Refactor client into separate files

Add a main loop, move I/O code into io.go, move configuration into config.go.
Use shared.Decode24BitLength instead of manually decoding block lengths.
Fix response block decoding and checksum validation.
Add constants for commonly used byte values and use these in the signer and
the client.

client/config.go

@@ -0,0 +1,84 @@
+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+	"time"
+
+	"github.com/goburrow/serial"
+	"github.com/sirupsen/logrus"
+	"gopkg.in/yaml.v2"
+)
+
+type SerialConfig struct {
+	Address  string `yaml:"address"`
+	BaudRate int    `yaml:"baudrate"`
+	DataBits int    `yaml:"databits"`
+	StopBits int    `yaml:"stopbits"`
+	Parity   string `yaml:"parity"`
+}
+
+type ClientConfig struct {
+	Serial        SerialConfig `yaml:"serial_config"`
+	Paranoid      bool         `yaml:"paranoid"`
+	Debug         bool         `yaml:"debug"`
+	GNUPGBinary   string       `yaml:"gnupg_bin"`
+	OpenSSLBinary string       `yaml:"openssl_bin"`
+	MySQLDSN      string       `yaml:"mysql_dsn"`
+}
+
+var defaultConfig = ClientConfig{
+	Serial: SerialConfig{
+		Address:  "/dev/ttyUSB0",
+		BaudRate: 115200,
+		DataBits: 8,
+		StopBits: 1,
+		Parity:   "N",
+	},
+	Paranoid:      false,
+	Debug:         false,
+	OpenSSLBinary: "/usr/bin/openssl",
+	GNUPGBinary:   "/usr/bin/gpg",
+	MySQLDSN:      "<username>:<password>@/database?parseTime=true",
+}
+
+func generateExampleConfig(configFile string) (config *ClientConfig, err error) {
+	config = &defaultConfig
+	configBytes, err := yaml.Marshal(config)
+	if err != nil {
+		logrus.Errorf("could not generate configuration data")
+		return
+	}
+
+	logrus.Infof("example data for %s:\n\n---\n%s\n", configFile, configBytes)
+	return
+}
+
+func readConfig(configFile string) (config *ClientConfig, err error) {
+	source, err := ioutil.ReadFile(configFile)
+	if err != nil {
+		logrus.Errorf("opening configuration file failed: %v", err)
+		if exampleConfig, err := generateExampleConfig(configFile); err != nil {
+			return nil, err
+		} else {
+			logrus.Info("starting with default config")
+			return exampleConfig, nil
+		}
+	}
+	if err := yaml.Unmarshal(source, &config); err != nil {
+		return nil, fmt.Errorf("loading configuration file failed: %v", err)
+	}
+
+	return config, nil
+}
+
+func fillSerialConfig(clientConfig *ClientConfig) *serial.Config {
+	return &serial.Config{
+		Address:  clientConfig.Serial.Address,
+		BaudRate: clientConfig.Serial.BaudRate,
+		DataBits: clientConfig.Serial.DataBits,
+		StopBits: clientConfig.Serial.StopBits,
+		Parity:   clientConfig.Serial.Parity,
+		Timeout:  30 * time.Second,
+	}
+}

