Optimizing Memory Mapping; Table 3. Sbsfu Code-Size Reduction - ST X-CUBE-SBSFU STM32Cube Integration Manual

Adapting SBSFU
7.2

Optimizing memory mapping

Several options exist to reduce SBSFU code size to maximize the size of the user
application slot. Some of these options are summarized in
Option
Select 1-image variant
Select AES-GCM symmetric
cryptographic scheme
Disable SFU_DEBUG_MODE
Disable SECBOOT_USE_LOCAL_LOADER
Implement a hardware decryption
If all the code running on STM32 is fully
trusted and robust then Secure Engine
internal isolation based on MPU for
STM32F4/F7/G0/G4/H7/L1 can be
removed.
The total gain depends on the mapping constraints described in
mapping definition on page
As an example,
images with a symmetric cryptographic scheme, the SFFU_DEBUG_MODE and
SECBOOT_USE_LOCAL_LOADER switches are disabled, resulting in a 16-Kbyte increase of
the user application size.
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Table 3. SBSFU code-size reduction

Description / Consequence
Download a new firmware image from the
user application is no more possible.
Shared symmetric key secret stored in the
device.
No more information displayed on the
terminal during SBSFU execution
No more local loader inside the SBSFU
application. This is not compatible with 1-
image variant.
Select STM32 devices integrating
cryptographic hardware IP.
Removes alignment constraints with MPU
regions.
10.
Figure 32 highlights the mapping modifications to be done. Starting from 2
AN5056 Rev 8
Table 3.
Gain
Slot size is doubled
vs. 2-image projects
~ 9 Kbytes
~ 9 Kbytes
~3 Kbytes
This depends on the user
implementation
Up to 12 Kbytes
depending on products
Section 3.2: Memory
AN5056