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

4 Freescale Semiconductor

i.MX28 security architecture overview

1.5 References

The references listed below are available for download at freescale.com using the keyword provided.

• i.MX28 Applications Processor Reference Manual. Keyword: MCIMX28RM.

• HAB Code-Signing Tool User’s Guide available in the Code Signing Tool package. Keyword:

IMX_CST_TOOL.

• Fuse programming guides, Programming OTP Bits and OTP Burner Documentation, are available

in the OTP tools package. Keyword: IMX_OTP_TOOLS.

• Elftosb documentation is available in the ELFTOSB package. Keyword: IMX_ELFTOSB_TOOL.

• High Assurance Boot Version 4 Application Programming Interface Reference Manual in the Code

Signing Tool package (version 2.0 and later). Keyword: IMX_CST_TOOL.

2 i.MX28 security architecture overview

This section gives a technical overview of the i.MX28 security architecture, providing the background

information needed for understanding the use cases and processes described in later sections.

2.1 ROM bootstrap code and HAB library

To design a correctly signed boot image, it is necessary to understand both the components which make

up HAB and the basic boot-time authentication process. This section gives an architectural level overview

of these elements which should be sufficient for most purposes.

The ROM bootstrap code is the first software executed after reset, and controls the initial phase of the boot

process, using the HAB library. HAB is used to authenticate the boot image in external memory prior to

its execution.

Based on pin or fuse settings, the ROM Bootstrap executes different boot modes to locate, load and execute

the boot image from various boot peripherals (for example, NAND flash, SD/MMC card, serial

EEPROM/flash, and USB recovery mode).

To ensure a secure boot, correct execution of the ROM Bootstrap must be guaranteed. To this end, its

integrity is protected by virtue of its location in masked processor-internal ROM. Execution of the Boot

ROM is also protected through disabling of external boot modes and interrupts.

SRK Super Root Key, an RSA key pair which forms the start of the boot-time authentication chain.

The SRK private keys are held by the CA. Unless explicitly noted, SRK in this document refers

to the public key only.

SRK Table Super Root Key Table, HAB v4 uses an SRK Table with maximum length of up to four (4) keys,

allowing selection of the SRK used for a particular image. The hash of the SRK Table is

embedded in the processor using OTP hardware.

Table 1. Definitions, acronyms, and abbreviations (continued)

Term/Acronym Definition

1 2 3 4 5 6 7 8 9 ... 30 31

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