8.12.6.5.4 Scrambling Key Protections

To enhance the security level, the scrambling key applied to the erasable memories is modified immediately before and also just after the physical erase sequence. Note that in case of empty memory, despite the fact that the sequential erasing process is not launched as previously described, the scrambling key of erasable memories is modified. The global key, which is configured in SECUMOD_SCRKEY, is not modified since the non-erasable part of the memory must be preserved. The figure below illustrates the chosen implementation.

The use of trimming bit XORs differentiates parts in the key final values reaching the scrambler/descrambler.

The key used for the BUREG256b scrambler/descrambler is derived from the BUSRAM4KB key and thus benefits from the same protection.