path: root/README.md

# Android Verified Boot 2.0
---

This repository contains tools and libraries for working with Android
Verified Boot 2.0. Usually AVB is used to refer to this codebase.

# Table of Contents

* [What is it?](#what-is-it)
    + [The VBMeta struct](#the-vbmeta-struct)
    + [Rollback Protection](#rollback-Protection)
    + [A/B Support](#a_b-Support)
    + [The VBMeta Digest](#the-vbmeta-digest)
* [Tools and Libraries](#tools-and-libraries)
    + [avbtool and libavb](#avbtool-and-libavb)
    + [Files and Directories](#files-and-directories)
    + [Portability](#portability)
    + [Versioning and Compatibility](#versioning-and-compatibility)
    + [Adding New Features](#adding-new-features)
    + [Using avbtool](#using-avbtool)
    + [Build System Integration](#build-system-integration)
* [Device Integration](#device-integration)
    + [System Dependencies](#system-dependencies)
    + [Locked and Unlocked mode](#locked-and-unlocked-mode)
    + [Tamper-evident Storage](#tamper_evident-storage)
    + [Named Persistent Values](#named-persistent-values)
    + [Persistent Digests](#persistent-digests)
    + [Updating Stored Rollback Indexes](#updating-stored-rollback-indexes)
    + [Recommended Bootflow](#recommended-bootflow)
      + [Booting Into Recovery](#booting-into-recovery)
    + [Handling dm-verity Errors](#handling-dm_verity-errors)
    + [Android Specific Integration](#android-specific-integration)
    + [Device Specific Notes](#device-specific-notes)
* [Version History](#version-history)

# What is it?

Verified boot is the process of assuring the end user of the integrity
of the software running on a device. It typically starts with a
read-only portion of the device firmware which loads code and executes
it only after cryptographically verifying that the code is authentic
and doesn't have any known security flaws. AVB is one implementation
of verified boot.

## The VBMeta struct

The central data structure used in AVB is the VBMeta struct. This data
structure contains a number of descriptors (and other metadata) and
all of this data is cryptographically signed. Descriptors are used for
image hashes, image hashtree metadata, and so-called *chained
partitions*. A simple example is the following:

![AVB with boot, system, and vendor](docs/avb-integrity-data-in-vbmeta.png)

where the `vbmeta` partition holds the hash for the `boot` partition
in a hash descriptor. For the `system` and `vendor` partitions a
hashtree follows the filesystem data and the `vbmeta` partition holds
the root hash, salt, and offset of the hashtree in hashtree
descriptors. Because the VBMeta struct in the `vbmeta` partition is
cryptographically signed, the boot loader can check the signature and
verify it was made by the owner of `key0` (by e.g. embedding the
public part of `key0`) and thereby trust the hashes used for `boot`,
`system`, and `vendor`.

A chained partition descriptor is used to delegate authority - it
contains the name of the partition where authority is delegated as
well as the public key that is trusted for signatures on this
particular partition. As an example, consider the following setup:

![AVB with a chained partition](docs/avb-chained-partition.png)

In this setup the `xyz` partition has a hashtree for
integrity-checking. Following the hashtree is a VBMeta struct which
contains the hashtree descriptor with hashtree metadata (root hash,
salt, offset, etc.) and this struct is signed with `key1`. Finally, at
the end of the partition is a footer which has the offset of the
VBMeta struct.

This setup allows the bootloader to use the chain partition descriptor
to find the footer at the end of the partition (using the name in the
chain partition descriptor) which in turns helps locate the VBMeta
struct and verify that it was signed by `key1` (using `key1_pub` stored in the
chain partition descriptor). Crucially, because there's a footer with
the offset, the `xyz` partition can be updated without the `vbmeta`
partition needing any changes.

The VBMeta struct is flexible enough to allow hash descriptors and hashtree
descriptors for any partition to live in the `vbmeta` partition, the partition
that they are used to integrity check (via a chain partition descriptor), or any
other partition (via a chain partition descriptor). This allows for a wide range
of organizational and trust relationships.

Chained partitions need not use a footer - it is permissible to have a chained
partition point to a partition where the VBMeta struct is at the beginning
(e.g. just like the `vbmeta` partition). This is useful for use-cases where all
hash- and hashtree-descriptors for the partitions owned by an entire
organization are stored in a dedicated partition, for example `vbmeta_google`.
In this example the hashtree descriptor for `system` is in the `vbmeta_google`
partition meaning that the bootloader doesn't need to access the `system`
partition at all which is helpful if the `system` partition is managed as a
logical partition (via e.g. [LVM
techniques](https://en.wikipedia.org/wiki/Logical_volume_management) or
similar).

## Rollback Protection

AVB includes Rollback Protection which is used to protect against
known security flaws. Each VBMeta struct has a *rollback index* baked
into it like the following:

![AVB rollback indexes](docs/avb-rollback-indexes.png)

These numbers are referred to as `rollback_index[n]` and are increased
for each image as security flaws are discovered and
fixed. Additionally the device stores the last seen rollback index in
tamper-evident storage:

![AVB stored rollback indexes](docs/avb-stored-rollback-indexes.png)

and these are referred to as `stored_rollback_index[n]`.

Rollback protection is having the device reject an image unless
`rollback_index[n]` >= `stored_rollback_index[n]` for all `n`, and
having the device increase `stored_rollback_index[n]` over
time. Exactly how this is done is discussed in
the
[Updating Stored Rollback Indexes](#updating-stored-rollback-indexes)
section.

## A/B Support

AVB has been designed to work with A/B by requiring that the A/B
suffix is never used in any partition names stored in
descriptors. Here's an example with two slots:

![AVB with A/B partitions](docs/avb-ab-partitions.png)

Note how the rollback indexes differ between slots - for slot A the
rollback indexes are `[42, 101]` and for slot B they are `[43, 103]`.

In version 1.1 or later, avbtool supports `--do_not_use_ab` for
`add_hash_footer` and `add_hashtree_footer` operations. This makes it
possible to work with a partition that does not use A/B and should
never have the prefix. This corresponds to the
`AVB_HASH[TREE]_DESCRIPTOR_FLAGS_DO_NOT_USE_AB` flags.

In version 1.3, avbtool supports `chain_partition_do_not_use_ab` for
`make_vbmeta_image` operations. This makes it possible to work with
a chain partition that does not use A/B and should not have the suffix.
This corresponds to the `AVB_CHAIN_PARTITION_DESCRIPTOR_FLAGS_DO_NOT_USE_AB` flag.

