11. Setup U-Boot Image for Encrypted Boot
2----------------------------------------
3An authenticated U-Boot image is used as starting point for
4Encrypted Boot. The image is encrypted by i.MX Code Signing
5Tool (CST). The CST replaces only the image data of
6u-boot-dtb.imx with the encrypted data. The Initial Vector Table,
7DCD, and Boot data, remains in plaintext.
8
9The image data is encrypted with a Encryption Key (DEK).
10Therefore, this key is needed to decrypt the data during the
11booting process. The DEK is protected by wrapping it in a Blob,
12which needs to be appended to the U-Boot image and specified in
13the CSF file.
14
15The DEK blob is generated by an authenticated U-Boot image with
16the dek_blob cmd enabled. The image used for DEK blob generation
17needs to have the following configurations enabled in Kconfig:
18
19CONFIG_SECURE_BOOT=y
20CONFIG_CMD_DEKBLOB=y
21
22Note: The encrypted boot feature is only supported by HABv4 or
23greater.
24
25The dek_blob command then can be used to generate the DEK blob of
26a DEK previously loaded in memory. The command is used as follows:
27
28dek_blob <DEK address> <Output Address> <Key Size in Bits>
29example: dek_blob 0x10800000 0x10801000 192
30
31The resulting DEK blob then is used to construct the encrypted
32U-Boot image. Note that the blob needs to be transferred back
33to the host.Then the following commands are used to construct
34the final image.
35
36cat u-boot-dtb.imx csf-u-boot.bin > u-boot-signed.imx
37objcopy -I binary -O binary --pad-to <blob_dst> --gap-fill=0x00 \
38    u-boot-signed.imx u-boot-signed-pad.bin
39cat u-boot-signed-pad.imx DEK_blob.bin > u-boot-encrypted.imx
40
41    NOTE: u-boot-signed.bin needs to be padded to the value
42    equivalent to the address in which the DEK blob is specified
43    in the CSF.
44