docs/design.md

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# Boot Loader

## Summary

mcuboot comprises two packages:

* The bootutil library (boot/bootutil)
* The boot application (each port has its own at boot/<port>)

The bootutil library performs most of the functions of a boot loader.  In
particular, the piece that is missing is the final step of actually jumping to
the main image.  This last step is instead implemented by the boot application.
Boot loader functionality is separated in this manner to enable unit testing of
the boot loader.  A library can be unit tested, but an application can't.
Therefore, functionality is delegated to the bootutil library when possible.

## Limitations

The boot loader currently only supports images with the following
characteristics:
* Built to run from flash.
* Built to run from a fixed location (i.e., not position-independent).

## Image Format

The following definitions describe the image format.

``` c
#define IMAGE_MAGIC                 0x96f3b83d

#define IMAGE_HEADER_SIZE           32

struct image_version {
    uint8_t iv_major;
    uint8_t iv_minor;
    uint16_t iv_revision;
    uint32_t iv_build_num;
};

/** Image header.  All fields are in little endian byte order. */
struct image_header {
    uint32_t ih_magic;
    uint32_t ih_load_addr;
    uint16_t ih_hdr_size; /* Size of image header (bytes). */
    uint16_t _pad2;
    uint32_t ih_img_size; /* Does not include header. */
    uint32_t ih_flags;    /* IMAGE_F_[...]. */
    struct image_version ih_ver;
    uint32_t _pad3;
};

/** Image TLV header.  All fields in little endian. */
struct image_tlv_info {
    uint16_t it_magic;
    uint16_t it_tlv_tot;  /* size of TLV area (including tlv_info header) */
};

/** Image trailer TLV format. All fields in little endian. */
struct image_tlv {
    uint8_t  it_type;   /* IMAGE_TLV_[...]. */
    uint8_t  _pad;
    uint16_t it_len;    /* Data length (not including TLV header). */
};

/*
 * Image header flags.
 */
#define IMAGE_F_PIC                      0x00000001 /* Not supported. */
#define IMAGE_F_NON_BOOTABLE             0x00000010 /* Split image app. */
#define IMAGE_F_RAM_LOAD                 0x00000020

/*
 * Image trailer TLV types.
 */
#define IMAGE_TLV_KEYHASH           0x01   /* hash of the public key */
#define IMAGE_TLV_SHA256            0x10   /* SHA256 of image hdr and body */
#define IMAGE_TLV_RSA2048_PSS       0x20   /* RSA2048 of hash output */
#define IMAGE_TLV_ECDSA224          0x21   /* ECDSA of hash output */
#define IMAGE_TLV_ECDSA256          0x22   /* ECDSA of hash output */
#define IMAGE_TLV_RSA3072_PSS       0x23   /* RSA3072 of hash output */
#define IMAGE_TLV_ED25519           0x24   /* ED25519 of hash output */
```

Optional type-length-value records (TLVs) containing image metadata are placed
after the end of the image.

The `ih_hdr_size` field indicates the length of the header, and therefore the
offset of the image itself.  This field provides for backwards compatibility in
case of changes to the format of the image header.

## Flash Map

A device's flash is partitioned according to its _flash map_.  At a high
level, the flash map maps numeric IDs to _flash areas_.  A flash area is a
region of disk with the following properties:
1. An area can be fully erased without affecting any other areas.
2. A write to one area does not restrict writes to other areas.

The boot loader uses the following flash area IDs:
```c
/* Independent from multiple image boot */
#define FLASH_AREA_BOOTLOADER         0
#define FLASH_AREA_IMAGE_SCRATCH      3
```
```c
/* If the boot loader is working with the first image */
#define FLASH_AREA_IMAGE_PRIMARY      1
#define FLASH_AREA_IMAGE_SECONDARY    2
```
```c
/* If the boot loader is working with the second image */
#define FLASH_AREA_IMAGE_PRIMARY      5
#define FLASH_AREA_IMAGE_SECONDARY    6
```

The bootloader area contains the bootloader image itself. The other areas are
described in subsequent sections. The flash could contain multiple executable
images therefore the flash area IDs of primary and secondary areas are mapped
based on the number of the active image (on which the bootloader is currently
working).