client/io.go

@@ -0,0 +1,98 @@
+package main
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/goburrow/serial"
+	log "github.com/sirupsen/logrus"
+
+	"git.cacert.org/cacert-gosigner/datastructures"
+	"git.cacert.org/cacert-gosigner/shared"
+)
+
+func sendHandShake(port serial.Port) error {
+	log.Debug("Shaking hands ...")
+	if length, err := port.Write([]byte{shared.HandshakeByte}); err != nil {
+		return fmt.Errorf("could not write handshake byte: %v", err)
+	} else {
+		log.Tracef("wrote %d handshake bytes", length)
+	}
+	handShakeResponse := make([]byte, 1)
+	if length, err := port.Read(handShakeResponse); err != nil {
+		return fmt.Errorf("failed to read handshake response: %v", err)
+	} else {
+		log.Tracef("read %d bytes", length)
+	}
+	if handShakeResponse[0] != shared.AckByte {
+		return fmt.Errorf("invalid handshake response expected 0x10 received %x", handShakeResponse[0])
+	}
+	log.Debug("Handshake successful")
+	return nil
+}
+
+func receiveResponse(port *serial.Port, responseChan *chan datastructures.SignerResponse, errorChan *chan error) {
+	header, err := shared.ReceiveBytes(port, 1, 20)
+	if err != nil {
+		*errorChan <- err
+		return
+	}
+	if header[0] != shared.HandshakeByte {
+		*errorChan <- fmt.Errorf("unexpected byte 0x%x expected 0x%x", header, shared.HandshakeByte)
+	}
+	log.Tracef("received handshake byte")
+
+	if _, err := (*port).Write([]byte{shared.AckByte}); err != nil {
+		*errorChan <- errors.New("could not write ACK")
+		return
+	}
+	log.Tracef("sent ACK byte")
+
+	lengthBytes, err := shared.ReceiveBytes(port, 3, 2)
+	if err != nil {
+		*errorChan <- err
+		return
+	}
+	blockLength := datastructures.Decode24BitLength(lengthBytes)
+	log.Tracef("received block length %d", blockLength)
+
+	blockData, err := shared.ReceiveBytes(port, blockLength, 5)
+	if err != nil {
+		*errorChan <- err
+		return
+	}
+	log.Tracef("received bytes %v", blockData)
+
+	checkSum, err := shared.ReceiveBytes(port, 1, 2)
+	if err != nil {
+		*errorChan <- err
+		return
+	}
+	log.Tracef("received checksum 0x%x", checkSum[0])
+
+	trailer, err := shared.ReceiveBytes(port, len(shared.MagicTrailer), 2)
+	if err != nil {
+		*errorChan <- err
+		return
+	}
+	if string(trailer) != shared.MagicTrailer {
+		*errorChan <- errors.New("expected trailer bytes not found")
+		return
+	}
+	log.Tracef("received valid trailer bytes")
+
+	if _, err := (*port).Write([]byte{shared.AckByte}); err != nil {
+		*errorChan <- fmt.Errorf("could not write ACK byte: %v", err)
+		return
+	}
+	log.Tracef("sent ACK byte")
+
+	signerResponse, err := datastructures.SignerResponseFromData(lengthBytes, blockData, checkSum[0])
+	if err != nil {
+		*errorChan <- err
+		return
+	}
+	log.Infof("received response of type %s", signerResponse.Action)
+
+	*responseChan <- *signerResponse
+}

client/main.go

@@ -1,86 +1,17 @@
 package main
 
 import (
-	"encoding/binary"
-	"errors"
 	"flag"
 	"fmt"
+	"time"
+
 	"git.cacert.org/cacert-gosigner/datastructures"
 	"git.cacert.org/cacert-gosigner/shared"
-	"io/ioutil"
-	"time"
 
 	"github.com/goburrow/serial"
 	log "github.com/sirupsen/logrus"
-	"gopkg.in/yaml.v2"
 )
 
-type SerialConfig struct {
-	Address  string `yaml:"address"`
-	BaudRate int    `yaml:"baudrate"`
-	DataBits int    `yaml:"databits"`
-	StopBits int    `yaml:"stopbits"`
-	Parity   string `yaml:"parity"`
-}
-
-type ClientConfig struct {
-	Serial        SerialConfig `yaml:"serial_config"`
-	Paranoid      bool         `yaml:"paranoid"`
-	Debug         bool         `yaml:"debug"`
-	GNUPGBinary   string       `yaml:"gnupg_bin"`
-	OpenSSLBinary string       `yaml:"openssl_bin"`
-	MySQLDSN      string       `yaml:"mysql_dsn"`
-}
-
-var defaultConfig = ClientConfig{
-	Serial: SerialConfig{
-		Address:  "/dev/ttyUSB0",
-		BaudRate: 115200,
-		DataBits: 8,
-		StopBits: 1,
-		Parity:   "N",
-	},
-	Paranoid:      false,
-	Debug:         false,
-	OpenSSLBinary: "/usr/bin/openssl",
-	GNUPGBinary:   "/usr/bin/gpg",
-	MySQLDSN:      "<username>:<password>@/database?parseTime=true",
-}
-
-const HandshakeByte = 0x02
-const AckByte = 0x10
-const NackByte = 0x11
-
-func readConfig(configFile string) (config *ClientConfig, err error) {
-	source, err := ioutil.ReadFile(configFile)
-	if err != nil {
-		log.Errorf("opening configuration file failed: %v", err)
-		if exampleConfig, err := generateExampleConfig(configFile); err != nil {
-			return nil, err
-		} else {
-			log.Info("starting with default config")
-			return exampleConfig, nil
-		}
-	}
-	if err := yaml.Unmarshal(source, &config); err != nil {
-		return nil, fmt.Errorf("loading configuration file failed: %v", err)
-	}
-
-	return config, nil
-}
-
-func generateExampleConfig(configFile string) (config *ClientConfig, err error) {
-	config = &defaultConfig
-	configBytes, err := yaml.Marshal(config)
-	if err != nil {
-		log.Errorf("could not generate configuration data")
-		return
-	}
-
-	log.Infof("example data for %s:\n\n---\n%s\n", configFile, configBytes)
-	return
-}
-
 func main() {
 	var configFile string
 
