feat(QMCv2): support static cipher (legacy)

algo/qmc/qmc.go

@@ -0,0 +1,185 @@
+package qmc
+
+import (
+	"encoding/binary"
+	"errors"
+	"io"
+	"strconv"
+	"strings"
+
+	"github.com/unlock-music/cli/algo/common"
+)
+
+type Decoder struct {
+	r       io.ReadSeeker
+	fileExt string
+
+	audioLen   int
+	decodedKey []byte
+	cipher     streamCipher
+	offset     int
+
+	rawMetaExtra1 int
+	rawMetaExtra2 int
+}
+
+// Read implements io.Reader, offer the decrypted audio data.
+// Validate should call before Read to check if the file is valid.
+func (d *Decoder) Read(p []byte) (int, error) {
+	n := len(p)
+	if d.audioLen-d.offset <= 0 {
+		return 0, io.EOF
+	} else if d.audioLen-d.offset < n {
+		n = d.audioLen - d.offset
+	}
+	m, err := d.r.Read(p[:n])
+	if m == 0 {
+		return 0, err
+	}
+
+	d.cipher.Decrypt(p[:m], d.offset)
+	d.offset += m
+	return m, err
+}
+
+func NewDecoder(r io.ReadSeeker) (*Decoder, error) {
+	d := &Decoder{r: r}
+	err := d.searchKey()
+	if err != nil {
+		return nil, err
+	}
+
+	if len(d.decodedKey) > 300 {
+		d.cipher, err = NewRC4Cipher(d.decodedKey)
+		if err != nil {
+			return nil, err
+		}
+	} else if len(d.decodedKey) != 0 {
+		d.cipher, err = NewMapCipher(d.decodedKey)
+		if err != nil {
+			return nil, err
+		}
+	} else {
+		d.cipher = NewStaticCipher()
+	}
+
+	_, err = d.r.Seek(0, io.SeekStart)
+	if err != nil {
+		return nil, err
+	}
+
+	return d, nil
+}
+
+func (d *Decoder) Validate() error {
+	buf := make([]byte, 16)
+	if _, err := io.ReadFull(d.r, buf); err != nil {
+		return err
+	}
+	_, err := d.r.Seek(0, io.SeekStart)
+	if err != nil {
+		return err
+	}
+
+	d.cipher.Decrypt(buf, 0)
+	fileExt, ok := common.SniffAll(buf)
+	if !ok {
+		return errors.New("detect file type failed")
+	}
+	d.fileExt = fileExt
+	return nil
+}
+
+func (d Decoder) GetFileExt() string {
+	return d.fileExt
+}
+
+func (d *Decoder) searchKey() error {
+	fileSizeM4, err := d.r.Seek(-4, io.SeekEnd)
+	if err != nil {
+		return err
+	}
+	buf, err := io.ReadAll(io.LimitReader(d.r, 4))
+	if err != nil {
+		return err
+	}
+	if string(buf) == "QTag" {
+		if err := d.readRawMetaQTag(); err != nil {
+			return err
+		}
+	} else {
+		size := binary.LittleEndian.Uint32(buf)
+		if size < 0x300 && size != 0 {
+			return d.readRawKey(int64(size))
+		} else {
+			// try to use default static cipher
+			d.audioLen = int(fileSizeM4 + 4)
+			return nil
+		}
+	}
+	return nil
+}
+
+func (d *Decoder) readRawKey(rawKeyLen int64) error {
+	audioLen, err := d.r.Seek(-(4 + rawKeyLen), io.SeekEnd)
+	if err != nil {
+		return err
+	}
+	d.audioLen = int(audioLen)
+
+	rawKeyData, err := io.ReadAll(io.LimitReader(d.r, rawKeyLen))
+	if err != nil {
+		return err
+	}
+
+	d.decodedKey, err = DecryptKey(rawKeyData)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func (d *Decoder) readRawMetaQTag() error {
+	// get raw meta data len
+	if _, err := d.r.Seek(-8, io.SeekEnd); err != nil {
+		return err
+	}
+	buf, err := io.ReadAll(io.LimitReader(d.r, 4))
+	if err != nil {
+		return err
+	}
+	rawMetaLen := int64(binary.BigEndian.Uint32(buf))
+
+	// read raw meta data
+	audioLen, err := d.r.Seek(-(8 + rawMetaLen), io.SeekEnd)
+	if err != nil {
+		return err
+	}
+	d.audioLen = int(audioLen)
+	rawMetaData, err := io.ReadAll(io.LimitReader(d.r, rawMetaLen))
+	if err != nil {
+		return err
+	}
+
+	items := strings.Split(string(rawMetaData), ",")
+	if len(items) != 3 {
+		return errors.New("invalid raw meta data")
+	}
+
+	d.decodedKey, err = DecryptKey([]byte(items[0]))
+	if err != nil {
+		return err
+	}
+
+	d.rawMetaExtra1, err = strconv.Atoi(items[1])
+	if err != nil {
+		return err
+	}
+	d.rawMetaExtra2, err = strconv.Atoi(items[2])
+	if err != nil {
+		return err
+	}
+
+	return nil
+}