Doc ID024209 Rev 2

17/30

AN4240

Application code structure

5.2.5

Load Crypto Keys function

The update blank part function allows to get the UID value of the device, a 120-bit read only

unique identification number which is programmed during device fabrication. The UID is

used in the memory update procedure and is available for application specific uses.

Figure 10. Get UID function

The UID value is derived here in order to be used later to check if the updating procedure

has been processed correctly.

Furthermore, few lines of code is used to load the Cryptographic keys in order to update

eleven keys: ten USER KEYS and BOOT_MAC_KEY.

Figure 11. Load Key function

In this case, M1 to M5 parameters have been calculated off line. In order to check that the

updating procedure was performed correctly, CSE calculates M4 and M5 to compare those

values with the values obtained off line.

How to obtain those parameters is explained in the Appendix A.

5.2.6

App code to be added to have a secure boot

In order to issue a Secure Boot OK it is necessary to add in the crt0.ppc file, the following

lines: