118 lines
2.2 KiB
Go
118 lines
2.2 KiB
Go
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package apdu
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import (
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"bytes"
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"encoding/binary"
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"errors"
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"fmt"
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"io"
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)
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var (
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ErrUnsupportedLenth80 = errors.New("length cannot be 0x80")
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ErrLengthTooBig = errors.New("length cannot be more than 3 bytes")
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)
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// ErrTagNotFound is an error returned if a tag is not found in a TLV sequence.
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type ErrTagNotFound struct {
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tag uint8
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}
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// Error implements the error interface
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func (e *ErrTagNotFound) Error() string {
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return fmt.Sprintf("tag %x not found", e.tag)
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}
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// FindTag searches for a tag value within a TLV sequence.
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func FindTag(raw []byte, tags ...uint8) ([]byte, error) {
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return findTag(raw, 0, tags...)
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}
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// FindTagN searches for a tag value within a TLV sequence and returns the n occurrence
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func FindTagN(raw []byte, n int, tags ...uint8) ([]byte, error) {
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return findTag(raw, n, tags...)
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}
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func findTag(raw []byte, occurrence int, tags ...uint8) ([]byte, error) {
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if len(tags) == 0 {
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return raw, nil
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}
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target := tags[0]
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buf := bytes.NewBuffer(raw)
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var (
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tag uint8
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length uint32
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err error
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)
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for {
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tag, err = buf.ReadByte()
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switch {
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case err == io.EOF:
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return []byte{}, &ErrTagNotFound{target}
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case err != nil:
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return nil, err
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}
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length, buf, err = parseLength(buf)
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if err != nil {
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return nil, err
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}
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data := make([]byte, length)
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if length != 0 {
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_, err = buf.Read(data)
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if err != nil {
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return nil, err
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}
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}
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if tag == target {
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// if it's the last tag in the search path, we start counting the occurrences
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if len(tags) == 1 && occurrence > 0 {
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occurrence--
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continue
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}
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if len(tags) == 1 {
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return data, nil
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}
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return findTag(data, occurrence, tags[1:]...)
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}
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}
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}
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func parseLength(buf *bytes.Buffer) (uint32, *bytes.Buffer, error) {
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length, err := buf.ReadByte()
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if err != nil {
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return 0, nil, err
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}
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if length == 0x80 {
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return 0, nil, ErrUnsupportedLenth80
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}
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if length > 0x80 {
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lengthSize := length - 0x80
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if lengthSize > 3 {
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return 0, nil, ErrLengthTooBig
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}
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data := make([]byte, lengthSize)
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_, err = buf.Read(data)
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if err != nil {
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return 0, nil, err
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}
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num := make([]byte, 4)
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copy(num[4-lengthSize:], data)
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return binary.BigEndian.Uint32(num), buf, nil
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}
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return uint32(length), buf, nil
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}
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