## Image Slots

A portion of the flash memory can be partitioned into multiple image areas, each
contains two image slots: a primary slot and a secondary slot.
The boot loader will only run an image from the primary slot, so images must be
built such that they can run from that fixed location in flash.  If the boot
loader needs to run the image resident in the secondary slot, it must copy its
contents into the primary slot before doing so, either by swapping the two
images or by overwriting the contents of the primary slot. The bootloader
supports either swap- or overwrite-based image upgrades, but must be configured
at build time to choose one of these two strategies.

In addition to the slots of image areas, the boot loader requires a scratch
area to allow for reliable image swapping. The scratch area must have a size
that is enough to store at least the largest sector that is going to be swapped.
Many devices have small equally sized flash sectors, eg 4K, while others have
variable sized sectors where the largest sectors might be 128K or 256K, so the
scratch must be big enough to store that. The scratch is only ever used when
swapping firmware, which means only when doing an upgrade. Given that, the main
reason for using a larger size for the scratch is that flash wear will be more
evenly distributed, because a single sector would be written twice the number of
times than using two sectors, for example. To evaluate the ideal size of the
scratch for your use case the following parameters are relevant:

* the ratio of image size / scratch size
* the number of erase cycles supported by the flash hardware

The image size is used (instead of slot size) because only the slot's sectors
that are actually used for storing the image are copied. The image/scratch ratio
is the number of times the scratch will be erased on every upgrade. The number
of erase cycles divided by the image/scratch ratio will give you the number of
times an upgrade can be performed before the device goes out of spec.

```
num_upgrades = number_of_erase_cycles / (image_size / scratch_size)
```

Let's assume, for example, a device with 10000 erase cycles, an image size of
150K and a scratch of 4K (usual minimum size of 4K sector devices). This would
result in a total of:

`10000 / (150 / 4) ~ 267`

Increasing the scratch to 16K would give us:

`10000 / (150 / 16) ~ 1067`

There is no *best* ratio, as the right size is use-case dependent. Factors to
consider include the number of times a device will be upgraded both in the field
and during development, as well as any desired safety margin on the
manufacturer's specified number of erase cycles. In general, using a ratio that
allows hundreds to thousands of field upgrades in production is recommended.

The overwrite upgrade strategy is substantially simpler to implement than the
image swapping strategy, especially since the bootloader must work properly
even when it is reset during the middle of an image swap. For this reason, the
rest of the document describes its behavior when configured to swap images
during an upgrade.

## Boot Swap Types

When the device first boots under normal circumstances, there is an up-to-date
firmware image in each primary slot, which mcuboot can validate and then
chain-load. In this case, no image swaps are necessary. During device upgrades,
however, new candidate image(s) is present in the secondary slot(s), which
mcuboot must swap into the primary slot(s) before booting as discussed above.

Upgrading an old image with a new one by swapping can be a two-step process. In
this process, mcuboot performs a "test" swap of image data in flash and boots
the new image or it will be executed during operation. The new image can then
update the contents of flash at runtime to mark itself "OK", and mcuboot will
then still choose to run it during the next boot. When this happens, the swap is
made "permanent". If this doesn't happen, mcuboot will perform a "revert" swap
during the next boot by swapping the image(s) back into its original location(s)
, and attempting to boot the old image(s).

Depending on the use case, the first swap can also be made permanent directly.
In this case, mcuboot will never attempt to revert the images on the next reset.

Test swaps are supported to provide a rollback mechanism to prevent devices
from becoming "bricked" by bad firmware.  If the device crashes immediately
upon booting a new (bad) image, mcuboot will revert to the old (working) image
at the next device reset, rather than booting the bad image again. This allows
device firmware to make test swaps permanent only after performing a self-test
routine.

On startup, mcuboot inspects the contents of flash to decide for each images
which of these "swap types" to perform; this decision determines how it
proceeds.

The possible swap types, and their meanings, are:

- `BOOT_SWAP_TYPE_NONE`: The "usual" or "no upgrade" case; attempt to boot the
  contents of the primary slot.

- `BOOT_SWAP_TYPE_TEST`: Boot the contents of the secondary slot by swapping
  images.  Unless the swap is made permanent, revert back on the next boot.

- `BOOT_SWAP_TYPE_PERM`: Permanently swap images, and boot the upgraded image
  firmware.

