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Secure Boot with i.MX28 HAB Version 4, Rev. 1

Freescale Semiconductor 11

Designing for code signing

3.1.4 Image layout

When performing a secure boot on an i.MX28 processor, the image must contain a correctly formatted

image vector table (IVT) with a valid header and pointers.

The loader inside ROM first loads the image from boot media. The image data is then passed through the

DCP (Data Co-Processor) where it will be decrypted (if it is an encrypted image) and placed at the

destination address.

As mentioned earlier in Section 2.2 the bootable section of the SB file consists of a sequence of boot

commands. Typically it consists of number of LOAD commands followed by HAB CALL commands to

execute bootlets and then finally a HAB JUMP command. The HAB CALL and HAB JUMP commands

require an address where an IVT was loaded.

At this stage, the rest of the boot process diverges depending on the security configuration:

non-secure/open or secure/closed which is determined by the SEC_CONFIG fuse field.

3.1.5 Nonsecure boot—image layout

When performing a non-secure boot with the SEC_CONFIG fuse field set to Open, it is not necessary to

provide the CSF data as part of the image. When no CSF is present, the CSF field of the IVT should be set

to NULL. Regardless of whether a valid CSF present or not, HAB will attempt to authenticate the image

performing the same steps as it would do for a secure boot in closed configuration. If authentication fails

then HAB will log events that can be later used for debugging purpose and continue execution of the

normal boot flow. Eventually ROM code will jump to the image pointed by *entry.

Note that when SEC_CONFIG fuse field is set to Open, all HAB failures are considered to be non- fatal

and the boot process is allowed to continue. The Open configuration should also be used for development

purposes of secure products where CSFs and other data components for secure boot can be debugged. The

Open configuration is the end configuration for non-secure products.

Figure 4. Typical memory layout of an unsigned image

1 2 ... 6 7 8 9 10 11 12 13 14 15 16 ... 30 31

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Crypto D100 Betriebsanweisung Seite 11