## The VBMeta Digest

The VBMeta digest is a digest over all VBMeta structs including the root struct
(e.g. in the `vbmeta` partition) and all VBMeta structs in chained
partitions. This digest can be calculated at build time using `avbtool
calculate_vbmeta_digest` and also at runtime using the
`avb_slot_verify_data_calculate_vbmeta_digest()` function. It is also set on the
kernel command-line as `androidboot.vbmeta.digest`, see the `avb_slot_verify()`
documentation for exact details.

This digest can be used together with `libavb` in userspace inside the loaded
operating system to verify authenticity of the loaded vbmeta structs. This is
useful if the root-of-trust and/or stored rollback indexes are only available
while running in the boot loader.

Additionally, if the VBMeta digest is included in [hardware-backed attestation
data](https://developer.android.com/training/articles/security-key-attestation)
a relying party can extract the digest and compare it with list of digests for
known good operating systems which, if found, provides additional assurance
about the device the application is running on.

For [factory images of Pixel 3 and later
devices](https://developers.google.com/android/images), the
`pixel_factory_image_verify.py` located in `tools/transparency` is a convenience
tool for downloading, verifying and calcuating VBMeta Digests.

    $ pixel_factory_image_verify.py https://dl.google.com/dl/android/aosp/image.zip
    Fetching file from: https://dl.google.com/dl/android/aosp/image.zip
    Successfully downloaded file.
    Successfully unpacked factory image.
    Successfully unpacked factory image partitions.
    Successfully verified VBmeta.
    Successfully calculated VBMeta Digest.
    The VBMeta Digest for factory image is: 1f329b20a2dd69425e7a29566ca870dad51d2c579311992d41c9ba9ba05e170e

If the given argument is not an URL it considered to be a local file:

    $ pixel_factory_image_verify.py image.zip

# Tools and Libraries

This section contains information about the tools and libraries
included in AVB.

## avbtool and libavb

The main job of `avbtool` is to create `vbmeta.img` which is the
top-level object for verified boot. This image is designed to go into
the `vbmeta` partition (or, if using A/B, the slot in question
e.g. `vbmeta_a` or `vbmeta_b`) and be of minimal size (for out-of-band
updates). The vbmeta image is cryptographically signed and contains
verification data (e.g. cryptographic digests) for verifying
`boot.img`, `system.img`, and other partitions/images.

The vbmeta image can also contain references to other partitions where
verification data is stored as well as a public key indicating who
should sign the verification data. This indirection provides
delegation, that is, it allows a 3rd party to control content on a
given partition by including their public key in `vbmeta.img`. By
design, this authority can be easily revoked by simply updating
`vbmeta.img` with new descriptors for the partition in question.

Storing signed verification data on other images - for example
`boot.img` and `system.img` - is also done with `avbtool`.

The minimum requirement for running `avbtool` is to either have
Python 3.5 installed or build the avbtool with the embedded launcher
using `m avbtool` and then run it out of the build artifact directory:
`out/soong/host/linux-x86/bin/avbtool`

In addition to `avbtool`, a library - `libavb` - is provided. This
library performs all verification on the device side e.g. it starts by
loading the `vbmeta` partition, checks the signature, and then goes on
to load the `boot` partition for verification. This library is
intended to be used in both boot loaders and inside Android. It has a
simple abstraction for system dependencies (see `avb_sysdeps.h`) as
well as operations that the boot loader or OS is expected to implement
(see `avb_ops.h`). The main entry point for verification is
`avb_slot_verify()`.

An optional extension `libavb_cert` additionally provides a scalable
certificate-based authorization mechanism. The base `libavb` requires
the device to implement public key validation manually (see
`avb_validate_vbmeta_public_key()` in `avb_ops.h`), which can be
complicated when working with anything other than a single hardcoded
key. `libavb_cert` provides an implementation of this function which
provides built-in support for features such as key rotation.

`libavb_cert` was previously named `libavb_atx` (Android Things eXtension) but
it has been renamed to better represent its usefulness as a general-purpose
extension rather than anything specific to the Android Things project.

## Files and Directories

* `libavb/`
    + An implementation of image verification. This code is designed
      to be highly portable so it can be used in as many contexts as
      possible. This code requires a C99-compliant C compiler. Part of
      this code is considered internal to the implementation and
      should not be used outside it. For example, this applies to the
      `avb_rsa.[ch]` and `avb_sha.[ch]` files. System dependencies
      expected to be provided by the platform is defined in
      `avb_sysdeps.h`. If the platform provides the standard C runtime
      `avb_sysdeps_posix.c` can be used.
* `libavb_cert/`
    + A libavb extension for certificate-based authorization.
* `libavb_user/`
    + Contains an `AvbOps` implementation suitable for use in Android
      userspace. This is used in `boot_control.avb` and `avbctl`.
* `libavb_ab/`
    + An experimental A/B implementation for use in boot loaders and
      AVB examples. **NOTE**: This code is *DEPRECATED* and you must
      define `AVB_AB_I_UNDERSTAND_LIBAVB_AB_IS_DEPRECATED` to use
      it. The code will be removed Jun 1 2018.
* `boot_control/`
    + An implementation of the Android `boot_control` HAL for use with
      boot loaders using the experimental `libavb_ab` A/B stack.
      **NOTE**: This code is *DEPRECATED* and will be removed Jun 1
      2018.
* `Android.bp`
    + Build instructions for building `libavb` (a static library for use
      on the device), host-side libraries (for unit tests), and unit
      tests.
* `avbtool`
    + A tool written in Python for working with images related to
      verified boot.
* `test/`
    + Unit tests for `abvtool`, `libavb`, `libavb_ab`, and
      `libavb_cert`.
* `tools/avbctl/`
    + Contains the source-code for a tool that can be used to control
      AVB at runtime in Android.
* `examples/uefi/`
    + Contains the source-code for a UEFI-based boot-loader utilizing
      `libavb/` and `libavb_ab/`.
* `examples/cert/`
    + Contains example source-code for using the `avb_cert` extension
* `README.md`
    + This file.
* `docs/`
    + Contains documentation files.

## Portability

The `libavb` code is intended to be used in bootloaders in devices
that will load Android or other operating systems. The suggested
approach is to copy the appropriate header and C files mentioned in
the previous section into the boot loader and integrate as
appropriate.

As the `libavb/` codebase will evolve over time integration should be
as non-invasive as possible. The intention is to keep the API of the
library stable however it will be broken if necessary. As for
portability, the library is intended to be highly portable, work on
both little- and big-endian architectures and 32- and 64-bit. It's
also intended to work in non-standard environments without the
standard C library and runtime.

