On Fri, Jul 24, 2020 at 12:06 PM Patrick Oppenlander <patrick.oppenlan...@gmail.com> wrote: > > Hi, > > I recently posted some patches to the list [1], [2], [3] to address > some issues with the cipher support in mkimage. Hopefully someone gets > a chance to review these patches as I think mkimage is a bit broken > without them. > > While considering using U-Boot cipher support in a product I work on, > I have convinced myself that the handling of the encryption IV could > be better, especially given that mkimage is using AES-CBC mode. > Please, correct me if I have missed something. > > Issue #1 > ======== > > Currently, mkimage treats the IV in the same manner as the encryption > key. There is an iv-name-hint property which mkimage uses to read the > IV from a file in the keys directory. This can then be written to > u-boot.dtb along with the encryption key. > > The problem with that is that u-boot.dtb is baked in at production > time and is generally not field upgradable. That means that the IV is > also baked in which is considered bad practice especially when using > CBC mode (see CBC IV attack). In general it is my understanding that > you should never use a key+IV twice regardless of cipher or mode. > > In my opinion a better solution would have been to write the IV into > the FIT image instead of iv-name-hint (it's only 16 bytes!), and > regenerate it (/dev/random?) each and every time the data is ciphered. > > An even better solution is to use AES-GCM (or something similar) as > this includes the IV with the ciphertext, simplifying the above, and > also provides authentication addressing another issue (see below). > > Issue #2 > ======= > > The current implementation uses encrypt-then-sign. I like this > approach as it means that the FIT image can be verified outside of > U-Boot without requiring encryption keys. It is also considered best > practise. > > However, for this to be secure, the details of the cipher need to be > included in the signature, otherwise an attacker can change the cipher > or key/iv properties. > > I do not believe that properties in the cipher node are currently > included when signing a FIT configuration including an encrypted > image. That should be a simple fix. Fixing it for image signatures > might be a bit more tricky. > > Issue #3 > ======= > > Due to the nature of encrypt-then-sign U-Boot can verify that the > ciphertext is unmodified, but it has no way of making sure that the > key used to encrypt the image matches the key in u-boot.fit used for > decryption. This can result in an attempt to boot gibberish and I > think it can open up certain attack vectors. > > The best way I know of to fix this is to use an authenticated > encryption mode such as AES-GCM or something similar. > > > Kind regards, > > Patrick > > [1] https://lists.denx.de/pipermail/u-boot/2020-July/420399.html > [2] https://lists.denx.de/pipermail/u-boot/2020-July/420400.html > [3] https://lists.denx.de/pipermail/u-boot/2020-July/420401.html
Hi Simon, I posted this writeup to the u-boot list and forgot to CC you. Sorry about that. Patrick