@@ -94,177 +25,127 @@ func main() {
 	if clientConfig, err = readConfig(configFile); err != nil {
 		log.Panic(err)
 	}
-	serialConfig = &serial.Config{
+	serialConfig = fillSerialConfig(clientConfig)
-		Address:  clientConfig.Serial.Address,
-		BaudRate: clientConfig.Serial.BaudRate,
-		DataBits: clientConfig.Serial.DataBits,
-		StopBits: clientConfig.Serial.StopBits,
-		Parity:   clientConfig.Serial.Parity,
-		Timeout:  30 * time.Second,
-	}
 	if clientConfig.Debug {
 		log.SetLevel(log.DebugLevel)
 	}
 
-	log.Debugf("connecting %+v", serialConfig)
+	log.Infof("connecting to %s", serialConfig.Address)
+	log.Tracef("serial parameters %v", serialConfig)
 	port, err := serial.Open(serialConfig)
 	if err != nil {
 		log.Fatal(err)
 	}
-	log.Debug("connected")
+	log.Debug("serial port connected")
 	defer func() {
 		err := port.Close()
 		if err != nil {
 			log.Fatal(err)
 		}
-		log.Debug("closed")
+		log.Debug("serial port closed")
 	}()
 
-	request := datastructures.NewNulRequest()
-	timeout := time.After(2700 * time.Millisecond)
 	errorChannel := make(chan error, 1)
-	responseChannel := make(chan *datastructures.SignerResponse, 1)
+	responseChannel := make(chan datastructures.SignerResponse, 1)
+	crlCheck := 0
 
-	go func() {
+	log.Debug("starting main loop")
-		if response, err := SendRequest(port, request); err != nil {
+
-			errorChannel <- err
+	for {
-		} else {
+		requestChannel := make(chan datastructures.SignerRequest, 1)
-			responseChannel <- response
+
+		go HandleRequests(&port, &responseChannel, &errorChannel, &requestChannel)
+
+		log.Debug("handling GPG database ...")
+		// HandleGPG(&requestChannel)
+		log.Debug("issuing certificates ...")
+		// HandleCertificates(&requestChannel)
+		log.Debug("revoking certificates ...")
+		// RevokeCertificates(&requestChannel)
+
+		crlCheck++
+		if crlCheck%100 == 0 {
+			log.Debug("refresh CRLs ...")
+			// RefreshCRLs(&requestChannel)
 		}
-	}()
 
-	select {
+		log.Debug("send NUL request to keep connection open")
-	case <-timeout:
+		requestChannel <- *datastructures.NewNulRequest()
-		log.Fatal("timeout")
+
-	case err := <-errorChannel:
+		select {
-		log.Fatal(err)
+		case response := <-responseChannel:
-	case response := <-responseChannel:
+			if err := Process(response); err != nil {
-		if err := Process(response); err != nil {
+				log.Error(err)
-			log.Fatal(err)
+			}
+		case err := <-errorChannel:
+			log.Error(err)
+		}
+
+		log.Debug("sleep for 2.7 seconds")
+		time.Sleep(2700 * time.Millisecond)
+	}
+}
+
+func Process(response datastructures.SignerResponse) (err error) {
+	log.Infof("process response of type %s", response.Action)
+	log.Tracef("process response %v", response)
+
+	switch response.Action {
+	case datastructures.ActionNul:
+		log.Trace("received response for NUL request")
+		return
+	default:
+		return fmt.Errorf("unsupported action in response 0x%x", response.Action)
+	}
+}
+
+func HandleRequests(port *serial.Port, responseChan *chan datastructures.SignerResponse, errorChan *chan error, requestChan *chan datastructures.SignerRequest) {
+	for {
+		select {
+		case request := <-*requestChan:
+			SendRequest(port, responseChan, errorChan, &request)
 		}
 	}
 }
 
