fix: Handle musicex tag correctly

algo/qmc/qmc.go

@@ -137,10 +137,10 @@ func (d *Decoder) searchKey() (err error) {
 		d.decodedKey, err = deriveKey([]byte(key))
 		if err == nil {
 			d.audioLen = fileSize
-			return nil
+		} else {
+			d.decodedKey = nil
+			d.logger.Warn("could not derive key, skip", zap.Error(err))
 		}
-		d.decodedKey = nil
-		d.logger.Warn("could not derive key, skip", zap.Error(err))
 	}
 
 	suffixBuf := make([]byte, 4)
@@ -153,7 +153,7 @@ func (d *Decoder) searchKey() (err error) {
 		return d.readRawMetaQTag()
 	case "STag":
 		return errors.New("qmc: file with 'STag' suffix doesn't contains media key")
-	// MusicEx\0
+	// speculative guess for "musicex\0"
 	case "cex\x00":
 		footer, err := NewMusicExTag(d.raw)
 		if err != nil {
@@ -161,17 +161,26 @@ func (d *Decoder) searchKey() (err error) {
 		}
 		d.audioLen = fileSize - int(footer.TagSize)
 		if key, ok := d.params.CryptoParams.QmcKeys.Get(footer.MediaFileName); ok {
+			d.logger.Debug("searchKey: using key from MediaFileName", zap.String("MediaFileName", footer.MediaFileName), zap.String("key", key))
 			d.decodedKey, err = deriveKey([]byte(key))
+		} else if d.decodedKey == nil {
+			return errors.New("searchKey: no key found for musicex tag")
 		}
 		return err
 	default:
-		size := binary.LittleEndian.Uint32(suffixBuf)
+		// if we already have a key from legacy mmkv, use it
+		if d.decodedKey != nil {
+			return nil
+		}
 
+		// try to use suffix as key length
+		size := binary.LittleEndian.Uint32(suffixBuf)
 		if size <= 0xFFFF && size != 0 { // assume size is key len
 			return d.readRawKey(int64(size))
 		}
 
-		// try to use default static cipher
+		// try to use default static cipher,
+		//   or the key read from the legacy mmkv
 		d.audioLen = fileSize
 		return nil
 	}