If the `AVB_ENABLE_DEBUG` preprocessor symbol is set, the code will
include useful debug information and run-time checks. Production
builds should not use this. The preprocessor symbol `AVB_COMPILATION`
should be set only when compiling the libraries. The code must be
compiled into a separate library.

Applications using the compiled `libavb` library must only include the
`libavb/libavb.h` file (which will include all public interfaces) and
must not have the `AVB_COMPILATION` preprocessor symbol set. This is
to ensure that internal code that may be change in the future (for
example `avb_sha.[ch]` and `avb_rsa.[ch]`) will not be visible to
application code.

## Versioning and Compatibility

AVB uses a version number with three fields - the major, minor, and
sub version. Here's an example version number

                         1.4.3
                         ^ ^ ^
                         | | |
    the major version ---+ | |
    the minor version -----+ |
      the sub version -------+

The major version number is bumped only if compatibility is broken,
e.g. a struct field has been removed or changed. The minor version
number is bumped only if a new feature is introduced, for example a
new algorithm or descriptor has been added. The sub version number is
bumped when bugs are fixed or other changes not affecting
compatibility are made.

The `AvbVBMetaImageHeader` struct (as defined in the
`avb_vbmeta_image.h`) carries the major and minor version number of
`libavb` required to verify the struct in question. This is stored in
the `required_libavb_version_major` and
`required_libavb_version_minor` fields. Additionally this struct
contains a textual field with the version of `avbtool` used to create
the struct, for example "avbtool 1.4.3" or "avbtool 1.4.3 some_board
Git-4589fbec".

Note that it's entirely possible to have a `AvbVBMetaImageHeader`
struct with

    required_libavb_version_major = 1
    required_libavb_version_minor = 0
    avbtool_release_string = "avbtool 1.4.3"

if, for example, creating an image that does not use any features
added after AVB version 1.0.

## Adding New Features

If adding a new feature for example a new algorithm or a new
descriptor then `AVB_VERSION_MINOR` in `avb_version.h` and `avbtool`
must be bumped and `AVB_VERSION_SUB` should be set to zero.

Unit tests **MUST** be added to check that

* The feature is used if - and only if - suitable commands/options are
  passed to `avbtool`.
* The `required_version_minor` field is set to the bumped value if -
  and only if - the feature is used. Also add tests to check that the
  correct value is output when `--print_required_libavb_version` is
  used.

If `AVB_VERSION_MINOR` has already been bumped since the last release
there is obviously no need to bump it again.

## Using avbtool

The content for the vbmeta partition can be generated as follows:

    $ avbtool make_vbmeta_image                                                                  \
        [--output OUTPUT]                                                                        \
        [--algorithm ALGORITHM] [--key /path/to/key_used_for_signing_or_pub_key]                 \
        [--public_key_metadata /path/to/pkmd.bin]                                                \
        [--rollback_index NUMBER] [--rollback_index_location NUMBER]                             \
        [--include_descriptors_from_image /path/to/image.bin]                                    \
        [--setup_rootfs_from_kernel /path/to/image.bin]                                          \
        [--chain_partition part_name:rollback_index_location:/path/to/key1.bin]                  \
        [--chain_partition_do_not_use_ab part_name:rollback_index_location:/path/to/key.bin]     \
        [--signing_helper /path/to/external/signer]                                              \
        [--signing_helper_with_files /path/to/external/signer_with_files]                        \
        [--print_required_libavb_version]                                                        \
        [--append_to_release_string STR]

An integrity footer containing the hash for an entire partition can be
added to an existing image as follows:

    $ avbtool add_hash_footer                                                      \
        --partition_name PARTNAME --partition_size SIZE                            \
        [--image IMAGE]                                                            \
        [--algorithm ALGORITHM] [--key /path/to/key_used_for_signing_or_pub_key]   \
        [--public_key_metadata /path/to/pkmd.bin]                                  \
        [--rollback_index NUMBER] [--rollback_index_location NUMBER]               \
        [--hash_algorithm HASH_ALG] [--salt HEX]                                   \
        [--include_descriptors_from_image /path/to/image.bin]                      \
        [--setup_rootfs_from_kernel /path/to/image.bin]                            \
        [--output_vbmeta_image OUTPUT_IMAGE] [--do_not_append_vbmeta_image]        \
        [--signing_helper /path/to/external/signer]                                \
        [--signing_helper_with_files /path/to/external/signer_with_files]          \
        [--print_required_libavb_version]                                          \
        [--append_to_release_string STR]                                           \
        [--calc_max_image_size]                                                    \
        [--do_not_use_ab]                                                          \
        [--use_persistent_digest]

Valid values for `HASH_ALG` above include `sha1` and `sha256`.

An integrity footer containing the root digest and salt for a hashtree
for a partition can be added to an existing image as follows. The
hashtree is also appended to the image.

    $ avbtool add_hashtree_footer                                                  \
        --partition_name PARTNAME --partition_size SIZE                            \
        [--image IMAGE]                                                            \
        [--algorithm ALGORITHM] [--key /path/to/key_used_for_signing_or_pub_key]   \
        [--public_key_metadata /path/to/pkmd.bin]                                  \
        [--rollback_index NUMBER] [--rollback_index_location NUMBER]               \
        [--hash_algorithm HASH_ALG] [--salt HEX] [--block_size SIZE]               \
        [--include_descriptors_from_image /path/to/image.bin]                      \
        [--setup_rootfs_from_kernel /path/to/image.bin]                            \
        [--setup_as_rootfs_from_kernel]                                            \
        [--output_vbmeta_image OUTPUT_IMAGE] [--do_not_append_vbmeta_image]        \
        [--do_not_generate_fec] [--fec_num_roots FEC_NUM_ROOTS]                    \
        [--signing_helper /path/to/external/signer]                                \
        [--signing_helper_with_files /path/to/external/signer_with_files]          \
        [--print_required_libavb_version]                                          \
        [--append_to_release_string STR]                                           \
        [--calc_max_image_size]                                                    \
        [--do_not_use_ab]                                                          \
        [--no_hashtree]                                                            \
        [--use_persistent_digest]                                                  \
        [--check_at_most_once]

Valid values for `HASH_ALG` above include `sha1`, `sha256`, and `blake2b-256`.

The size of an image with integrity footers can be changed using the
`resize_image` command:

    $ avbtool resize_image                                                         \
        --image IMAGE                                                              \
        --partition_size SIZE

The integrity footer on an image can be removed from an image. The
hashtree can optionally be kept in place.

