Cloak/internal/client/TLS.go

package client

import (
	"crypto/rand"
	"encoding/binary"
	"github.com/cbeuw/Cloak/internal/util"
	"net"
	"time"

	log "github.com/sirupsen/logrus"
)

type clientHelloFields struct {
	random         []byte
	sessionId      []byte
	x25519KeyShare []byte
	sni            []byte
}

type browser interface {
	composeClientHello(clientHelloFields) []byte
}

func makeServerName(serverName string) []byte {
	serverNameListLength := make([]byte, 2)
	binary.BigEndian.PutUint16(serverNameListLength, uint16(len(serverName)+3))
	serverNameType := []byte{0x00} // host_name
	serverNameLength := make([]byte, 2)
	binary.BigEndian.PutUint16(serverNameLength, uint16(len(serverName)))
	ret := make([]byte, 2+1+2+len(serverName))
	copy(ret[0:2], serverNameListLength)
	copy(ret[2:3], serverNameType)
	copy(ret[3:5], serverNameLength)
	copy(ret[5:], serverName)
	return ret
}

// addExtensionRecord, add type, length to extension data
func addExtRec(typ []byte, data []byte) []byte {
	length := make([]byte, 2)
	binary.BigEndian.PutUint16(length, uint16(len(data)))
	ret := make([]byte, 2+2+len(data))
	copy(ret[0:2], typ)
	copy(ret[2:4], length)
	copy(ret[4:], data)
	return ret
}

func addRecordLayer(input []byte, typ []byte, ver []byte) []byte {
	length := make([]byte, 2)
	binary.BigEndian.PutUint16(length, uint16(len(input)))
	ret := make([]byte, 5+len(input))
	copy(ret[0:1], typ)
	copy(ret[1:3], ver)
	copy(ret[3:5], length)
	copy(ret[5:], input)
	return ret
}

func genStegClientHello(ai authenticationPayload, serverName string) (ret clientHelloFields) {
	// random is marshalled ephemeral pub key 32 bytes
	// The authentication ciphertext and its tag are then distributed among SessionId and X25519KeyShare
	ret.random = ai.randPubKey[:]
	ret.sessionId = ai.ciphertextWithTag[0:32]
	ret.x25519KeyShare = ai.ciphertextWithTag[32:64]
	ret.sni = makeServerName(serverName)
	return
}

type DirectTLS struct {
	browser
}

func (DirectTLS) HasRecordLayer() bool                              { return true }
func (DirectTLS) UnitReadFunc() func(net.Conn, []byte) (int, error) { return util.ReadTLS }

// PrepareConnection handles the TLS handshake for a given conn and returns the sessionKey
// if the server proceed with Cloak authentication
func (tls DirectTLS) PrepareConnection(authInfo *authInfo, conn net.Conn) (preparedConn net.Conn, sessionKey [32]byte, err error) {
	preparedConn = conn
	payload, sharedSecret := makeAuthenticationPayload(authInfo, rand.Reader, time.Now())
	chOnly := tls.browser.composeClientHello(genStegClientHello(payload, authInfo.MockDomain))
	chWithRecordLayer := addRecordLayer(chOnly, []byte{0x16}, []byte{0x03, 0x01})
	_, err = preparedConn.Write(chWithRecordLayer)
	if err != nil {
		return
	}
	log.Trace("client hello sent successfully")

	buf := make([]byte, 1024)
	log.Trace("waiting for ServerHello")
	_, err = util.ReadTLS(preparedConn, buf)
	if err != nil {
		return
	}

	encrypted := append(buf[11:43], buf[89:121]...)
	nonce := encrypted[0:12]
	ciphertextWithTag := encrypted[12:60]
	sessionKeySlice, err := util.AESGCMDecrypt(nonce, sharedSecret[:], ciphertextWithTag)
	if err != nil {
		return
	}
	copy(sessionKey[:], sessionKeySlice)

	for i := 0; i < 2; i++ {
		// ChangeCipherSpec and EncryptedCert (in the format of application data)
		_, err = util.ReadTLS(preparedConn, buf)
		if err != nil {
			return
		}
	}

	return preparedConn, sessionKey, nil

}