diff --git a/scripts/imgtool/keys/rsa.py b/scripts/imgtool/keys/rsa.py
index 8d5d048..96951c9 100644
--- a/scripts/imgtool/keys/rsa.py
+++ b/scripts/imgtool/keys/rsa.py
@@ -50,6 +50,9 @@
     def sig_tlv(self):
         return "RSA2048"
 
+    def sig_len(self):
+        return 256
+
 class RSA2048(RSA2048Public):
     """
     Wrapper around an 2048-bit RSA key, with imgtool support.