To provide confidentiality of image data while in transport to the
device or while residing on an external flash,
MCUBoot has support
for encrypting/decrypting images on-the-fly while upgrading.
The image header needs to flag this image as
ENCRYPTED (0x04) and
a TLV with the key must be present in the image. When upgrading the
image from the
secondary slot to the
primary slot it is automatically
decrypted (after validation). If swap upgrades are enabled, the image
located in the
primary slot, also having the
ENCRYPTED flag set and the
TLV present, is re-encrypted while swapping to the
The encrypted image support is supposed to allow for confidentiality if the image is not residing on the device or is written to external storage, eg a SPI flash being used for the secondary slot.
It does not protect against the possibility of attaching a JTAG and reading the internal flash memory, or using some attack vector that enables dumping the internal flash in any way.
Since decrypting requires a private key (or secret if using symetric crypto) to reside inside the device, it is the responsibility of the device manufacturer to guarantee that this key is already in the device and not possible to extract.
When encrypting an image, only the payload (FW) is encrypted. The header, TLVs are still sent as plain data.
Hashing and signing also remain functionally the same way as before, applied over the un-encrypted data. Validation on encrypted images, checks that the encrypted flag is set and TLV data is OK, then it decrypts each image block before sending the data to the hash routines.
The image is encrypted using AES-CTR-128, with a counter that starts from zero (over the payload blocks) and increments by 1 for each 16-byte block. AES-CTR-128 was chosen for speed/simplicity and allowing for any block to be encrypted/decrypted without requiring knowledge of any other block (allowing for simple resume operations on swap interruptions).
The key used is a randomized when creating a new image, by
newt. This key should never be reused and no checks are done for this,
but randomizing a 16-byte block with a TRNG should make it highly
improbable that duplicates ever happen.
To distribute this AES-CTR-128 key, new TLVs were defined. The key can be encrypted using either RSA-OAEP or AES-KW-128. Also in the future support for EICES (using EC) can be added.
For RSA-OAEP a new TLV with value
0x30 is added to the image, for
AES-KW-128 a new TLV with value
0x31 is added to the image. The contents
of both TLVs are the results of applying the given operations over the
When starting a new upgrade process,
MCUBoot checks that the image in the
secondary slot has the
ENCRYPTED flag set and has the required TLV with the
encrypted key. It then uses its internal private/secret key to decrypt
the TLV containing the key. Given that no errors are found, it will then
start the validation process, decrypting the blocks before check. A good
image being determined, the upgrade consists in reading the blocks from
secondary slot, decrypting and writing to the
If swap is used for the upgrade process, the encryption happens when
copying the sectors of the
secondary slot to the scratch area.
scratch area is not encrypted, so it must reside in the internal
flash of the MCU to avoid attacks that could interrupt the upgrade and
dump the data.
Also when swap is used, the image in the
primary slot is checked for
presence of the
ENCRYPTED flag and the key TLV. If those are present the
sectors are re-encrypted when copying from the
primary slot to
PS: Each encrypted image must have its own key TLV that should be unique and used only for this particular image.
Also when swap method is employed, the sizes of both images are saved to the status area just before starting the upgrade process, because it would be very hard to determine this information when an interruption occurs and the information is spread across multiple areas.
Creating your keys
- If using RSA-OAEP, generating a keypair follows steps similar to those described in signed_images
- If using AES-KW-128 (
kekcan be generated with a command like
dd if=/dev/urandom bs=1 count=16 | base64 > my_kek.b64