+==========================================================+
+   i.MX6, i.MX7 U-Boot Encrypted Boot guide using HABv4   +
+==========================================================+

1. HABv4 Encrypted Boot process
-------------------------------

This document describes a step-by-step procedure on how to encrypt and
sign an U-Boot image. It is assumed that the reader is familiar
with basic HAB concepts and has already followed the mx6_mx7_secure_boot.txt
guide and got a working closed device.

Details about HAB and encrypted boot process can be found in application
notes AN4581[1] and AN12056[2] and in the introduction_habv4.txt document.

Before continuing, be sure to have fatwrite and dek_blob commands
available in U-Boot. If not, enable them in Kconfig and rebuild
U-Boot:

- Defconfig

  CONFIG_FAT_WRITE=y
  CONFIG_CMD_DEKBLOB=y
  CONFIG_CMD_PRIBLOB=y

- Kconfig

  File systems -> Enable FAT filesystem support-> Enable FAT filesystem
  write support
  ARM architecture -> Support the 'dek_blob' command
  ARM architecture -> Support the set_priblob_bitfield command

1.1 Building an encrypted U-Boot image
--------------------------------------

This U-Boot is built the same way the one from the secure document is,
so it provides the same access the the HAB APIs, extra functions
for HAB, etc...

However, the layout of the new image is different, as a part of it is
encrypted, and a DEK blob is appended at the end. The diagram below
illustrates an encrypted u-boot-dtb.imx image layout:

  ----------------------- +-----------------------------+ <--- *start
         ^            ^   |      Image Vector Table     |
         |            |   +-----------------------------+ <--- *boot_data
         |            |   |          Boot Data          |
         |      Plain |   +-----------------------------+ <--- *dcd
         |       text |   |          DCD Table          |
         |            |   +-----------------------------+
  Signed |            v   |           Padding           |
   data  |        ------- +-----------------------------+ <--- *entry
         |            ^   |                             |
         |            |   |                             |
         |  Encrypted |   |        u-boot-dtb.bin       |
         |       data |   |                             |
         |            |   |                             |
         |            |   +-----------------------------+
         v            v   |           Padding           |
  ----------------------- +-----------------------------+ <--- *csf
                      ^   |    Command Sequence File    |
               0x2000 |   |   (commands + SRK table +   |
           (in bytes) |   | signatures + certificates + |
                      v   |        Nonce + MAC)         |
                  ------- +-----------------------------+
                          |           Padding           |
                          +-----------------------------+ <--- *csf + 0x2000
                          |           DEK Blob          |
                          +-----------------------------+
                          |           Padding           |
                          +-----------------------------+

1.2 Get a secure boot working
-----------------------------

You need to go through all the steps described into the
mx6_mx7_secure_boot.txt guide, and get a signed U-Boot which can
boot successfully on a closed target. Otherwise, the following
steps will not work.

1.3 Compile the CST to enable the encrypting feature
----------------------------------------------------

The encrypting feature is not enabled by default. You need to
execute the following commands to enable it:

  $ sudo apt-get install libssl-dev openssl
  $ cd <CST install directory>/code/back_end/src
  $ gcc -o cst_encrypted -I ../hdr -L ../../../linux64/lib *.c
    -lfrontend -lcrypto
  $ cp cst_encrypted ../../../<where your original CST executable is>

1.4 Creating the CSF description files
--------------------------------------

The CSF contains all the commands that the ROM executes during the
secure boot. These commands instruct the HAB on which memory areas
of the image to authenticate, which keys to install, use, etc...

CSF examples for encrypted boot are available under
doc/imx/habv4/csf_examples/ directory.

For both CSF, first part is same compared to the CSF used for
the secure boot step.
Here we describe how to encrypt the U-Boot image and then sign it.

1.4.1 csf_u-boot_enc.txt
-------------------------

This first CSF is used to encrypt the U-Boot image and generate the
dek.bin file. The Authenticate Data command has to be modified, and
two new commands have to be added:

- Modify the Authenticate Data command to only cover IVT and DCD:

  Blocks = 0x877ff400 0x00000000 0x00000c00 "u-boot-dtb.imx"

- Add the new Install Secret Key command to generate the dek.bin
  file and install the blob. The parameter which depends of your
  configuration is the Blob Address. Padding of 0x2000 is
  recommended. Following the csf_uboot.txt data for instance:

  Blob Address = Authenticate Start Address + Padding + length
               = 0x877ff400 + 0x2000 + 0x9ec00 = 0x878a0000

- Add the new Decrypt Data command to encrypt the file. As the file
  specified in parameter will be modified, we suggest to copy it.
  Then modify the Blocks command depending of your U-Boot image.
  In our example:

  $ cp u-boot-dtb.imx u-boot-dtb.imx-enc
  Block = (Authenticate start addr + 0xc00) 0xc00 (length - 0xc00)
          u-boot-dtb.imx-enc
        = (0x877ff400 + 0xc00) 0xc00 (0x9ec00 - 0xc00)
          u-boot-dtb.imx-enc
        = 0x87800000 0xc00 0x9e000 u-boot-dtb.imx-enc

1.4.2 csf_u-boot_sign_enc.txt
-----------------------------

This second CSF is used to sign the encrypted U-Boot image previously
generated (u-boot-dtb.imx-enc). The Authenticate Data part has also
to be changed, the modifications are the following:

- The Authenticate Data command is same compared to the one in
  csf_uboot.txt file, except that this time, the file parameter
  is the file previously encrypted: u-boot-dtb.imx-enc.

