api/bootloader: add ParseSignedFirwmare() util function

api/bootloader/device.go

@@ -233,18 +233,12 @@ func (device *Device) flashUnsignedFirmware(firmware []byte, progressCallback fu
 // format is invalid, or the firmware magic does not match the expected magic according to the
 // device product.
 func (device *Device) parseSignedFirmware(firmware []byte) ([]byte, []byte, error) {
-	if len(firmware) <= magicLen+sigDataLen {
-		return nil, nil, errp.New("firmware too small")
-	}
-	magic, firmware := firmware[:magicLen], firmware[magicLen:]
-	sigData, firmware := firmware[:sigDataLen], firmware[sigDataLen:]
-
-	expectedMagic, ok := sigDataMagic[device.product]
-	if !ok {
-		return nil, nil, errp.New("unrecognized product")
+	product, sigData, firmware, err := ParseSignedFirmware(firmware)
+	if err != nil {
+		return nil, nil, err
 	}
-	if binary.BigEndian.Uint32(magic) != expectedMagic {
-		return nil, nil, errp.New("invalid signing pubkeys data magic")
+	if product != device.product {
+		return nil, nil, errp.New("signed firmware binary does not match device product")
 	}
 	return sigData, firmware, nil
 }

api/bootloader/util.go

@@ -18,6 +18,9 @@ import (
 	"bytes"
 	"crypto/sha256"
 	"encoding/binary"
+
+	"github.com/digitalbitbox/bitbox02-api-go/api/common"
+	"github.com/digitalbitbox/bitbox02-api-go/util/errp"
 )
 
 // HashFirmware returns the hash of `<firmware version><firmware>`, as computed by the bootloader to
@@ -35,3 +38,28 @@ func HashFirmware(firmwareVersion uint32, unsignedFirmware []byte) []byte {
 	copy(padded, unsignedFirmware)
 	return doubleHash(append(firmwareVersionLE, padded...))
 }
+
+// ParseSignedFirmware parses a signed firmware file and returns (sigdata, firmware). Errors if the
+// format is invalid, or the firmware magic does not match the expected magic according to the
+// device product.
+func ParseSignedFirmware(firmware []byte) (common.Product, []byte, []byte, error) {
+	if len(firmware) <= magicLen+sigDataLen {
+		return "", nil, nil, errp.New("firmware too small")
+	}
+	magic, firmware := firmware[:magicLen], firmware[magicLen:]
+	sigData, firmware := firmware[:sigDataLen], firmware[sigDataLen:]
+
+	var product common.Product
+	magicInt := binary.BigEndian.Uint32(magic)
+	for p, productMagic := range sigDataMagic {
+		if magicInt == productMagic {
+			product = p
+			break
+		}
+	}
+	if product == "" {
+		return "", nil, nil, errp.Newf("unrecognized magic")
+	}
+
+	return product, sigData, firmware, nil
+}