add option for rollback protection
Depends on 'MCUBOOT_OVERWRITE_ONLY' option since swap info is not protected
by signature
Signed-off-by: Håkon Øye Amundsen <haakon.amundsen@nordicsemi.no>
diff --git a/docs/design.md b/docs/design.md
index 6c2bc26..b779778 100644
--- a/docs/design.md
+++ b/docs/design.md
@@ -942,3 +942,11 @@
For more information on adding dependency entries to an image,
see: [imgtool](imgtool.md).
+
+## [Downgrade Prevention](#downgrade-prevention)
+
+Downgrade prevention is a feature which enforces that the new image must have a
+higher version number than the image it is replacing. This feature is enabled
+with the `MCUBOOT_DOWNGRADE_PREVENTION` option. Downgrade prevention is only
+available when the overwrite-based image update strategy is used
+(i.e. `MCUBOOT_OVERWRITE_ONLY` is set).