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device.go
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device.go
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/***
Copyright (c) 2020, Hector Sanjuan
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.
***/
package nfctype4
import (
"bytes"
"errors"
"fmt"
"github.com/hsanjuan/go-ndef"
"github.com/hsanjuan/go-nfctype4/capabilitycontainer"
"github.com/hsanjuan/go-nfctype4/helpers"
)
// Device represents an NFC Forum device, that is, an application
// which allows to perform Read and Update operations on a NFC Type 4 Tag,
// by following the operation instructions stated in the specification.
//
// Interaction with physical and software Tags is done via CommandDrivers.
// A device is configured with a CommandDriver which is
// in charge of sending and receiving bytes from the Tags.
// The `nfctype4/drivers/libnfc` driver, for example, supports using a
// libnfc-supported reader to talk to a real NFC Type 4 Tag.
type Device struct {
MajorVersion byte // 2
MinorVersion byte // 0
commander *Commander
}
// tagState is used to store the relevant information obtained from a
// NDEF Detection Procedure
type tagState struct {
NLEN uint16
MaxReadBinaryLen uint16
MaxUpdateBinaryLen uint16
MaxNDEFLen uint16
ReadOnly bool
}
// New returns a pointer to a new Device configured
// with the provided CommandDriver to perform
// operations on the Tags.
func New(cmdDriver CommandDriver) *Device {
return &Device{
MajorVersion: NFCForumMajorVersion,
MinorVersion: NFCForumMinorVersion,
commander: &Commander{
Driver: cmdDriver,
},
}
}
// Setup [re]configures this device to use the provided
// command driver to perform operations on the tags.
func (dev *Device) Setup(cmdDriver CommandDriver) {
dev.commander = &Commander{
Driver: cmdDriver,
}
}
// Read performs a full read operation on a NFC Type 4 tag.
//
// The CommandDriver provided with Setup is initialized and
// closed at the end of the operation.
//
// Read performs the NDEF Detect Procedure and, if successful,
// performs a read operation on the NDEF File.
//
// It returns the NDEFMessage stored in the tag, or an error
// if something went wrong.
func (dev *Device) Read() (*ndef.Message, error) {
if err := dev.checkReady(); err != nil {
return nil, err
}
// Initialize driver and make sure we close it at the end
err := dev.commander.Driver.Initialize()
defer dev.commander.Driver.Close()
if err != nil {
return nil, err
}
detectState, err := dev.ndefDetectProcedure()
if err != nil {
return nil, err
}
if detectState.NLEN == 0 {
return nil, errors.New(
"Device.Read: no NDEF Message detected.")
}
// Message detected
// readLen represents what is the maximum amount of data we are going
// to read from the Tag in one go.
// It needs to be the minimum between maxReadBinaryLen and nlen
readLen := detectState.MaxReadBinaryLen
nlen := detectState.NLEN
if nlen < readLen {
readLen = nlen
}
// Read messages doing as many ReadBinary calls as necessary
totalRead := uint16(0)
var buffer bytes.Buffer // to hold what we are reading
for totalRead < nlen {
if nlen-totalRead < readLen { //last round
readLen = nlen - totalRead
}
// Always offset the nlen bytes (2)
chunk, err := dev.commander.ReadBinary(2+totalRead, readLen)
if err != nil {
return nil, err
}
buffer.Write(chunk)
totalRead += readLen
}
ndefBytes := buffer.Bytes()
// We finally have the NDEF Message. Parse it.
ndefMessage := new(ndef.Message)
if _, err := ndefMessage.Unmarshal(ndefBytes); err != nil {
return nil, err
}
// Finally, return the parsed NDEF Message
return ndefMessage, nil
}
// Update performs an update operation on a NFC Type 4 tag.
//
// The CommandDriver provided with Setup is initialized and
// closed at the end of the operation.
//
// The update operation starts by performing the NDEF
// Detection Procedure and the writing the provided NDEF Message
// to the NDEF File in the Tag, when the tag is not read-only.
//
// Note that update cannot be used to format a tag (clear the
// NDEF Message). For that, use Format().
//
// Update returns an error when there is a problem at some point
// in the process.
func (dev *Device) Update(m *ndef.Message) error {
if err := dev.checkReady(); err != nil {
return err
}
// Initialize driver and make sure we close it at the end
err := dev.commander.Driver.Initialize()
defer dev.commander.Driver.Close()
if err != nil {
return err
}
detectState, err := dev.ndefDetectProcedure()
if err != nil {
return err
}
if detectState.ReadOnly {
return errors.New("Device.Update: the tag is read-only")
}
messageBytes, err := m.Marshal()
if err != nil {
return err
}
if len(messageBytes) > int(detectState.MaxNDEFLen-2) {
return fmt.Errorf("Message is too large. Max size is %d",
detectState.MaxNDEFLen-2)
}
// Per above, this can be done without risking overflows
msgLen := uint16(len(messageBytes))
// The number of bytes to write will be the maximum or,
// if that's more than the message, just the message size
writeLen := detectState.MaxUpdateBinaryLen
if msgLen < writeLen {
writeLen = msgLen
}
// If the msgLen + 2 fits inside the MaxUpdateBinaryLen
// then we could do this in a single UpdateBinary command.