-func Process(response *datastructures.SignerResponse) error {
+func SendRequest(port *serial.Port, responseChan *chan datastructures.SignerResponse, errorChan *chan error, request *datastructures.SignerRequest) {
-	log.Debugf("process %v", response)
+	log.Tracef("send request %v to serial port %v", *request, *port)
-	return nil
+	if err := sendHandShake(*port); err != nil {
-}
+		*errorChan <- err
+		return
+	}
 
-func sendHandShake(port serial.Port) error {
+	requestBytes := request.Serialize()
-	log.Debug("Shaking hands ...")
+	if length, err := (*port).Write(requestBytes); err != nil {
-	if length, err := port.Write([]byte{HandshakeByte}); err != nil {
+		*errorChan <- err
-		return fmt.Errorf("could not write handshake byte: %v", err)
+		return
 	} else {
-		log.Debugf("wrote %d handshake bytes", length)
+		log.Tracef("wrote %d request bytes", length)
+	}
+
+	if length, err := (*port).Write([]byte{datastructures.CalculateXorCheckSum([][]byte{requestBytes})}); err != nil {
+		*errorChan <- err
+		return
+	} else {
+		log.Tracef("wrote %d checksum bytes", length)
+	}
+
+	if length, err := (*port).Write([]byte(shared.MagicTrailer)); err != nil {
+		*errorChan <- err
+		return
+	} else {
+		log.Tracef("wrote %d trailer bytes", length)
 	}
-	handShakeResponse := make([]byte, 1)
-	if length, err := port.Read(handShakeResponse); err != nil {
-		return fmt.Errorf("failed to read handshake response: %v", err)
-	} else {
-		log.Debugf("read %d bytes", length)
-	}
-	if handShakeResponse[0] != AckByte {
-		return fmt.Errorf("invalid handshake response expected 0x10 received %x", handShakeResponse[0])
-	}
-	log.Debug("Handshake successful")
-	return nil
-}
 
-func receiveResponse(port *serial.Port, responseChan *chan []byte, errorChan *chan error) {
 	header, err := shared.ReceiveBytes(port, 1, 20)
 	if err != nil {
 		*errorChan <- err
 		return
 	}
-	if header[0] != HandshakeByte {
+	if header[0] != shared.AckByte {
-		*errorChan <- fmt.Errorf("unexpected byte 0x%x expected 0x%x", header, HandshakeByte)
+		*errorChan <- fmt.Errorf("unexpected byte 0x%x expected 0x%x", header, shared.AckByte)
-	}
-
-	if _, err := (*port).Write([]byte{AckByte}); err != nil {
-		*errorChan <- errors.New("could not write ACK")
 		return
 	}
 
-	lengthBytes, err := shared.ReceiveBytes(port, 3, 2)
+	receiveResponse(port, responseChan, errorChan)
-	if err != nil {
-		*errorChan <- err
-		return
-	}
-	blockLength := binary.BigEndian.Uint32([]byte{0x0, lengthBytes[0], lengthBytes[1], lengthBytes[2]})
-	blockData, err := shared.ReceiveBytes(port, int(blockLength), 5)
-	if err != nil {
-		*errorChan <- err
-		return
-	}
-
-	checkSum, err := shared.ReceiveBytes(port, 1, 2)
-	if err != nil {
-		*errorChan <- err
-		return
-	}
-	log.Debugf("block checksum is %d", checkSum)
-	trailer, err := shared.ReceiveBytes(port, len(shared.MagicTrailer), 2)
-	if err != nil {
-		*errorChan <- err
-		return
-	}
-	if string(trailer) != shared.MagicTrailer {
-		*errorChan <- errors.New("expected trailer bytes not found")
-		return
-	}
-
-	if _, err := (*port).Write([]byte{AckByte}); err != nil {
-		*errorChan <- fmt.Errorf("could not write ACK byte: %v", err)
-	}
-	*responseChan <- blockData
-
-	return
-}
-
-func SendRequest(port serial.Port, request *datastructures.SignerRequest) (response *datastructures.SignerResponse, err error) {
-	log.Debugf("send request %v to serial port %v", request, port)
-
-	if err = sendHandShake(port); err != nil {
-		return nil, err
-	}
-
-	requestBytes := request.Serialize()
-	if length, err := port.Write(requestBytes); err != nil {
-		log.Fatal(err)
-	} else {
-		log.Debugf("wrote %d request bytes", length)
-	}
-
-	if length, err := port.Write([]byte{datastructures.CalculateXorCheckSum([][]byte{requestBytes})}); err != nil {
-		log.Fatal(err)
-	} else {
-		log.Debugf("wrote %d checksum bytes", length)
-	}
-
-	if length, err := port.Write([]byte(shared.MagicTrailer)); err != nil {
-		log.Fatal(err)
-	} else {
-		log.Debugf("wrote %d trailer bytes", length)
-	}
-
-	header, err := shared.ReceiveBytes(&port, 1, 20)
-	if err != nil {
-		return nil, err
-	}
-	if header[0] != AckByte {
-		return nil, fmt.Errorf("unexpected byte 0x%x expected 0x%x", header, AckByte)
-	}
-
-	responseChan := make(chan []byte, 1)
-	errChan := make(chan error, 1)
-	go receiveResponse(&port, &responseChan, &errChan)
-
-	select {
-	case responseData := <-responseChan:
-		log.Debugf("response data: %v", responseData)
-	case err := <-errChan:
-		log.Errorf("%v", err)
-	}
-
-	return nil, nil
 }