- `BOOT_SWAP_TYPE_REVERT`: A previous test swap was not made permanent;
  swap back to the old image whose data are now in the secondary slot.  If the
  old image marks itself "OK" when it boots, the next boot will have swap type
  `BOOT_SWAP_TYPE_NONE`.

- `BOOT_SWAP_TYPE_FAIL`: Swap failed because image to be run is not valid.

- `BOOT_SWAP_TYPE_PANIC`: Swapping encountered an unrecoverable error.

The "swap type" is a high-level representation of the outcome of the
boot. Subsequent sections describe how mcuboot determines the swap type from
the bit-level contents of flash.

## Image Trailer

For the bootloader to be able to determine the current state and what actions
should be taken during the current boot operation, it uses metadata stored in
the image flash areas. While swapping, some of this metadata is temporarily
copied into and out of the scratch area.

This metadata is located at the end of the image flash areas, and is called an
image trailer. An image trailer has the following structure:

```
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                                                               ~
    ~    Swap status (BOOT_MAX_IMG_SECTORS * min-write-size * 3)    ~
    ~                                                               ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                 Encryption key 0 (16 octets) [*]              |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                 Encryption key 1 (16 octets) [*]              |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      Swap size (4 octets)                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Swap info   |           0xff padding (7 octets)             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Copy done   |           0xff padding (7 octets)             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Image OK    |           0xff padding (7 octets)             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       MAGIC (16 octets)                       |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
```

[*]: Only present if the encryption option is enabled (`MCUBOOT_ENC_IMAGES`).

The offset immediately following such a record represents the start of the next
flash area.

Note: "min-write-size" is a property of the flash hardware.  If the hardware
allows individual bytes to be written at arbitrary addresses, then
min-write-size is 1.  If the hardware only allows writes at even addresses,
then min-write-size is 2, and so on.

An image trailer contains the following fields:

1. Swap status: A series of records which records the progress of an image
   swap.  To swap entire images, data are swapped between the two image areas
   one or more sectors at a time, like this:

   - sector data in the primary slot is copied into scratch, then erased
   - sector data in the secondary slot is copied into the primary slot,
     then erased
   - sector data in scratch is copied into the secondary slot

As it swaps images, the bootloader updates the swap status field in a way that
allows it to compute how far this swap operation has progressed for each
sector.  The swap status field can thus used to resume a swap operation if the
bootloader is halted while a swap operation is ongoing and later reset. The
`BOOT_MAX_IMG_SECTORS` value is the configurable maximum number of sectors
mcuboot supports for each image; its value defaults to 128, but allows for
either decreasing this size, to limit RAM usage, or to increase it in devices
that have massive amounts of Flash or very small sized sectors and thus require
a bigger configuration to allow for the handling of all slot's sectors.
The factor of min-write-sz is due to the behavior of flash hardware. The factor
of 3 is explained below.

2. Encryption keys: key-encrypting keys (KEKs).  These keys are needed for
   image encryption and decryption.  See the
   [encrypted images](encrypted_images.md) document for more information.

3. Swap size: When beginning a new swap operation, the total size that needs
   to be swapped (based on the slot with largest image + tlvs) is written to
   this location for easier recovery in case of a reset while performing the
   swap.

4. Swap info: A single byte which encodes the following informations:
    - Swap type: Stored in bits 0-3. Indicating the type of swap operation in
    progress. When mcuboot resumes an interrupted swap, it uses this field to
    determine the type of operation to perform. This field contains one of the
    following values in the table below.
    - Image number: Stored in bits 4-7. It has always 0 value at single image
    boot. In case of multi image boot it indicates, which image was swapped when
    interrupt happened. The same scratch area is used during in case of all
    image swap operation. Therefore this field is used to determine which image
    the trailer belongs to if boot status is found on scratch area when the swap
    operation is resumed.

| Name                      | Value |
| ------------------------- | ----- |
| `BOOT_SWAP_TYPE_TEST`     | 2     |
| `BOOT_SWAP_TYPE_PERM`     | 3     |
| `BOOT_SWAP_TYPE_REVERT`   | 4     |


5. Copy done: A single byte indicating whether the image in this slot is
   complete (0x01=done; 0xff=not done).

6. Image OK: A single byte indicating whether the image in this slot has been
   confirmed as good by the user (0x01=confirmed; 0xff=not confirmed).

