zuc: eea XORKeyStreamAt fix issue

Author: emmansun

zuc/eea.go

@@ -126,6 +126,9 @@ func (c *eea) XORKeyStreamAt(dst, src []byte, offset uint64) {
 	if offsetDiff <= uint64(c.xLen) {
 		c.xLen -= int(offsetDiff)
 		c.used += offsetDiff
+		if c.xLen > 0 {
+			copy(c.x[:], c.x[offsetDiff:])
+		}
 		c.XORKeyStream(dst, src)
 		return
 	}

zuc/eea_test.go

@@ -122,6 +122,19 @@ func TestXORStreamAt(t *testing.T) {
 		}
 	})
 
+	t.Run("Jump and forward (incomplete word): offsetDiff <= xLen", func(t *testing.T) {
+		c.XORKeyStreamAt(dst[:1], src[:1], 0)
+		c.XORKeyStreamAt(dst[3:16], src[3:16], 3)
+		if !bytes.Equal(dst[3:16], expected[3:16]) {
+			t.Errorf("expected=%x, result=%x\n", expected[3:16], dst[3:16])
+		}
+		c.XORKeyStreamAt(dst[:1], src[:1], 0)
+		c.XORKeyStreamAt(dst[4:16], src[4:16], 4)
+		if !bytes.Equal(dst[4:16], expected[4:16]) {
+			t.Errorf("expected=%x, result=%x\n", expected[3:16], dst[3:16])
+		}
+	})
+
 	t.Run("Jump and forward (skipped keys more than 128)", func(t *testing.T) {
 		// test offset - used > 128 bytes case
 		c.XORKeyStreamAt(dst[:16], src[:16], 0)