diff --git a/scripts/imgtool/keys/privatebytes.py b/scripts/imgtool/keys/privatebytes.py
new file mode 100644
index 0000000..8027ac8
--- /dev/null
+++ b/scripts/imgtool/keys/privatebytes.py
@@ -0,0 +1,16 @@
+# SPDX-License-Identifier: Apache-2.0
+
+from cryptography.hazmat.primitives import serialization
+
+
+class PrivateBytesMixin():
+    def _get_private_bytes(self, minimal, format, exclass):
+        if format is None:
+            format = self._DEFAULT_FORMAT
+        if format not in self._VALID_FORMATS:
+            raise exclass("{} does not support {}".format(
+                self.shortname(), format))
+        return format, self.key.private_bytes(
+                encoding=serialization.Encoding.DER,
+                format=self._VALID_FORMATS[format],
+                encryption_algorithm=serialization.NoEncryption())