7. MAGIC: The following 16 bytes, written in host-byte-order:

``` c
    const uint32_t boot_img_magic[4] = {
        0xf395c277,
        0x7fefd260,
        0x0f505235,
        0x8079b62c,
    };
```

## IMAGE TRAILERS

At startup, the boot loader determines the boot swap type by inspecting the
image trailers.  When using the term "image trailers" what is meant is the
aggregate information provided by both image slot's trailers.

### New swaps (non-resumes)

For new swaps, mcuboot must inspect a collection of fields to determine which
swap operation to perform.

The image trailers records are structured around the limitations imposed by
flash hardware. As a consequence, they do not have a very intuitive design, and
it is difficult to get a sense of the state of the device just by looking at the
image trailers.  It is better to map all the possible trailer states to the swap
types described above via a set of tables.  These tables are reproduced below.

Note: An important caveat about the tables described below is that they must
be evaluated in the order presented here. Lower state numbers must have a
higher priority when testing the image trailers.

```
    State I
                     | primary slot | secondary slot |
    -----------------+--------------+----------------|
               magic | Any          | Good           |
            image-ok | Any          | Unset          |
           copy-done | Any          | Any            |
    -----------------+--------------+----------------'
     result: BOOT_SWAP_TYPE_TEST                     |
    -------------------------------------------------'


    State II
                     | primary slot | secondary slot |
    -----------------+--------------+----------------|
               magic | Any          | Good           |
            image-ok | Any          | 0x01           |
           copy-done | Any          | Any            |
    -----------------+--------------+----------------'
     result: BOOT_SWAP_TYPE_PERM                     |
    -------------------------------------------------'


    State III
                     | primary slot | secondary slot |
    -----------------+--------------+----------------|
               magic | Good         | Unset          |
            image-ok | 0xff         | Any            |
           copy-done | 0x01         | Any            |
    -----------------+--------------+----------------'
     result: BOOT_SWAP_TYPE_REVERT                   |
    -------------------------------------------------'
```

Any of the above three states results in mcuboot attempting to swap images.

Otherwise, mcuboot does not attempt to swap images, resulting in one of the
other three swap types, as illustrated by State IV.

```
    State IV
                     | primary slot | secondary slot |
    -----------------+--------------+----------------|
               magic | Any          | Any            |
            image-ok | Any          | Any            |
           copy-done | Any          | Any            |
    -----------------+--------------+----------------'
     result: BOOT_SWAP_TYPE_NONE,                    |
             BOOT_SWAP_TYPE_FAIL, or                 |
             BOOT_SWAP_TYPE_PANIC                    |
    -------------------------------------------------'
```

In State IV, when no errors occur, mcuboot will attempt to boot the contents of
the primary slot directly, and the result is `BOOT_SWAP_TYPE_NONE`. If the image
in the primary slot is not valid, the result is `BOOT_SWAP_TYPE_FAIL`. If a
fatal error occurs during boot, the result is `BOOT_SWAP_TYPE_PANIC`. If the
result is either `BOOT_SWAP_TYPE_FAIL` or `BOOT_SWAP_TYPE_PANIC`, mcuboot hangs
rather than booting an invalid or compromised image.

Note: An important caveat to the above is the result when a swap is requested
      and the image in the secondary slot fails to validate, due to a hashing or
      signing error. This state behaves as State IV with the extra action of
      marking the image in the primary slot as "OK", to prevent further attempts
      to swap.

### Resumed swaps

If mcuboot determines that it is resuming an interrupted swap (i.e., a reset
occurred mid-swap), it fully determines the operation to resume by reading the
`swap info` field from the active trailer and extracting the swap type from bits
0-3. The set of tables in the previous section are not necessary in the resume
case.

## High-Level Operation

With the terms defined, we can now explore the boot loader's operation.  First,
a high-level overview of the boot process is presented.  Then, the following
sections describe each step of the process in more detail.

Procedure:

1. Inspect swap status region; is an interrupted swap being resumed?
    Yes: Complete the partial swap operation; skip to step 3.
    No: Proceed to step 2.

