session_srtp.go

// SPDX-FileCopyrightText: 2023 The Pion community <https://pion.ly>
// SPDX-License-Identifier: MIT

package srtp

import (
	"net"
	"sync"
	"time"

	"github.com/pion/logging"
	"github.com/pion/rtp"
)

const defaultSessionSRTPReplayProtectionWindow = 64

// SessionSRTP implements io.ReadWriteCloser and provides a bi-directional SRTP session
// SRTP itself does not have a design like this, but it is common in most applications
// for local/remote to each have their own keying material. This provides those patterns
// instead of making everyone re-implement.
type SessionSRTP struct {
	session
	writeStream *WriteStreamSRTP
}

// NewSessionSRTP creates a SRTP session using conn as the underlying transport.
func NewSessionSRTP(conn net.Conn, config *Config) (*SessionSRTP, error) { //nolint:dupl
	if config == nil {
		return nil, errNoConfig
	} else if conn == nil {
		return nil, errNoConn
	}

	loggerFactory := config.LoggerFactory
	if loggerFactory == nil {
		loggerFactory = logging.NewDefaultLoggerFactory()
	}

	localOpts := append(
		[]ContextOption{},
		config.LocalOptions...,
	)
	remoteOpts := append(
		[]ContextOption{
			// Default options
			SRTPReplayProtection(defaultSessionSRTPReplayProtectionWindow),
		},
		config.RemoteOptions...,
	)

	srtpSession := &SessionSRTP{
		session: session{
			nextConn:            conn,
			localOptions:        localOpts,
			remoteOptions:       remoteOpts,
			readStreams:         map[uint32]readStream{},
			newStream:           make(chan readStream),
			acceptStreamTimeout: config.AcceptStreamTimeout,
			started:             make(chan interface{}),
			closed:              make(chan interface{}),
			bufferFactory:       config.BufferFactory,
			log:                 loggerFactory.NewLogger("srtp"),
		},
	}
	srtpSession.writeStream = &WriteStreamSRTP{srtpSession}

	err := srtpSession.session.start(
		config.Keys.LocalMasterKey, config.Keys.LocalMasterSalt,
		config.Keys.RemoteMasterKey, config.Keys.RemoteMasterSalt,
		config.Profile,
		srtpSession,
	)
	if err != nil {
		return nil, err
	}

	return srtpSession, nil
}

// OpenWriteStream returns the global write stream for the Session.
func (s *SessionSRTP) OpenWriteStream() (*WriteStreamSRTP, error) {
	return s.writeStream, nil
}

// OpenReadStream opens a read stream for the given SSRC, it can be used
// if you want a certain SSRC, but don't want to wait for AcceptStream.
func (s *SessionSRTP) OpenReadStream(ssrc uint32) (*ReadStreamSRTP, error) {
	r, _ := s.session.getOrCreateReadStream(ssrc, s, newReadStreamSRTP)

	if readStream, ok := r.(*ReadStreamSRTP); ok {
		return readStream, nil
	}

	return nil, errFailedTypeAssertion
}

// AcceptStream returns a stream to handle RTCP for a single SSRC.
func (s *SessionSRTP) AcceptStream() (*ReadStreamSRTP, uint32, error) {
	stream, ok := <-s.newStream
	if !ok {
		return nil, 0, errStreamAlreadyClosed
	}

	readStream, ok := stream.(*ReadStreamSRTP)
	if !ok {
		return nil, 0, errFailedTypeAssertion
	}

	return readStream, stream.GetSSRC(), nil
}

// Close ends the session.
func (s *SessionSRTP) Close() error {
	return s.session.close()
}

func (s *SessionSRTP) write(b []byte) (int, error) {
	packet := &rtp.Packet{}

	if err := packet.Unmarshal(b); err != nil {
		return 0, err
	}

	return s.writeRTP(&packet.Header, packet.Payload)
}

// bufferpool is a global pool of buffers used for encrypted packets in
// writeRTP below.  Since it's global, buffers can be shared between
// different sessions, which amortizes the cost of allocating the pool.
//
// 1472 is the maximum Ethernet UDP payload.  We give ourselves 20 bytes
// of slack for any authentication tags, which is more than enough for
// either CTR or GCM.  If the buffer is too small, no harm, it will just
// get expanded by growBuffer.
var bufferpool = sync.Pool{ // nolint:gochecknoglobals
	New: func() interface{} {
		return make([]byte, 1492)
	},
}

func (s *SessionSRTP) writeRTP(header *rtp.Header, payload []byte) (int, error) {
	if _, ok := <-s.session.started; ok {
		return 0, errStartedChannelUsedIncorrectly
	}

	// encryptRTP will either return our buffer, or, if it is too
	// small, allocate a new buffer itself.  In either case, it is
	// safe to put the buffer back into the pool, but only after
	// nextConn.Write has returned.
	ibuf := bufferpool.Get()
	defer bufferpool.Put(ibuf)

	s.session.localContextMutex.Lock()
	encrypted, err := s.localContext.encryptRTP(ibuf.([]byte), header, payload)
	s.session.localContextMutex.Unlock()

	if err != nil {
		return 0, err
	}

	return s.session.nextConn.Write(encrypted)
}

func (s *SessionSRTP) setWriteDeadline(t time.Time) error {
	return s.session.nextConn.SetWriteDeadline(t)
}

func (s *SessionSRTP) decrypt(buf []byte) error {
	header := &rtp.Header{}
	headerLen, err := header.Unmarshal(buf)
	if err != nil {
		return err
	}

	r, isNew := s.session.getOrCreateReadStream(header.SSRC, s, newReadStreamSRTP)
	if r == nil {
		return nil // Session has been closed
	} else if isNew {
		if !s.session.acceptStreamTimeout.IsZero() {
			_ = s.session.nextConn.SetReadDeadline(time.Time{})
		}
		s.session.newStream <- r // Notify AcceptStream
	}

	readStream, ok := r.(*ReadStreamSRTP)
	if !ok {
		return errFailedTypeAssertion
	}

	decrypted, err := s.remoteContext.decryptRTP(buf, buf, header, headerLen)
	if err != nil {
		return err
	}

	_, err = readStream.write(decrypted)
	if err != nil {
		return err
	}

	return nil
}