Doc ID024209 Rev 2

19/30

AN4240

Application code structure

where:

Figure 14. Locator memory setup

Once the BOOT_MAC_KEY is loaded in the device, the CSE is able to calculate the CMAC

of the secure boot as defined in the startup file after a first reset. With a second reset the

CSE will check if the content of the flash has been modified, comparing the CMAC

previously calculated over the authenticated code versus the new CMAC calculated over

the same flash content. If they match the CSE release a Secure Boot OK.

5.2.7

Demo application code

As described before the last project allows, once CSE has released a secure boot, to use

the cryptographic keys to encrypt and decrypt a logo.

The following lines of code permit the encryption and decryption commands using the ECB

and CBC cipher modes over a preloaded image.

The Cryptographic key used, is the KEY1 which has the KEY USAGE flag equal to 0 in

order to be used for encryption/decryption feature.

For the CBC cipher mode, an initialize vector is used to add more entropy to the process.

The st_Bitmap is the ST logo of 1800 blocks size obtained using an image bitmap converter.