1High Assurance Boot (HAB) for i.MX6 CPUs 2 3To authenticate U-Boot only by the CPU there is no code required in 4U-Boot itself. However, the U-Boot image to be programmed into the 5boot media needs to be properly constructed, i.e. it must contain a 6proper Command Sequence File (CSF). 7 8The Initial Vector Table contains a pointer to the CSF. Please see 9doc/README.imximage for how to prepare u-boot.imx. 10 11The CSF itself is being generated by Freescale HAB tools. 12 13mkimage will output additional information about "HAB Blocks" 14which can be used in the Freescale tooling to authenticate U-Boot 15(entries in the CSF file). 16 17Image Type: Freescale IMX Boot Image 18Image Ver: 2 (i.MX53/6 compatible) 19Data Size: 327680 Bytes = 320.00 kB = 0.31 MB 20Load Address: 177ff420 21Entry Point: 17800000 22HAB Blocks: 177ff400 00000000 0004dc00 23 ^^^^^^^^ ^^^^^^^^ ^^^^^^^^ 24 | | | 25 | | -------- (1) 26 | | 27 | ------------------- (2) 28 | 29 --------------------------- (3) 30 31(1) Size of area in file u-boot.imx to sign 32 This area should include the IVT, the Boot Data the DCD 33 and U-Boot itself. 34(2) Start of area in u-boot.imx to sign 35(3) Start of area in RAM to authenticate 36 37CONFIG_SECURE_BOOT currently enables only an additional command 38'hab_status' in U-Boot to retrieve the HAB status and events. This 39can be useful while developing and testing HAB. 40 41Commands to generate a signed U-Boot using Freescale HAB tools: 42cst --o U-Boot_CSF.bin < U-Boot.CSF 43objcopy -I binary -O binary --pad-to 0x2000 --gap-fill=0x00 \ 44 U-Boot_CSF.bin U-Boot_CSF_pad.bin 45cat u-boot.imx U-Boot_CSF_pad.bin > u-boot-signed.imx 46 47NOTE: U-Boot_CSF.bin needs to be padded to the value specified in 48the imximage.cfg file. 49 50Setup U-Boot Image for Encrypted Boot 51------------------------------------- 52An authenticated U-Boot image is used as starting point for 53Encrypted Boot. The image is encrypted by Freescale's Code 54Signing Tool (CST). The CST replaces only the image data of 55u-boot.imx with the encrypted data. The Initial Vector Table, 56DCD, and Boot data, remains in plaintext. 57 58The image data is encrypted with a Encryption Key (DEK). 59Therefore, this key is needed to decrypt the data during the 60booting process. The DEK is protected by wrapping it in a Blob, 61which needs to be appended to the U-Boot image and specified in 62the CSF file. 63 64The DEK blob is generated by an authenticated U-Boot image with 65the dek_blob cmd enabled. The image used for DEK blob generation 66needs to have the following configurations enabled: 67 68CONFIG_SECURE_BOOT 69CONFIG_SYS_FSL_SEC_COMPAT 4 /* HAB version */ 70CONFIG_FSL_CAAM 71CONFIG_CMD_DEKBLOB 72CONFIG_SYS_FSL_LE 73 74Note: The encrypted boot feature is only supported by HABv4 or 75greater. 76 77The dek_blob command then can be used to generate the DEK blob of 78a DEK previously loaded in memory. The command is used as follows: 79 80dek_blob <DEK address> <Output Address> <Key Size in Bits> 81example: dek_blob 0x10800000 0x10801000 192 82 83The resulting DEK blob then is used to construct the encrypted 84U-Boot image. Note that the blob needs to be transferred back 85to the host.Then the following commands are used to construct 86the final image. 87 88objcopy -I binary -O binary --pad-to 0x2000 --gap-fill=0x00 \ 89 U-Boot_CSF.bin U-Boot_CSF_pad.bin 90cat u-boot.imx U-Boot_CSF_pad.bin > u-boot-signed.imx 91objcopy -I binary -O binary --pad-to <blob_dst> --gap-fill=0x00 \ 92 u-boot-signed.imx u-boot-signed-pad.bin 93cat u-boot-signed-pad.imx DEK_blob.bin > u-boot-encrypted.imx 94 95 NOTE: u-boot-signed.bin needs to be padded to the value 96 equivalent to the address in which the DEK blob is specified 97 in the CSF. 98