    $ avbtool erase_footer --image IMAGE [--keep_hashtree]

For hash- and hashtree-images the vbmeta struct can also be written to
an external file via the `--output_vbmeta_image` option and one can
also specify that the vbmeta struct and footer not be added to the
image being operated on.

The hashtree and FEC data in an image can be zeroed out with the following
command:

    $ avbtool zero_hashtree --image IMAGE

This is useful for trading compressed image size for having to reculculate the
hashtree and FEC at runtime. If this is done the hashtree and FEC data is set
to zero except for the first eight bytes which are set to the magic
`ZeRoHaSH`. Either the hashtree or FEC data or both may be zeroed this way
so applications should check for the magic both places. Applications can
use the magic to detect if recalculation is needed.

To calculate the maximum size of an image that will fit in a partition
of a given size after having used the `avbtool add_hash_footer` or
`avbtool add_hashtree_footer` commands on it, use the
`--calc_max_image_size` option:

    $ avbtool add_hash_footer --partition_size $((10*1024*1024)) \
        --calc_max_image_size
    10416128

    $ avbtool add_hashtree_footer --partition_size $((10*1024*1024)) \
        --calc_max_image_size
    10330112

To calculate the required libavb version that would be put in the
vbmeta struct when using `make_vbmeta_image`, `add_hash_footer`, and
`add_hashtree_footer` commands use the
`--print_required_libavb_version` option:

    $ avbtool make_vbmeta_image \
        --algorithm SHA256_RSA2048 --key /path/to/key.pem \
        --include_descriptors_from_image /path/to/boot.img \
        --include_descriptors_from_image /path/to/system.img \
        --print_required_libavb_version
    1.0

Alternatively, `--no_hashtree` can be used with `avbtool add_hashtree_footer`
command. If `--no_hashtree` is given, the hashtree blob is omitted and only
its descriptor is added to the vbmeta struct. The descriptor says the size
of hashtree is 0, which tells an application the need to recalculate
hashtree.

The `--signing_helper` option can be used in `make_vbmeta_image`,
`add_hash_footer` and `add_hashtree_footer` commands to specify any
external program for signing hashes. The data to sign (including
padding e.g. PKCS1-v1.5) is fed via `STDIN` and the signed data is
returned via `STDOUT`. If `--signing_helper` is present in a command
line, the `--key` option need only contain a public key. Arguments for
a signing helper are `algorithm` and `public key`. If the signing
helper exits with a non-zero exit code, it means failure.

Here's an example invocation:

    /path/to/my_signing_program SHA256_RSA2048 /path/to/publickey.pem

The `--signing_helper_with_files` is similar to `--signing_helper`
except that a temporary file is used to communicate with the helper
instead of `STDIN` and `STDOUT`. This is useful in situations where
the signing helper is using code which is outputting diagnostics on
`STDOUT` instead of `STDERR`. Here's an example invocation

    /path/to/my_signing_program_with_files SHA256_RSA2048 \
      /path/to/publickey.pem /tmp/path/to/communication_file

where the last positional argument is a file that contains the data to
sign. The helper should write the signature in this file.

The `append_vbmeta_image` command can be used to append an entire
vbmeta blob to the end of another image. This is useful for cases when
not using any vbmeta partitions, for example:

    $ cp boot.img boot-with-vbmeta-appended.img
    $ avbtool append_vbmeta_image                       \
        --image boot-with-vbmeta-appended.img           \
        --partition_size SIZE_OF_BOOT_PARTITION         \
        --vbmeta_image vbmeta.img
    $ fastboot flash boot boot-with-vbmeta-appended.img

Information about an image can be obtained using the `info_image` command. The
output of this command should not be relied on and the way information is
structured may change.

The `verify_image` command can be used to verify the contents of
several image files at the same time. When invoked on an image the
following checks are performed:

* If the image has a VBMeta struct the signature is checked against
  the embedded public key. If the image doesn't look like `vbmeta.img`
  then a footer is looked for and used if present.

* If the option `--key` is passed then a `.pem` file is expected and
  it's checked that the embedded public key in said VBMeta struct
  matches the given key.

* All descriptors in the VBMeta struct are checked in the following
  way:
    + For a hash descriptor the image file corresponding to the
      partition name is loaded and its digest is checked against that
      in the descriptor.
    + For a hashtree descriptor the image file corresponding to the
      partition name is loaded and the hashtree is calculated and its
      root digest compared to that in the descriptor.
    + For a chained partition descriptor its contents is compared
      against content that needs to be passed in via the
      `--expected_chain_partition` options. The format for this option
      is similar to that of the `--chain_partition` option. If there
      is no `--expected_chain_partition` descriptor for the chain
      partition descriptor the check fails.

Here's an example for a setup where the digests for `boot.img` and
`system.img` are stored in `vbmeta.img` which is signed with
`my_key.pem`. It also checks that the chain partition for partition
`foobar` uses rollback index 8 and that the public key in AVB format
matches that of the file `foobar_vendor_key.avbpubkey`:

    $ avbtool verify_image \
         --image /path/to/vbmeta.img \
         --key my_key.pem \
         --expect_chained_partition foobar:8:foobar_vendor_key.avbpubkey

    Verifying image /path/to/vbmeta.img using key at my_key.pem
    vbmeta: Successfully verified SHA256_RSA4096 vbmeta struct in /path_to/vbmeta.img
    boot: Successfully verified sha256 hash of /path/to/boot.img for image of 10543104 bytes
    system: Successfully verified sha1 hashtree of /path/to/system.img for image of 1065213952 bytes
    foobar: Successfully verified chain partition descriptor matches expected data

In this example the `verify_image` command verifies the files
`vbmeta.img`, `boot.img`, and `system.img` in the directory
`/path/to`. The directory and file extension of the given image
(e.g. `/path/to/vbmeta.img`) is used together with the partition name
in the descriptor to calculate the filenames of the images holding
hash and hashtree images.

The `verify_image` command can also be used to check that a custom
signing helper works as intended.

The `calculate_vbmeta_digest` command can be used to calculate the vbmeta digest
of several image files at the same time. The result is printed as a hexadecimal
string either on `STDOUT` or a supplied path (using the `--output` option).

    $ avbtool calculate_vbmeta_digest \
         --hash_algorithm sha256 \
         --image /path/to/vbmeta.img
    a20fdd01a6638c55065fe08497186acde350d6797d59a55d70ffbcf41e95c2f5

In this example the `calculate_vbmeta_digest` command loads the `vbmeta.img`
file. If this image has one or more chain partition descriptors, the same logic
as the `verify_image` command is used to load files for these (e.g. it assumes
the same directory and file extension as the given image). Once all vbmeta
structs have been loaded, the digest is calculated (using the hash algorithm
given by the `--hash_algorithm` option) and printed out.

