parse BER-TLV with multiple length bytes

apdu/utils.go

@@ -2,10 +2,17 @@ package apdu
 
 import (
 	"bytes"
+	"encoding/binary"
+	"errors"
 	"fmt"
 	"io"
 )
 
+var (
+	ErrUnsupportedLenth80 = errors.New("length cannot be 0x80")
+	ErrLengthTooBig       = errors.New("length cannot be more than 3 bytes")
+)
+
 // ErrTagNotFound is an error returned if a tag is not found in a TLV sequence.
 type ErrTagNotFound struct {
 	tag uint8
@@ -36,7 +43,7 @@ func findTag(raw []byte, occurrence int, tags ...uint8) ([]byte, error) {
 
 	var (
 		tag    uint8
-		length uint8
+		length uint32
 		err    error
 	)
 
@@ -49,11 +56,7 @@ func findTag(raw []byte, occurrence int, tags ...uint8) ([]byte, error) {
 			return nil, err
 		}
 
-		length, err = buf.ReadByte()
-		if err != nil {
-			return nil, err
-		}
-
+		length, buf, err = parseLength(buf)
 		data := make([]byte, length)
 		if length != 0 {
 			_, err = buf.Read(data)
@@ -77,3 +80,34 @@ func findTag(raw []byte, occurrence int, tags ...uint8) ([]byte, error) {
 		}
 	}
 }
+
+func parseLength(buf *bytes.Buffer) (uint32, *bytes.Buffer, error) {
+	length, err := buf.ReadByte()
+	if err != nil {
+		return 0, nil, err
+	}
+
+	if length == 0x80 {
+		return 0, nil, ErrUnsupportedLenth80
+	}
+
+	if length > 0x80 {
+		lengthSize := length - 0x80
+		if lengthSize > 3 {
+			return 0, nil, ErrLengthTooBig
+		}
+
+		data := make([]byte, lengthSize)
+		_, err = buf.Read(data)
+		if err != nil {
+			return 0, nil, err
+		}
+
+		num := make([]byte, 4)
+		copy(num[4-lengthSize:], data)
+
+		return binary.BigEndian.Uint32(num), buf, nil
+	}
+
+	return uint32(length), buf, nil
+}

apdu/utils_test.go

@@ -1,6 +1,7 @@
 package apdu
 
 import (
+	"bytes"
 	"testing"
 
 	"github.com/status-im/keycard-go/hexutils"
@@ -42,6 +43,61 @@ func TestFindTag(t *testing.T) {
 	assert.Equal(t, &ErrTagNotFound{uint8(0xC3)}, err)
 }
 
+func TestParseLength(t *testing.T) {
+	scenarios := []struct {
+		data           []byte
+		expectedLength uint32
+		err            error
+	}{
+		{
+			data:           []byte{0x01, 0xAA},
+			expectedLength: 1,
+			err:            nil,
+		},
+		{
+			data:           []byte{0x7F, 0xAA},
+			expectedLength: 127,
+			err:            nil,
+		},
+		{
+			data:           []byte{0x81, 0x80, 0xAA},
+			expectedLength: 128,
+			err:            nil,
+		},
+		{
+			data:           []byte{0x82, 0x80, 0x80, 0xAA},
+			expectedLength: 32896,
+			err:            nil,
+		},
+		{
+			data:           []byte{0x83, 0x80, 0x80, 0x80, 0xAA},
+			expectedLength: 8421504,
+			err:            nil,
+		},
+		{
+			data:           []byte{0x80, 0xAA},
+			expectedLength: 0,
+			err:            ErrUnsupportedLenth80,
+		},
+		{
+			data:           []byte{0x84, 0xAA},
+			expectedLength: 0,
+			err:            ErrLengthTooBig,
+		},
+	}
+
+	for _, s := range scenarios {
+		buf := bytes.NewBuffer(s.data)
+		length, _, err := parseLength(buf)
+		if s.err == nil {
+			assert.NoError(t, err)
+			assert.Equal(t, s.expectedLength, length)
+		} else {
+			assert.Equal(t, s.err, err)
+		}
+	}
+}
+
 func TestFindTagN(t *testing.T) {
 	data := hexutils.HexToBytes("0A 01 A1 0A 01 A2")