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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 image being determined, the upgrade consists in reading the blocks from
the `secondary slot`, decrypting and writing to the `primary slot`. the `secondary slot`, decrypting and writing to the `primary slot`.
If swap is used for the upgrade process, the encryption happens when If swap using scratch is used for the upgrade process, the decryption happens
copying the sectors of the `secondary slot` to the scratch area. 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
The `scratch` area is not encrypted, so it must reside in the internal `MCUBOOT_SWAP_SAVE_ENCTLV` is enabled, the decryption keys are stored in
flash of the MCU to avoid attacks that could interrupt the upgrade and plaintext in the scratch area. Therefore, `MCUBOOT_SWAP_SAVE_ENCTLV` must be
dump the data. 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 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 presence of the `ENCRYPTED` flag and the key TLV. If those are present the