2. Inspect image trailers; is a swap requested?
    Yes.
    ​    1. Is the requested image valid (integrity and security check)?
    ​        Yes.
    ​            a. Perform swap operation.
    ​            b. Persist completion of swap procedure to image trailers.
    ​            c. Proceed to step 3.
    ​        No.
    ​            a. Erase invalid image.
    ​            b. Persist failure of swap procedure to image trailers.
    ​            c. Proceed to step 3.
    No: Proceed to step 3.

3. Boot into image in primary slot.

### Multiple Image Boot

When the flash contains multiple executable images the boot loader's operation
is a bit more complex but similar to the previously described procedure with
one image. Every image can be updated independently therefore the flash is
partitioned further to arrange two slots for each image.
```
+--------------------+
| MCUBoot            |
+--------------------+
        ~~~~~            <- memory might be not contiguous
+--------------------+
| Image 0            |
| primary   slot     |
+--------------------+
| Image 0            |
| secondary slot     |
+--------------------+
        ~~~~~            <- memory might be not contiguous
+--------------------+
| Image N            |
| primary   slot     |
+--------------------+
| Image N            |
| secondary slot     |
+--------------------+
| Scratch            |
+--------------------+
```
The multiple image boot procedure is organized in loops which iterate over all
the firmware images. The high-level overview of the boot process is presented
below.

Procedure:

+ ###### Loop 1. Iterate over all images
    1. Inspect swap status region of current image; is an interrupted swap being
       resumed?
        + Yes:
            + Review the validity of previously determined swap types
              of other images.
            + Complete the partial swap operation.
            + Mark the swap type as `None`.
            + Skip to next image.
        + No: Proceed to step 2.

    2. Inspect image trailers in the primary and secondary slot; is an image
       swap requested?
        + Yes: Review the validity of previously determined swap types of other
               images. Is the requested image valid (integrity and security
               check)?
            + Yes:
                + Set the previously determined swap type for the current image.
                + Skip to next image.
            + No:
                + Erase invalid image.
                + Persist failure of swap procedure to image trailers.
                + Mark the swap type as `Fail`.
                + Skip to next image.
        + No:
            + Mark the swap type as `None`.
            + Skip to next image.

+ ###### Loop 2. Iterate over all images
    At this point there are no aborted swaps and the swap types are determined
    for each image.

    1. Is an image swap requested?
        + Yes:
            + Perform image update operation.
            + Persist completion of swap procedure to image trailers.
            + Skip to next image.
        + No: Skip to next image.

+ ###### Loop 3. Iterate over all images

    1. Validate image in the primary slot (integrity and security check) or
       at least do a basic sanity check to avoid booting into an empty flash
       area.

+ Boot into image in the primary slot of the 0th image position\
  (other image in the boot chain is started by another image).

## Image Swapping

The boot loader swaps the contents of the two image slots for two reasons:
    * User has issued a "set pending" operation; the image in the secondary slot
      should be run once (state II) or repeatedly (state III), depending on
      whether a permanent swap was specified.
​          * Test image rebooted without being confirmed; the boot loader should
        revert to the original image currently in the secondary slot (state IV).

If the image trailers indicates that the image in the secondary slot should be
run, the boot loader needs to copy it to the primary slot.  The image currently
in the primary slot also needs to be retained in flash so that it can be used
later.  Furthermore, both images need to be recoverable if the boot loader
resets in the middle of the swap operation.  The two images are swapped
according to the following procedure:

<!-- Markdown doesn't do nested numbered lists.  It will do nested
bulletted lists, so maybe that is better. -->
​    1. Determine how many flash sectors each image slot consists of.  This
​       number must be the same for both slots.
​    2. Iterate the list of sector indices in descending order (i.e., starting
​       with the greatest index); current element = "index".
​        b. Erase scratch area.
​        c. Copy secondary_slot[index] to scratch area.
​            - If these are the last sectors (i.e., first swap being perfomed),
​              copy the full sector *except* the image trailer.
​            - Else, copy entire sector contents.
​        d. Write updated swap status (i).
​        e. Erase secondary_slot[index]
​        f. Copy primary_slot[index] to secondary_slot[index]
​            - If these are the last sectors (i.e., first swap being perfomed),
​              copy the full sector *except* the image trailer.
​            - Else, copy entire sector contents.
​        g. Write updated swap status (ii).
​        h. Erase primary_slot[index].
​        i. Copy scratch area to primary_slot[index].
​            - If these are the last sectors (i.e., first swap being perfomed),
​              copy the full sector *except* the image trailer.
​            - Else, copy entire sector contents.
​        j. Write updated swap status (iii).
​    3. Persist completion of swap procedure to the primary slot image trailer.