  Blocks = 0x877ff400 0x000 0x0009ec00 "u-boot-dtb.imx-enc"

- For the two new commands, we do not want to they modify our previously
  signed/generated files. Therefore, for the Key parameter of the
  Install Secret Key command, the value is now dek-dummy.bin, which
  will generate a new dek file instead of erasing the previous one.
  About the decrypt data command, you need to copy the u-boot-dtb.imx
  file again in a u-boot-dtb.imx-dummy file, to not replace the
  original encrypted file with an encrypted one:

  Key = "dek-dummy.bin"
  Blocks = 0x87800000 0x00000c00 0x9e000 "u-boot-dtb.imx-dummy"

1.5 Encrypt the U-Boot image
----------------------------

The image is encrypted using the Code Signing Tool. It generates also
a CSF binary and a dek.bin file, which will be used on the future
steps below.

- Create the CSF binary file and encrypt the U-Boot image

  $ ./cst_encrypted -i csf_u-boot_enc.txt -o csf_u-boot_enc.bin

1.6 Sign the encrypted U-Boot image
-----------------------------------

The image is then signed using the Code Signing Tool. It also
generate a CSF binary, which will be used on the future steps below.

- Create the CSF binary file and sign the encrypted U-Boot image

  $ ./cst_encrypted -i csf_u-boot_sign_enc.txt -o csf_u-boot_sign_enc.bin

1.7 Swap Nonce/MAC from csf_u-boot_enc.bin to csf_u-boot_sign_enc.bin
---------------------------------------------------------------------

First, calculate Nonce/MAC size based on MAC bytes value
in CSF. As Mac bytes is 16:

  Nonce/MAC size = Nonce size + MAC bytes + CSF header for Nonce/Mac
                 = 12 + 16 + 8 = 36 bytes

Then, calculate Nonce/MAC offset in CSF:

  MAC offset = csf_u-boot-enc.bin size - Nonce/MAC size
             = 3972 - 36 = 3936 Bytes

In the next step, extract Nonce/NAC from the first CSF:

  $ dd if=csf_u-boot_enc.bin of=noncemac.bin bs=1 skip=3936 count=36

Finally, replace the MAC of csf_u-boot_sign_enc.bin with the extracted
one:

  $ dd if=noncemac.bin of=csf_u-boot_sign_enc.bin bs=1 seek=3936 count=36

1.8 Generate encryptedu-boot with no dek
----------------------------------------

As described in the layout in the first part of this document, the
final image is composed of these different parts padded to known
values, to make it compliant with the CSF.

First, pad the CSF to 0x2000:

  $ objcopy -I binary -O binary --pad-to 0x2000 --gap-fill=0xff
  csf_u-boot_sign_enc.bin csf_u-boot_sign_enc_padded.bin

Then, append this file to the encrypted U-Boot image:

  $ cat u-boot-dtb.imx-enc csf_u-boot_sign_enc_padded.bin >
  u-boot_encrypted_no_dek.bin

Pad this new file to width+offset (0x9ec00 + 0x2000 = 0xa0c00):

  $ objcopy -I binary -O binary --pad-to 0xa0c00 --gap-fill=0x00
  u-boot_encrypted_no_dek.bin u-boot_encrypted_no_dek_padded.bin

1.9 Generate the DEK Blob
-------------------------
The missing part to get our final U-Boot encrypted image is a DEK
blob. To generate it, copy the dek.bin file generated at step 1.5
on the Boot partition of your SD Card. Then interrupt the boot
and your chip. You need the fatwrite and dek_blob command, which
should be enabled by the secure boot. If not, you have to enable
them in Kconfig and recompile U-Boot

Run the following commands:

  => mmc list
  FSL_SDHC: 0 (SD)   #index to use for mmc in following commands
  => fatload mmc 0 0x80800000 dek.bin
  => dek_blob 0x80800000 0x80801000 128
  => fatwrite mmc 0 0x80801000 dek_blob.bin 0x48

1.10 Finalize the encrypted U-Boot image
----------------------------------------

Finally, copy the generated dek_blob.bin file from your SDCard to
your CST repository. Append it with the last padded file to get your
final image:

  $ cat u-boot_encrypted_no_dek_padded.bin dek_blob.bin >
  u-boot_encrypted.bin

If the image does not boot, please verify the size of your
U-Boot image, the length specified into CSF and the padding values.

2. About the PRIBLOB bitfield from CAAM SCFGR register
------------------------------------------------------

It is highly recommended to set the PRIBLOB bitfield from the CAAM
SCFGR register to 0x3 once your encrypted U-Boot image is completed.
To do so, a command has been implemented in u-boot:

  => set_priblob_bitfield

Once this bitfield is set to 0x3, it ensures cryptographic separation
of private blob types avoiding any modification or replacement of
DEK blobs. Newly created blobs will be incompatible with blobs
required to decrypt an encrypted boot image. When the HAB later
executes the command to decrypt the DEK, an incompatible DEK blob
will be detected and cause an error. A substitute encrypted boot image
will not be decrypted, and will not be executed.

References:
[1] AN4581: "Secure Boot on i.MX 50, i.MX 53, i.MX 6 and i.MX 7 Series
using HABv4" - Rev. 2
[2] AN12056: "Encrypted Boot on HABv4 and CAAM Enabled Devices" - Rev. 1
