Fix: algorithm bugs in `qmc`

Optimize: algorithm in `kgm`

algo/kgm/kgm.go

@@ -75,12 +75,8 @@ func (d *Decoder) Decode() error {
 	d.audio = make([]byte, lenData)
 
 	for i := 0; i < lenData; i++ {
-		med8 := d.key[i%17] ^ dataEncrypted[i]
-		med8 ^= (med8 & 0xf) << 4
-
-		msk8 := maskV2PreDef[i%272] ^ maskV2[i>>4]
-		msk8 ^= (msk8 & 0xf) << 4
-		d.audio[i] = med8 ^ msk8
+		med8 := dataEncrypted[i] ^ d.key[i%17] ^ maskV2PreDef[i%(16*17)] ^ maskV2[i>>4]
+		d.audio[i] = med8 ^ (med8&0xf)<<4
 	}
 	if d.isVpr {
 		for i := 0; i < lenData; i++ {

algo/kgm/mask.go

@@ -6,7 +6,7 @@ import (
 	"github.com/ulikunitz/xz"
 	"github.com/unlock-music/cli/internal/logging"
 	"go.uber.org/zap"
-	"io/ioutil"
+	"io"
 )
 
 var maskDiffVpr = []byte{
@@ -50,7 +50,7 @@ func initMask() {
 	if err != nil {
 		logging.Log().Fatal("load kgm mask failed", zap.Error(err))
 	}
-	maskV2, err = ioutil.ReadAll(maskReader)
+	maskV2, err = io.ReadAll(maskReader)
 	if err != nil {
 		logging.Log().Fatal("load kgm mask failed", zap.Error(err))
 	}

algo/qmc/mask_key256.go

@@ -39,7 +39,7 @@ func NewKey256FromMask44(mask44 []byte) (*Key256Mask, error) {
 	return q, nil
 }
 
-func (q *Key256Mask) getMatrix44() (mask44 []byte, err error) {
+func (q *Key256Mask) GetMatrix44() ([]byte, error) {
 	if len(q.matrix) != 128 {
 		return nil, ErrMaskLength128
 	}
@@ -53,7 +53,7 @@ func (q *Key256Mask) getMatrix44() (mask44 []byte, err error) {
 					return nil, ErrMaskDecode
 				}
 			}
-			q.matrix[idx44] = q.matrix[it256[0]]
+			matrix44[idx44] = q.matrix[it256[0]]
 			idx44++
 		}
 	}