diff --git a/mshal/ms213x/csum.go b/mshal/ms213x/csum.go
new file mode 100644
index 0000000..c3a8a16
--- /dev/null
+++ b/mshal/ms213x/csum.go
@@ -0,0 +1,42 @@
+package ms213x
+
+import (
+	"encoding/binary"
+	"errors"
+	"fmt"
+)
+
+func calcSum(f []byte) uint16 {
+	var csum uint16
+	for _, m := range f {
+		csum += uint16(m)
+	}
+	return csum
+}
+
+func CheckImage(f []byte) error {
+	if len(f) < 0x30+4 {
+		return errors.New("file too short (hdr)")
+	}
+
+	if t := binary.BigEndian.Uint16(f); t != 0x5aa5 && t != 0x6996 && t != 0x3cc3 {
+		return fmt.Errorf("unknown flash type: %x", t)
+	}
+
+	codeLen := int(binary.BigEndian.Uint16(f[2:]))
+	end := 0x30 + codeLen
+	if len(f) < end+4 {
+		return errors.New("file too short (code)")
+	}
+
+	hdrSum := calcSum(f[2:12]) + calcSum(f[16:0x30])
+	codeSum := calcSum(f[0x30:end])
+
+	if hdrImg := binary.BigEndian.Uint16(f[end:]); hdrSum != hdrImg {
+		return fmt.Errorf("header checksum mismatch: %x != %x", hdrSum, hdrImg)
+	} else if codeImg := binary.BigEndian.Uint16(f[end+2:]); codeSum != codeImg {
+		return fmt.Errorf("code checksum mismatch: %x != %x", codeSum, codeImg)
+	}
+
+	return nil
+}