Improvements to FIT ciphering

[Simon Glass]

+Philippe Reynes too


On Mon, 27 Jul 2020 at 16:50, Patrick Oppenlander
<patrick.oppenlander at gmail.com> wrote:
>
> On Fri, Jul 24, 2020 at 12:06 PM Patrick Oppenlander
> <patrick.oppenlander at 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