To print hash and hashtree digests embedded in the verified metadata, use the
`print_partition_digests` command like this:

    $ avbtool print_partition_digests --image /path/to/vbmeta.img
    system: ddaa513715fd2e22f3c1cea3c1a1f98ccb515fc6
    boot: 5cba9a418e04b5f9e29ee6a250f6cdbe30c6cec867c59d388f141c3fedcb28c1
    vendor: 06993a9e85e46e53d3892881bb75eff48ecadaa8

For partitions with hash descriptors, this prints out the digest and for
partitions with hashtree descriptors the root digest is printed out. Like the
`calculate_vbmeta_digest` and `verify_image` commands, chain partitions are
followed. To use JSON for the output, use the `--json` option.

In case you would like to log all command lines for all avbtool invocations for
debugging integrations with other tooling, you can configure the envirionment
variable AVB_INVOCATION_LOGFILE with the name of the log file:

    $ export AVB_INVOCATION_LOGFILE='/tmp/avb_invocation.log'
    $ ./avbtool.py version
    $ ./avbtool.py version
    $ cat /tmp/avb_invocation.log
    ./avbtool.py version
    ./avbtool.py version


## Build System Integration

In Android, AVB is enabled by the `BOARD_AVB_ENABLE` variable

    BOARD_AVB_ENABLE := true

This will make the build system create `vbmeta.img` which will contain
a hash descriptor for `boot.img`, a hashtree descriptor for
`system.img`, a kernel-cmdline descriptor for setting up `dm-verity`
for `system.img` and append a hash-tree to `system.img`. If the build
system is set up such that one or many of `vendor.img` / `product.img`
/ `system_ext.img` / `odm.img` are being built, the hash-tree for each
of them will also be appended to the image respectively, and their
hash-tree descriptors will be included into `vbmeta.img` accordingly.

By default, the algorithm `SHA256_RSA4096` is used with a test key
from the `external/avb/test/data` directory. This can be overriden by
the `BOARD_AVB_ALGORITHM` and `BOARD_AVB_KEY_PATH` variables to use
e.g. a 4096-bit RSA key and SHA-512:

    BOARD_AVB_ALGORITHM := SHA512_RSA4096
    BOARD_AVB_KEY_PATH := /path/to/rsa_key_4096bits.pem

Remember that the public part of this key needs to be available to the
bootloader of the device expected to verify resulting images. Use
`avbtool extract_public_key` to extract the key in the expected format
(`AVB_pk` in the following). If the device is using a different root
of trust than `AVB_pk` the `--public_key_metadata` option can be used
to embed a blob (`AVB_pkmd` in the following) that can be used to
e.g. derive `AVB_pk`. Both `AVB_pk` and `AVB_pkmd` are passed to the
`validate_vbmeta_public_key()` operation when verifying a slot.

Some devices may support the end-user configuring the root of trust to use, see
the [Device Specific Notes](#device-specific-notes) section for details.

Devices can be configured to create additional `vbmeta` partitions as
[chained partitions](#the-vbmeta-struct) in order to update a subset of
partitions without changing the top-level `vbmeta` partition. For example,
the following variables create `vbmeta_system.img` as a chained `vbmeta`
image that contains the hash-tree descriptors for `system.img`, `system_ext.img`
and `product.img`. `vbmeta_system.img` itself will be signed by the specified
key and algorithm.

    BOARD_AVB_VBMETA_SYSTEM := system system_ext product
    BOARD_AVB_VBMETA_SYSTEM_KEY_PATH := external/avb/test/data/testkey_rsa2048.pem
    BOARD_AVB_VBMETA_SYSTEM_ALGORITHM := SHA256_RSA2048
    BOARD_AVB_VBMETA_SYSTEM_ROLLBACK_INDEX_LOCATION := 1

Note that the hash-tree descriptors for `system.img`, `system_ext.img` and
`product.img` will be included only in `vbmeta_system.img`, but not
`vbmeta.img`. With the above setup, partitions `system.img`, `system_ext.img`,
`product.img` and `vbmeta_system.img` can be updated independently - but as a
group - of the rest of the partitions, *or* as part of the traditional updates
that update all the partitions.

Currently build system supports building chained `vbmeta` images of
`vbmeta_system.img` (`BOARD_AVB_VBMETA_SYSTEM`) and `vbmeta_vendor.img`
(`BOARD_AVB_VBMETA_VENDOR`).

To prevent rollback attacks, the rollback index should be increased on
a regular basis. The rollback index can be set with the
`BOARD_AVB_ROLLBACK_INDEX` variable:

     BOARD_AVB_ROLLBACK_INDEX := 5

If this is not set, the rollback index defaults to 0.

The variable `BOARD_AVB_MAKE_VBMETA_IMAGE_ARGS` can be used to specify
additional options passed to `avbtool make_vbmeta_image`. Typical
options to be used here include `--prop`, `--prop_from_file`,
`--chain_partition`, `--public_key_metadata`, and `--signing_helper`.

The variable `BOARD_AVB_BOOT_ADD_HASH_FOOTER_ARGS` can be used to
specify additional options passed to `avbtool add_hash_footer` for
`boot.img`. Typical options to be used here include `--hash_algorithm`
and `--salt`.

The variable `BOARD_AVB_SYSTEM_ADD_HASHTREE_FOOTER_ARGS` can be used
to specify additional options passed to `avbtool add_hashtree_footer`
for `system.img`. Typical options to be used here include
`--hash_algorithm`, `--salt`, `--block_size`, and
`--do_not_generate_fec`.

The variable `BOARD_AVB_VENDOR_ADD_HASHTREE_FOOTER_ARGS` can be used
to specify additional options passed to `avbtool add_hashtree_footer`
for `vendor.img`. Typical options to be used here include
`--hash_algorithm`, `--salt`, `--block_size`, and
`--do_not_generate_fec`.

The variable `BOARD_AVB_DTBO_ADD_HASH_FOOTER_ARGS` can be used to
specify additional options passed to `avbtool add_hash_footer` for
`dtbo.img`. Typical options to be used here include `--hash_algorithm`
and `--salt`.

Build system variables (such as `PRODUCT_SUPPORTS_VERITY_FEC`) used
for previous version of Verified Boot in Android are not used in AVB.

