result := <-l.conns: return result.conn, result.err case <-l.quit: return nil, errors.New("brontide connection closed") } } // Close closes the listener. Any blocked Accept operations will be unblocked // and return errors. // // Part of the net.Listener interface. func (l *Listener) Close() error { select { case <-l.quit: default: close(l.quit) } return l.tcp.Close() } // Addr returns the listener's network address. // // Part of the net.Listener interface. func (l *Listener) Addr() net.Addr { return l.tcp.Addr() }