The additional caveats in step 2f are necessary so that the secondary slot image
trailer can be written by the user at a later time.  With the image trailer
unwritten, the user can test the image in the secondary slot
(i.e., transition to state II).

Note1: If the sector being copied is the last sector, then swap status is
temporarily maintained on scratch for the duration of this operation, always
using the primary slot's area otherwise.

Note2: The bootloader tries to copy only used sectors (based on largest image
installed on any of the slots), minimizing the amount of sectors copied and
reducing the amount of time required for a swap operation.

The particulars of step 3 vary depending on whether an image is being tested,
permanently used, reverted or a validation failure of the secondary slot
happened when a swap was requested:

    * test:
        o Write primary_slot.copy_done = 1
        (swap caused the following values to be written:
            primary_slot.magic = BOOT_MAGIC
            primary_slot.image_ok = Unset)

    * permanent:
        o Write primary_slot.copy_done = 1
        (swap caused the following values to be written:
            primary_slot.magic = BOOT_MAGIC
            primary_slot.image_ok = 0x01)

    * revert:
        o Write primary_slot.copy_done = 1
        o Write primary_slot.image_ok = 1
        (swap caused the following values to be written:
            primary_slot.magic = BOOT_MAGIC)

    * failure to validate the secondary slot:
        o Write primary_slot.image_ok = 1

After completing the operations as described above the image in the primary slot
should be booted.

## Swap Status

The swap status region allows the boot loader to recover in case it restarts in
the middle of an image swap operation.  The swap status region consists of a
series of single-byte records.  These records are written independently, and
therefore must be padded according to the minimum write size imposed by the
flash hardware.  In the below figure, a min-write-size of 1 is assumed for
simplicity.  The structure of the swap status region is illustrated below.  In
this figure, a min-write-size of 1 is assumed for simplicity.

```
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |sec127,state 0 |sec127,state 1 |sec127,state 2 |sec126,state 0 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |sec126,state 1 |sec126,state 2 |sec125,state 0 |sec125,state 1 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |sec125,state 2 |                                               |
    +-+-+-+-+-+-+-+-+                                               +
    ~                                                               ~
    ~               [Records for indices 124 through 1              ~
    ~                                                               ~
    ~               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~               |sec000,state 0 |sec000,state 1 |sec000,state 2 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
```

The above is probably not helpful at all; here is a description in English.

Each image slot is partitioned into a sequence of flash sectors.  If we were to
enumerate the sectors in a single slot, starting at 0, we would have a list of
sector indices.  Since there are two image slots, each sector index would
correspond to a pair of sectors.  For example, sector index 0 corresponds to
the first sector in the primary slot and the first sector in the secondary slot.
Finally, reverse the list of indices such that the list starts with index
`BOOT_MAX_IMG_SECTORS - 1` and ends with 0.  The swap status region is a
representation of this reversed list.

During a swap operation, each sector index transitions through four separate
states:
```
0. primary slot: image 0,   secondary slot: image 1,   scratch: N/A
1. primary slot: image 0,   secondary slot: N/A,       scratch: image 1 (1->s, erase 1)
2. primary slot: N/A,       secondary slot: image 0,   scratch: image 1 (0->1, erase 0)
3. primary slot: image 1,   secondary slot: image 0,   scratch: N/A     (s->0)
```

Each time a sector index transitions to a new state, the boot loader writes a
record to the swap status region.  Logically, the boot loader only needs one
record per sector index to keep track of the current swap state.  However, due
to limitations imposed by flash hardware, a record cannot be overwritten when
an index's state changes.  To solve this problem, the boot loader uses three
records per sector index rather than just one.