datastructures/common.go

@@ -6,6 +6,15 @@ type Action uint8
 
 const ActionNul = Action(0)
 
+func (a Action) String() string {
+	switch a {
+	case ActionNul:
+		return "NUL"
+	default:
+		return "unknown"
+	}
+}
+
 func encode24BitLength(data []byte) []byte {
 	lengthBytes := make([]byte, 4)
 	binary.BigEndian.PutUint32(lengthBytes, uint32(len(data)))
@@ -13,7 +22,7 @@ func encode24BitLength(data []byte) []byte {
 }
 
 // calculate length from 24 bits of data in network byte order
-func decode24BitLength(bytes []byte) int {
+func Decode24BitLength(bytes []byte) int {
 	return int(binary.BigEndian.Uint32([]byte{0x0, bytes[0], bytes[1], bytes[2]}))
 }
 

datastructures/signerrequest.go

@@ -25,19 +25,19 @@ type SignerRequest struct {
 const protocolVersion = 1
 
 func SignerRequestFromData(lengthBytes []byte, blockData []byte, checkSum byte) (*SignerRequest, error) {
-	headerLength := decode24BitLength(blockData[0:3])
+	headerLength := Decode24BitLength(blockData[0:3])
 	headerBytes := blockData[3 : 3+headerLength]
 
 	contentBytes := blockData[3+headerLength:]
-	content1Length := decode24BitLength(contentBytes[0:3])
+	content1Length := Decode24BitLength(contentBytes[0:3])
 	content1 := string(contentBytes[3 : 3+content1Length])
 
 	content2Offset := 3 + content1Length
-	content2Length := decode24BitLength(contentBytes[content2Offset : content2Offset+3])
+	content2Length := Decode24BitLength(contentBytes[content2Offset : content2Offset+3])
 	content2 := string(contentBytes[3+content2Offset : 3+content2Offset+content2Length])
 
 	content3Offset := 3 + content2Offset + content2Length
-	content3Length := decode24BitLength(contentBytes[content3Offset : content3Offset+3])
+	content3Length := Decode24BitLength(contentBytes[content3Offset : content3Offset+3])
 	content3 := string(contentBytes[3+content3Offset : 3+content3Offset+content3Length])
 
 	calculated := CalculateXorCheckSum([][]byte{lengthBytes, blockData})
@@ -79,8 +79,8 @@ func (r SignerRequest) Serialize() []byte {
 
 func NewNulRequest() *SignerRequest {
 	return &SignerRequest{
-		Version: protocolVersion,
+		Version:  protocolVersion,
-		Action:  ActionNul,
+		Action:   ActionNul,
 		Content1: time.Now().UTC().Format("010203042006.05"),
 	}
 }

datastructures/signerresponse.go

@@ -17,20 +17,26 @@ type SignerResponse struct {
 }
 
 func SignerResponseFromData(lengthBytes []byte, blockData []byte, checkSum byte) (*SignerResponse, error) {
-	headerLength := decode24BitLength(lengthBytes)
+	if len(blockData) < 3 {
-	headerBytes := blockData[3 : 3+headerLength]
+		return nil, errors.New("begin of structure corrupt")
+	}
 