A/B related build system variables can be found [here](https://source.android.com/devices/tech/ota/ab_updates#build-variables).

# Device Integration

This section discusses recommendations and best practices for
integrating `libavb` with a device boot loader. It's important to
emphasize that these are just recommendations so the use of the word
`must` should be taken lightly.

Additionally term *HLOS* is used in this chapter to refer to the *High
Level Operating System*. This obviously includes Android (including
other form-factors than phones) but could also be other operating
systems.

## System Dependencies

The `libavb` library is written in a way so it's portable to any
system with a C99 compiler. It does not require the standard C library
however the boot loader must implement a simple set of system
primitives required by `libavb` such as `avb_malloc()`, `avb_free()`,
and `avb_print()`.

In addition to the system primitives, `libavb` interfaces with the boot
loader through the supplied `AvbOps` struct. This includes operations
to read and write data from partitions, read and write rollback
indexes, check if the public key used to make a signature should be
accepted, and so on.

## Locked and Unlocked mode

AVB has been designed to support the notion of the device being either
LOCKED state or UNLOCKED state as used in Android.

In the context of AVB, the LOCKED state means that verification errors
are fatal whereas in UNLOCKED state they are not. If the device is
UNLOCKED pass `AVB_SLOT_VERIFY_FLAGS_ALLOW_VERIFICATION_ERROR` flag in
the `flags` parameter of `avb_slot_verify()` and treat verification
errors including

* `AVB_SLOT_VERIFY_RESULT_ERROR_PUBLIC_KEY_REJECTED`
* `AVB_SLOT_VERIFY_RESULT_ERROR_VERIFICATION`
* `AVB_SLOT_VERIFY_RESULT_ERROR_ROLLBACK_INDEX`

as non-fatal. If the device is in the LOCKED state, don't pass the
`AVB_SLOT_VERIFY_FLAGS_ALLOW_VERIFICATION_ERROR` flag in the `flags`
parameter of `avb_slot_verify()` and only treat
`AVB_SLOT_VERIFY_RESULT_OK` as non-fatal.

On Android, device state may be altered through the fastboot interface
using, e.g. `fastboot flashing lock` (to transition to the LOCKED
state) and `fastboot flashing unlock` (to transition to the UNLOCKED
state).

The device must only allow state transitions (e.g. from LOCKED to
UNLOCKED or UNLOCKED to LOCKED) after asserting physical presence of
the user. If the device has a display and buttons this is typically
done by showing a dialog and requiring the user to confirm or cancel
using physical buttons.

All user data must be cleared when transitioning from the LOCKED to
the UNLOCKED state (including the `userdata` partition and any NVRAM
spaces). Additionally all `stored_rollback_index[n]` locations must be
cleared (all elements must be set to zero). Similar action (erasing
`userdata`, NVRAM spaces, and `stored_rollback_index[n]` locations)
shall also happening when transitioning from UNLOCKED to LOCKED. If
the device is required to use full disk encryption, then a less
intensive wipe is required for UNLOCKED to LOCKED. Depending on the
device form-factor and intended use, the user should be prompted to
confirm before any data is erased.

## Tamper-evident Storage

In this document, *tamper-evident* means that it's possible to detect
if the HLOS has tampered with the data, e.g. if it has been
overwritten.

Tamper-evident storage must be used for stored rollback indexes, keys
used for verification, device state (whether the device is LOCKED or
UNLOCKED), and named persistent values. If tampering has been detected
the corresponding `AvbOps` operation should fail by e.g. returning
`AVB_IO_RESULT_ERROR_IO`. It is especially important that verification
keys cannot be tampered with since they represent the root-of-trust.

If verification keys are mutable they must only be set by the end
user, e.g. it must never be set at the factory or store or any
intermediate point before the end user. Additionally, it must only be
possible to set or clear a key while the device is in the UNLOCKED
state.

## Named Persistent Values

AVB 1.1 introduces support for named persistent values which must be
tamper evident and allows AVB to store arbitrary key-value pairs.
Integrators may limit support for these values to a set of fixed
well-known names, a maximum value size, and / or a maximum number of
values.

## Persistent Digests

Using a persistent digest for a partition means the digest (or root
digest in the case of a hashtree) is not stored in the descriptor but
is stored in a named persistent value. This allows configuration data
which may differ from device to device to be verified by AVB. It must
not be possible to modify the persistent digest when the device is in
the LOCKED state, except if a digest does not exist it may be initialized.

To specify that a descriptor should use a persistent digest, use the
`--use_persistent_digest` option for the `add_hash_footer` or
`add_hashtree_footer` avbtool operations. Then, during verification of
the descriptor, AVB will look for the digest in the named persistent
value `avb.persistent_digest.$(partition_name)` instead of in the
descriptor itself.

For hashtree descriptors using a persistent digest, the digest value
will be available for substitution into kernel command line descriptors
using a token of the form `$(AVB_FOO_ROOT_DIGEST)` where 'FOO' is the
uppercase partition name, in this case for the partition named 'foo'.
The token will be replaced by the digest in hexadecimal form.

By default, when the `--use_persistent_digest` option is used with
`add_hash_footer` or `add_hashtree_footer`, avbtool will generate a
descriptor with no salt rather than the typical default of generating a
random salt equal to the digest length. This is because the digest
value is stored in persistent storage and thus cannot change over time.
An alternative option would be to manually provide a random salt using
`--salt`, but this salt would need to remain unchanged for the life
of the device once the persistent digest value was written.

## Updating Stored Rollback Indexes

In order for Rollback Protection to work the bootloader will need to
update the `stored_rollback_indexes[n]` array on the device prior to
transferring control to the HLOS. If not using A/B this is
straightforward - just update it to what's in the AVB metadata for the
slot before booting. In pseudo-code it would look like this:

```c++
// The |slot_data| parameter should be the AvbSlotVerifyData returned
// by avb_slot_verify() for the slot we're about to boot.
//
bool update_stored_rollback_indexes_for_slot(AvbOps* ops,
                                             AvbSlotVerifyData* slot_data) {
    for (int n = 0; n < AVB_MAX_NUMBER_OF_ROLLBACK_INDEX_LOCATIONS; n++) {
        uint64_t rollback_index = slot_data->rollback_indexes[n];
        if (rollback_index > 0) {
            AvbIOResult io_ret;
            uint64_t current_stored_rollback_index;

            io_ret = ops->read_rollback_index(ops, n, &current_stored_rollback_index);
            if (io_ret != AVB_IO_RESULT_OK) {
                return false;
            }

            if (rollback_index > current_stored_rollback_index) {
                io_ret = ops->write_rollback_index(ops, n, rollback_index);
                if (io_ret != AVB_IO_RESULT_OK) {
                    return false;
                }
            }
        }
    }
    return true;
}
```

However if using A/B more care must be taken to still allow the device
to fall back to the old slot if the update didn't work.