// For the moment we do the slow way which works always.
// Write 0000h in the NLEN field first
err = dev.commander.UpdateBinary([]byte{0x00, 0x00}, 0)
if err != nil {
return err
}
// Write the message doing as many UpdateBinary calls as necessary
totalWrite := uint16(0)
for totalWrite < msgLen {
if msgLen-totalWrite < writeLen { //last round
writeLen = msgLen - totalWrite
}
err = dev.commander.UpdateBinary(
messageBytes[totalWrite:totalWrite+writeLen],
totalWrite+2) // Always offset the 2 NLEN bytes
if err != nil {
return err
}
totalWrite += writeLen
}
// Finally write NLEN
msgLenBytes := helpers.Uint16ToBytes(msgLen)
err = dev.commander.UpdateBinary(msgLenBytes[:], 0)
if err != nil {
return err
}
return nil
}
// Format performs an update operation which erases a tag.
// It does this by writing to the first two bytes of the NDEF File
// and setting their value to 0 (zero-length for the file).
//
// Be aware that the memory is not wiped or overwritten. An attacker
// may likely recover the values stored in the tag by resetting
// the length of the NDEF File to the maximum.
//
// To wipe the memory, issue an Update() with a Message of the maximum
// length supported by the tag and a randomized/meaningless payload.
//
// Format returns an error when a problem happens.
func (dev *Device) Format() error {
if err := dev.checkReady(); err != nil {
return err
}
// Initialize driver and make sure we close it at the end
err := dev.commander.Driver.Initialize()
defer dev.commander.Driver.Close()
if err != nil {
return err
}
detectState, err := dev.ndefDetectProcedure()
if err != nil {
return err
}
if detectState.ReadOnly {
return errors.New("Device.Update: the tag is read-only")
}
err = dev.commander.UpdateBinary([]byte{0, 0}, 0)
if err != nil {
return err
}
return nil
}
func (dev *Device) ndefDetectProcedure() (*tagState, error) {
state := new(tagState)
// Select NDEF Application
if err := dev.commander.NDEFApplicationSelect(); err != nil {
return nil, err
}
// Select Capability Container
if err := dev.commander.Select(capabilitycontainer.CCID); err != nil {
return nil, err
}
// Read Capability Container start. It should have at least 15 bytes.
ccBytes, err := dev.commander.ReadBinary(0, 15)
if err != nil {
return nil, err
}
if len(ccBytes) < 15 {
return nil, errors.New(
"invalid Capability Container: should be 15 bytes")
}
// Read the remainder of the Capability Container based on CCLEN.
ccLen := helpers.BytesToUint16([2]byte{ccBytes[0], ccBytes[1]})
if ccLen > 15 {
ccBytesExtra, err := dev.commander.ReadBinary(15, ccLen-15)
if err != nil {
return nil, err
}
ccBytes = append(ccBytes, ccBytesExtra...)
}
// Parse the Capability Container
cc := new(capabilitycontainer.CapabilityContainer)
if _, err := cc.Unmarshal(ccBytes); err != nil {
return nil, err
}
// Check that we can read the tag
fcTlv := cc.NDEFFileControlTLV
if !(*capabilitycontainer.ControlTLV)(fcTlv).IsFileReadable() {
return nil, errors.New(
"Device.Read: NDEF File is marked as not readable.")
}
state.MaxReadBinaryLen = cc.MLe
state.MaxUpdateBinaryLen = cc.MLc
state.MaxNDEFLen = fcTlv.MaximumFileSize
state.ReadOnly = (*capabilitycontainer.ControlTLV)(fcTlv).IsFileReadOnly()
// Select the NDEF File
if err := dev.commander.Select(fcTlv.FileID); err != nil {
return nil, err
}
// Detect NDEF Message procedure 5.4.1
nlenBytes, err := dev.commander.ReadBinary(0, 2)
if err != nil {
return nil, err
}
nlen := helpers.BytesToUint16([2]byte{nlenBytes[0], nlenBytes[1]})
if nlen > state.MaxNDEFLen-2 {
return nil, errors.New(
"Device.Read: Device is not in a valid state")
}
state.NLEN = nlen
return state, nil
}
func (dev *Device) checkReady() error {
if dev.commander == nil {
return errors.New("The Device has not been setup. " +
"Please run Device.Setup(CommandDriver) first")
}
return nil
}