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docs: Update documentation regarding encrypted scratch area 

When using swap using scratch, the decryption now happens when copying
from the scratch area to the primary slot, which means the image is
stored encrypted in the scratch area. This commit updates the
documentation accordingly.

Signed-off-by: Thomas Altenbach <thomas.altenbach@legrand.com>
This commit is contained in:
Thomas Altenbach 2024-04-30 17:19:55 +02:00 committed by Fabio Utzig
parent 08d2d94c14
commit 44a935ebcc
1 changed files with 8 additions and 6 deletions
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14
docs/encrypted_images.md
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@ -120,12 +120,14 @@ start the validation process, decrypting the blocks before check. A good
image being determined, the upgrade consists in reading the blocks from
the `secondary slot`, decrypting and writing to the `primary slot`.
If swap is used for the upgrade process, the encryption happens when
copying the sectors of the `secondary slot` to the scratch area.
The `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.
If swap using scratch is used for the upgrade process, the decryption happens
when copying the content of the scratch area to the `primary slot`, which means
the scratch area does not contain the image unencrypted. However, unless
`MCUBOOT_SWAP_SAVE_ENCTLV` is enabled, the decryption keys are stored in
plaintext in the scratch area. Therefore, `MCUBOOT_SWAP_SAVE_ENCTLV` must be
enabled if the scratch area does not reside in the internal flash memory of the
MCU, to avoid attacks that could interrupt the upgrade and read the plaintext
decryption keys from external flash memory.
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