For an HLOS like Android where rollback is only supported if the
updated OS version is found to not work, `stored_rollback_index[n]`
should only be updated from slots that are marked as SUCCESSFUL in the
A/B metadata. The pseudo-code for that is as follows where
`is_slot_is_marked_as_successful()` comes from the A/B stack in use:

```c++
if (is_slot_is_marked_as_successful(slot->ab_suffix)) {
    if (!update_stored_rollback_indexes_for_slot(ops, slot)) {
        // TODO: handle error.
    }
}
```

This logic should ideally be implemented outside of the HLOS. One
possible implementation is to update rollback indices in the
bootloader when booting into a successful slot. This means that
when booting into a new OS not yet marked as successful, the
rollback indices would not be updated. The first reboot after the
slot succeeded would trigger an update of the rollback indices.

For an HLOS where it's possible to roll back to a previous version,
`stored_rollback_index[n]` should be set to the largest possible value
allowing all bootable slots to boot. This approach is implemented in
AVB's experimental (and now deprecated) A/B stack `libavb_ab`, see the
`avb_ab_flow()` implementation. Note that this requires verifying
*all* bootable slots at every boot and this may impact boot time.

## Recommended Bootflow

The recommended boot flow for a device using AVB is as follows:

![Recommended AVB boot flow](docs/avb-recommended-boot-flow.png)

Notes:

* The device is expected to search through all A/B slots until it
  finds a valid OS to boot. Slots that are rejected in the LOCKED
  state might not be rejected in the UNLOCKED state, (e.g. when
  UNLOCKED any key can be used and rollback index failures are
  allowed), so the algorithm used for selecting a slot varies
  depending on what state the device is in.

* If no valid OS (that is, no bootable A/B slot) can be found, the
  device cannot boot and has to enter repair mode. It is
  device-dependent what this looks like.  If the device has a screen
  it must convey this state to the user.

* If the device is LOCKED, only an OS signed by an embedded
  verification key (see the previous section) shall be
  accepted. Additionally, `rollback_index[n]` as stored in the
  verified image must be greater or equal than what's in
  `stored_rollback_index[n]` on the device (for all `n`) and the
  `stored_rollback_index[n]` array is expected to be updated as
  specified in the previous section.
    + If the key used for verification was set by the end user, and
      the device has a screen, it must show a warning with the key
      fingerprint to convey that the device is booting a custom
      OS. The warning must be shown for at least 10 seconds before the
      boot process continues. If the device does not have a screen,
      other ways must be used to convey that the device is booting a
      custom OS (lightbars, LEDs, etc.).

* If the device is UNLOCKED, there is no requirement to check the key
  used to sign the OS nor is there any requirement to check or update
  rollback `stored_rollback_index[n]` on the device. Because of this
  the user must always be shown a warning about verification not
  occurring.
    + It is device-dependent how this is implemented since it depends
      on the device form-factor and intended usage. If the device has
      a screen and buttons (for example if it's a phone) the warning
      is to be shown for at least 10 seconds before the boot process
      continues. If the device does not have a screen, other ways must
      be used to convey that the device is UNLOCKED (lightbars, LEDs,
      etc.).

### Booting Into Recovery

On Android devices not using A/B, the `recovery` partition usually isn't
updated along with other partitions and therefore can't be referenced
from the main `vbmeta` partition.

It's still possible to use AVB to protect this partition (and others)
by signing these partitions and passing the
`AVB_SLOT_VERIFY_FLAGS_NO_VBMETA_PARTITION` flag to `avb_slot_verify()`.
In this mode, the key used to sign each requested partition is verified
by the `validate_public_key_for_partition()` operation which is also
used to return the rollback index location to be used.

## Handling dm-verity Errors

By design, hashtree verification errors are detected by the HLOS and
not the bootloader. AVB provides a way to specify how the error should
be handled through the `hashtree_error_mode` parameter in the
`avb_slot_verify()` function. Possible values include

* `AVB_HASHTREE_ERROR_MODE_RESTART_AND_INVALIDATE` means that the HLOS
  will invalidate the current slot and restart. On devices with A/B
  this would lead to attempting to boot the other slot (if it's marked
  as bootable) or it could lead to a mode where no OS can be booted
  (e.g. some form of repair mode). In Linux this requires a kernel
  built with `CONFIG_DM_VERITY_AVB`.

* `AVB_HASHTREE_ERROR_MODE_RESTART` means that the OS will restart
  without the current slot being invalidated. Be careful using this
  mode unconditionally as it may introduce boot loops if the same
  hashtree verification error is hit on every boot.

* `AVB_HASHTREE_ERROR_MODE_EIO` means that an `EIO` error will be
  returned to the application.

* `AVB_HASHTREE_ERROR_MODE_MANAGED_RESTART_AND_EIO` means that either the **RESTART**
  or **EIO** mode is used, depending on state. This mode implements a state
  machine whereby **RESTART** is used by default and when the
  `AVB_SLOT_VERIFY_FLAGS_RESTART_CAUSED_BY_HASHTREE_CORRUPTION` is passed to
  `avb_slot_verify()` the mode transitions to **EIO**. When a new OS has been
  detected the device transitions back to the **RESTART** mode.
    + To do this persistent storage is needed - specifically this means that the
      passed in `AvbOps` will need to have the `read_persistent_value()` and
      `write_persistent_value()` operations implemented. The name of the
      persistent value used is **avb.managed_verity_mode** and 32 bytes of storage
      is needed.

* `AVB_HASHTREE_ERROR_MODE_LOGGING` means that errors will be logged
   and corrupt data may be returned to applications. This mode should
   be used for **ONLY** diagnostics and debugging. It cannot be used
   unless verification errors are allowed.

