Amazon has just changed the encryption used for Kindle books on older e-readers without a software update. Any Kindle with software 5.18.5 or higher is now downloading new e-books using the KFX-ZIP format, which is not compatible with the encryption broken by 3rd-party tools for popular software such as Calibre and the DeDRM plugin.

Prior e-books using old Kindle software were downloaded as KFX files, which were easily backed up to a user’s computer via USB cable, and then had their encryption broken to load the books on other devices or to create a local backup of prior purchases.

The new KFX-Zip files are available on Kindle e-readers from the 7th generation through the 11th generation. Prior Kindles from the 6th generation and below do not yet download e-books using the new book encryption, but it remains to be seen whether Amazon will issue a mandatory software update to plug the loophole that could break digital rights management.