-	contentBytes := blockData[3+headerLength:]
+	offset := 0
-	content1Length := decode24BitLength(contentBytes[0:3])
+	headerLength := Decode24BitLength(blockData[offset : offset+3])
-	content1 := string(contentBytes[3 : 3+content1Length])
+	offset += 3
+	headerBytes := blockData[offset : offset+headerLength]
+	offset += headerLength
 
-	content2Offset := 3 + content1Length
+	content := make([]string, 3)
-	content2Length := decode24BitLength(contentBytes[content2Offset : content2Offset+3])
+	for offset < len(blockData) {
-	content2 := string(contentBytes[3+content2Offset : 3+content2Offset+content2Length])
+		dataLength := Decode24BitLength(blockData[offset : offset+3])
-
+		if len(blockData)-3 < dataLength {
-	content3Offset := 3 + content2Offset + content2Length
+			return nil, errors.New("structure cut off")
-	content3Length := decode24BitLength(contentBytes[content3Offset : content3Offset+3])
+		}
-	content3 := string(contentBytes[3+content3Offset : 3+content3Offset+content3Length])
+		offset += 3
+		content = append(content, string(blockData[offset:offset+dataLength]))
+		offset += dataLength
+	}
 
 	calculated := CalculateXorCheckSum([][]byte{lengthBytes, blockData})
 	if checkSum != calculated {
@@ -42,9 +48,9 @@ func SignerResponseFromData(lengthBytes []byte, blockData []byte, checkSum byte)
 		Action:    Action(headerBytes[1]),
 		Reserved1: headerBytes[2],
 		Reserved2: headerBytes[3],
-		Content1:  content1,
+		Content1:  content[0],
-		Content2:  content2,
+		Content2:  content[1],
-		Content3:  content3,
+		Content3:  content[2],
 	}, nil
 }
 

shared/shared.go

@@ -1,3 +1,7 @@
 package shared
 
 const MagicTrailer = "rie4Ech7"
+
+const HandshakeByte = 0x02
+const AckByte = 0x10
+const NackByte = 0x11

signer/main.go

@@ -1,15 +1,15 @@
 package main
 
 import (
-	"encoding/binary"
 	"errors"
 	"flag"
 	"fmt"
-	"git.cacert.org/cacert-gosigner/datastructures"
-	"git.cacert.org/cacert-gosigner/shared"
 	"log"
 	"time"
 
+	"git.cacert.org/cacert-gosigner/datastructures"
+	"git.cacert.org/cacert-gosigner/shared"
+
 	"github.com/goburrow/serial"
 )
 
@@ -96,7 +96,7 @@ func SendResponse(port *serial.Port, response *datastructures.SignerResponse) er
 	}
 
 	tryAgain := true
-	for ; tryAgain; {
+	for tryAgain {
 		data := response.Serialize()
 		if _, err := (*port).Write(data); err != nil {
 			return err
@@ -150,11 +150,11 @@ func Receive(port *serial.Port, commandChan *chan datastructures.SignerRequest,
 		*errorChan <- err
 		return
 	}
-	if header[0] != 0x02 {
+	if header[0] != shared.HandshakeByte {
-		*errorChan <- fmt.Errorf("unexpected byte 0x%x expected 0x02", header)
+		*errorChan <- fmt.Errorf("unexpected byte 0x%x expected 0x%x", header[0], shared.HandshakeByte)
 	}
 
-	if _, err := (*port).Write([]byte{0x10}); err != nil {
+	if _, err := (*port).Write([]byte{shared.AckByte}); err != nil {
 		*errorChan <- errors.New("could not write ACK")
 		return
 	}
@@ -164,8 +164,8 @@ func Receive(port *serial.Port, commandChan *chan datastructures.SignerRequest,
 		*errorChan <- err
 		return
 	}
-	blockLength := binary.BigEndian.Uint32([]byte{0x0, lengthBytes[0], lengthBytes[1], lengthBytes[2]})
+	blockLength := datastructures.Decode24BitLength(lengthBytes)
-	blockData, err := shared.ReceiveBytes(port, int(blockLength), 5)
+	blockData, err := shared.ReceiveBytes(port, blockLength, 5)
 	if err != nil {
 		*errorChan <- err
 		return
@@ -199,4 +199,3 @@ func Receive(port *serial.Port, commandChan *chan datastructures.SignerRequest,
 
 	*commandChan <- *command
 }
-