* `AVB_HASHTREE_ERROR_MODE_PANIC` means that the OS will **panic** without
  the current slot being invalidated. Be careful using this mode as it may
  introduce boot panic if the same hashtree verification error is hit on
  every boot. This mode is available since: 1.7.0 (kernel 5.9)

The value passed in `hashtree_error_mode` is essentially just passed on through
to the HLOS through the the `androidboot.veritymode`,
`androidboot.veritymode.managed`, and `androidboot.vbmeta.invalidate_on_error`
kernel command-line parameters in the following way:

|      | `androidboot.veritymode` | `androidboot.veritymode.managed` | `androidboot.vbmeta.invalidate_on_error` |
|------|:------------------------:|:--------------------------------:|:----------------------------------------:|
| `AVB_HASHTREE_ERROR_MODE_RESTART_AND_INVALIDATE` | **enforcing** | (unset) | **yes** |
| `AVB_HASHTREE_ERROR_MODE_RESTART` | **enforcing** | (unset) | (unset) |
| `AVB_HASHTREE_ERROR_MODE_EIO` | **eio** | (unset) | (unset) |
| `AVB_HASHTREE_ERROR_MODE_MANAGED_RESTART_AND_EIO` | **eio** or **enforcing** | **yes** | (unset) |
| `AVB_HASHTREE_ERROR_MODE_LOGGING` | **ignore_corruption** | (unset) | (unset) |
| `AVB_HASHTREE_ERROR_MODE_PANIC` | **panicking** | (unset) | (unset) |

The only exception to this table is that if the
`AVB_VBMETA_IMAGE_FLAGS_HASHTREE_DISABLED` flag is set in the top-level vbmeta,
then `androidboot.veritymode` is set to **disabled** and
`androidboot.veritymode.managed` and `androidboot.vbmeta.invalidate_on_error`
are unset.

The different values of `hashtree_error_mode` parameter in the `avb_slot_verify()`
function can be categorized into three groups:

* `AVB_HASHTREE_ERROR_MODE_RESTART_AND_INVALIDATE`, which needs `CONFIG_DM_VERITY_AVB`
  in the kernel config for the kernel to invalidate the current slot and
  restart. This is kept here for legacy Android Things devices and is not
  recommended for other device form factors.

* The bootloader handles the switch between `AVB_HASHTREE_ERROR_MODE_RESTART`
  and `AVB_HASHTREE_ERROR_MODE_EIO`. This would need a persistent storage on the
  device to store the vbmeta digest, so the bootloader can detect if a device
  ever gets an update or not. Once the new OS is installed and if the device is
  in **EIO** mode, the bootloader should switch back to **RESTART** mode.

* `AVB_HASHTREE_ERROR_MODE_MANAGED_RESTART_AND_EIO`: `libavb` helps the
  bootloader manage **EIO**/**RESTART** state transition. The bootloader needs
  to implement the callbacks of `AvbOps->read_persistent_value()` and
  `AvbOps->write_persistent_value()` for `libavb` to store the vbmeta digest to
  detect whether a new OS is installed.

### Which mode should I use for my device?

This depends entirely on the device, how the device is intended to be
used, and the desired user experience.

For Android devices the `AVB_HASHTREE_ERROR_MODE_MANAGED_RESTART_AND_EIO` mode
should be used. Also see the [Boot Flow section on source.android.com](https://source.android.com/security/verifiedboot/boot-flow) for the kind of UX and UI the boot loader should implement.

If the device doesn't have a screen or if the HLOS supports multiple bootable
slots simultaneously it may make more sense to just use
`AVB_HASHTREE_ERROR_MODE_RESTART_AND_INVALIDATE`.

## Android Specific Integration

On Android, the boot loader must set the
`androidboot.verifiedbootstate` parameter on the kernel command-line
to indicate the boot state. It shall use the following values:

* **green**: If in LOCKED state and the key used for verification was not set by the end user.
* **yellow**: If in LOCKED state and the key used for verification was set by the end user.
* **orange**: If in the UNLOCKED state.

## GKI 2.0 Integration

Starting from Android 12, devices launching with kernel version 5.10 or higher
must ship with the GKI kernel. See [GKI 2.0](https://source.android.com/devices/architecture/kernel/generic-kernel-image#gki2)
for details.

While incorporating a certified GKI `boot.img` into a device codebase, the
following board variables should be configured. The setting shown below is just
an example to be adjusted per device.

```
# Uses a prebuilt boot.img
TARGET_NO_KERNEL := true
BOARD_PREBUILT_BOOTIMAGE := device/${company}/${board}/boot.img

# Enables chained vbmeta for the boot.img so it can be updated independently,
# without updating the vbmeta.img. The following configs are optional.
# When they're absent, the hash of the boot.img will be stored then signed in
# the vbmeta.img.
BOARD_AVB_BOOT_KEY_PATH := external/avb/test/data/testkey_rsa4096.pem
BOARD_AVB_BOOT_ALGORITHM := SHA256_RSA4096
BOARD_AVB_BOOT_ROLLBACK_INDEX := $(PLATFORM_SECURITY_PATCH_TIMESTAMP)
BOARD_AVB_BOOT_ROLLBACK_INDEX_LOCATION := 2
```

**NOTE**: The certified GKI `boot.img` isn't signed for verified boot.
A device-specific verified boot chain should still be configured for a prebuilt
GKI `boot.img`.

## Device Specific Notes

This section contains information about how AVB is integrated into specific
devices. This is not an exhaustive list.

### Pixel 2 and later

On the Pixel 2, Pixel 2 XL and later Pixel models, the boot loader supports a
virtual partition with the name `avb_custom_key`. Flashing and erasing this
partition only works in the UNLOCKED state. Setting the custom key is done like
this:

    avbtool extract_public_key --key key.pem --output pkmd.bin
    fastboot flash avb_custom_key pkmd.bin

Erasing the key is done by erasing the virtual partition:

    fastboot erase avb_custom_key

When the custom key is set and the device is in the LOCKED state it will boot
images signed with both the built-in key as well as the custom key. All other
security features (including rollback-protection) are in effect, e.g. the
**only** difference is the root of trust to use.

When booting an image signed with a custom key, a yellow screen will be shown as
part of the boot process to remind the user that the custom key is in use.

# Version History

### Version 1.3
Version 1.3 adds support for the following:
* A 32-bit `flags` element is added to a chain descriptor.
* Support for chain partitions which don't use [A/B](#a_b-support).

### Version 1.2

Version 1.2 adds support for the following:
* `rollback_index_location` field of the main vbmeta header.
* `check_at_most_once` parameter of dm-verity in a hashtree descriptor.

### Version 1.1

Version 1.1 adds support for the following:

* A 32-bit `flags` element is added to hash and hashtree descriptors.
* Support for partitions which don't use [A/B](#a_b-support).
* Tamper-evident [named persistent values](#named-persistent-values).
* [Persistent digests](#persistent-digests) for hash or hashtree descriptors.

### Version 1.0

All features not explicitly listed under a later version are supported by 1.0.