psiphon-tunnel-core/replace/dtls/flight4bhandler.go

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

package dtls

import (
	"bytes"
	"context"

	"github.com/pion/dtls/v2/pkg/crypto/prf"
	"github.com/pion/dtls/v2/pkg/protocol"
	"github.com/pion/dtls/v2/pkg/protocol/alert"
	"github.com/pion/dtls/v2/pkg/protocol/extension"
	"github.com/pion/dtls/v2/pkg/protocol/handshake"
	"github.com/pion/dtls/v2/pkg/protocol/recordlayer"
)

func flight4bParse(_ context.Context, _ flightConn, state *State, cache *handshakeCache, cfg *handshakeConfig) (flightVal, *alert.Alert, error) {
	_, msgs, ok := cache.fullPullMap(state.handshakeRecvSequence, state.cipherSuite,
		handshakeCachePullRule{handshake.TypeFinished, cfg.initialEpoch + 1, true, false},
	)
	if !ok {
		// No valid message received. Keep reading
		return 0, nil, nil
	}

	var finished *handshake.MessageFinished
	if finished, ok = msgs[handshake.TypeFinished].(*handshake.MessageFinished); !ok {
		return 0, &alert.Alert{Level: alert.Fatal, Description: alert.InternalError}, nil
	}

	plainText := cache.pullAndMerge(
		handshakeCachePullRule{handshake.TypeClientHello, cfg.initialEpoch, true, false},
		handshakeCachePullRule{handshake.TypeServerHello, cfg.initialEpoch, false, false},
		handshakeCachePullRule{handshake.TypeFinished, cfg.initialEpoch + 1, false, false},
	)

	expectedVerifyData, err := prf.VerifyDataClient(state.masterSecret, plainText, state.cipherSuite.HashFunc())
	if err != nil {
		return 0, &alert.Alert{Level: alert.Fatal, Description: alert.InternalError}, err
	}
	if !bytes.Equal(expectedVerifyData, finished.VerifyData) {
		return 0, &alert.Alert{Level: alert.Fatal, Description: alert.HandshakeFailure}, errVerifyDataMismatch
	}

	// Other party may re-transmit the last flight. Keep state to be flight4b.
	return flight4b, nil, nil
}

func flight4bGenerate(_ context.Context, _ flightConn, state *State, cache *handshakeCache, cfg *handshakeConfig) ([]*packet, *alert.Alert, error) {
	var pkts []*packet

	extensions := []extension.Extension{&extension.RenegotiationInfo{
		RenegotiatedConnection: 0,
	}}
	if (cfg.extendedMasterSecret == RequestExtendedMasterSecret ||
		cfg.extendedMasterSecret == RequireExtendedMasterSecret) && state.extendedMasterSecret {
		extensions = append(extensions, &extension.UseExtendedMasterSecret{
			Supported: true,
		})
	}
	if state.getSRTPProtectionProfile() != 0 {
		extensions = append(extensions, &extension.UseSRTP{
			ProtectionProfiles: []SRTPProtectionProfile{state.getSRTPProtectionProfile()},
		})
	}

	selectedProto, err := extension.ALPNProtocolSelection(cfg.supportedProtocols, state.peerSupportedProtocols)
	if err != nil {
		return nil, &alert.Alert{Level: alert.Fatal, Description: alert.NoApplicationProtocol}, err
	}
	if selectedProto != "" {
		extensions = append(extensions, &extension.ALPN{
			ProtocolNameList: []string{selectedProto},
		})
		state.NegotiatedProtocol = selectedProto
	}

	cipherSuiteID := uint16(state.cipherSuite.ID())
	serverHello := &handshake.Handshake{
		Message: &handshake.MessageServerHello{
			Version:           protocol.Version1_2,
			Random:            state.localRandom,
			SessionID:         state.SessionID,
			CipherSuiteID:     &cipherSuiteID,
			CompressionMethod: defaultCompressionMethods()[0],
			Extensions:        extensions,
		},
	}

	serverHello.Header.MessageSequence = uint16(state.handshakeSendSequence)

	if len(state.localVerifyData) == 0 {
		plainText := cache.pullAndMerge(
			handshakeCachePullRule{handshake.TypeClientHello, cfg.initialEpoch, true, false},
		)
		raw, err := serverHello.Marshal()
		if err != nil {
			return nil, &alert.Alert{Level: alert.Fatal, Description: alert.InternalError}, err
		}
		plainText = append(plainText, raw...)

		state.localVerifyData, err = prf.VerifyDataServer(state.masterSecret, plainText, state.cipherSuite.HashFunc())
		if err != nil {
			return nil, &alert.Alert{Level: alert.Fatal, Description: alert.InternalError}, err
		}
	}

	pkts = append(pkts,
		&packet{
			record: &recordlayer.RecordLayer{
				Header: recordlayer.Header{
					Version: protocol.Version1_2,
				},
				Content: serverHello,
			},
		},
		&packet{
			record: &recordlayer.RecordLayer{
				Header: recordlayer.Header{
					Version: protocol.Version1_2,
				},
				Content: &protocol.ChangeCipherSpec{},
			},
		},
		&packet{
			record: &recordlayer.RecordLayer{
				Header: recordlayer.Header{
					Version: protocol.Version1_2,
					Epoch:   1,
				},
				Content: &handshake.Handshake{
					Message: &handshake.MessageFinished{
						VerifyData: state.localVerifyData,
					},
				},
			},
			shouldEncrypt:            true,
			resetLocalSequenceNumber: true,
		},
	)

	return pkts, nil, nil
}