Posted on October 07, 2013 at 16:56http://www.st.com/web/en/resource/technical/document/application_note/DM00044227.pdf
http://www.st.com/st-web-ui/static/active/cn/resource/technical/document/application_note/DM00075930.pdf
Now you might be able to get the code/docs out of ST, but it will likely require you to jump through a lot of hoops and sign a lot of paperwork.
Secure firmware is not magic, but is complicated to do thoroughly, and AES encryption code is readily available in the public domain. The F21x and F41x have hardware encryption also.
Securely locking down the device plays off against being able to update it. Ideally you want to disable JTAG/SWD interfaces and make them difficult to access. This of course does not protect you from people with physical access to your device, and the industrial methods to break and destroy it, and desire to do so.
The method and ideas expressed by Atmel, TI and NXP, et al are applicable to most devices, there will be some trade-offs in the hw/sw split of the implementations.
I'd recommend becoming familiar with the process, the need to keep end-to-end security, and the weakness/vulnerabilities in the system, and those whom you wish to protect against, and their skills/resources.