Each sector-state pair is represented as a set of three records.  The record
values map to the above four states as follows

```
            | rec0 | rec1 | rec2
    --------+------+------+------
    state 0 | 0xff | 0xff | 0xff
    state 1 | 0x01 | 0xff | 0xff
    state 2 | 0x01 | 0x02 | 0xff
    state 3 | 0x01 | 0x02 | 0x03
```

The swap status region can accommodate `BOOT_MAX_IMG_SECTORS` sector indices.
Hence, the size of the region, in bytes, is
`BOOT_MAX_IMG_SECTORS * min-write-size * 3`. The only requirement for the index
count is that it is great enough to account for a maximum-sized image
(i.e., at least as great as the total sector count in an image slot).  If a
device's image slots have been configured with `BOOT_MAX_IMG_SECTORS: 128` and
use less than 128 sectors, the first record that gets written will be somewhere
in the middle of the region. For example, if a slot uses 64 sectors, the first
sector index that gets swapped is 63, which corresponds to the exact halfway
point within the region.

Note: since the scratch area only ever needs to record swapping of the last
sector, it uses at most min-write-size * 3 bytes for its own status area.

## Reset Recovery

If the boot loader resets in the middle of a swap operation, the two images may
be discontiguous in flash.  Bootutil recovers from this condition by using the
image trailers to determine how the image parts are distributed in flash.

The first step is determine where the relevant swap status region is located.
Because this region is embedded within the image slots, its location in flash
changes during a swap operation.  The below set of tables map image trailers
contents to swap status location.  In these tables, the "source" field
indicates where the swap status region is located. In case of multi image boot
the images primary area and the single scratch area is always examined in pairs.
If swap status found on scratch area then it might not belong to the current
image. The swap_info field of swap status stores the corresponding image number.
If it does not match then "source: none" is returned.

```
              | primary slot | scratch      |
    ----------+--------------+--------------|
        magic | Good         | Any          |
    copy-done | 0x01         | N/A          |
    ----------+--------------+--------------'
    source: none                            |
    ----------------------------------------'

              | primary slot | scratch      |
    ----------+--------------+--------------|
        magic | Good         | Any          |
    copy-done | 0xff         | N/A          |
    ----------+--------------+--------------'
    source: primary slot                    |
    ----------------------------------------'

              | primary slot | scratch      |
    ----------+--------------+--------------|
        magic | Any          | Good         |
    copy-done | Any          | N/A          |
    ----------+--------------+--------------'
    source: scratch                         |
    ----------------------------------------'

              | primary slot | scratch      |
    ----------+--------------+--------------|
        magic | Unset        | Any          |
    copy-done | 0xff         | N/A          |
    ----------+--------------+--------------|
    source: primary slot                    |
    ----------------------------------------+------------------------------+
    This represents one of two cases:                                      |
    o No swaps ever (no status to read, so no harm in checking).           |
    o Mid-revert; status in the primary slot.                              |
    For this reason we assume the primary slot as source, to trigger a     |
    check of the status area and find out if there was swapping under way. |
    -----------------------------------------------------------------------'
```

If the swap status region indicates that the images are not contiguous, mcuboot
determines the type of swap operation that was interrupted by reading the `swap
info` field in the active image trailer and extarcting the swap type from bits
0-3 then resumes the operation. In other words, it applies the procedure defined
in the previous section, moving image 1 into the primary slot and image 0 into
the secondary slot. If the boot status indicates that an image part is present
in the scratch area, this part is copied into the correct location by starting
at step e or step h in the area-swap procedure, depending on whether the part
belongs to image 0 or image 1.

After the swap operation has been completed, the boot loader proceeds as though
it had just been started.

## Integrity Check

An image is checked for integrity immediately before it gets copied into the
primary slot.  If the boot loader doesn't perform an image swap, then it can
perform an optional integrity check of the image in the primary slot if
`MCUBOOT_VALIDATE_PRIMARY_SLOT` is set, otherwise it doesn't perform an
integrity check.

During the integrity check, the boot loader verifies the following aspects of
an image:
​    * 32-bit magic number must be correct (0x96f3b83d).
​    * Image must contain an `image_tlv_info` struct, identified by its magic
​      (0x6907) exactly following the firmware (hdr_size + img_size).
​    * Image must contain a SHA256 TLV.
​    * Calculated SHA256 must match SHA256 TLV contents.
​    * Image *may* contain a signature TLV.  If it does, it must also have a
​      KEYHASH TLV with the hash of the key that was used to sign. The list of
​      keys will then be iterated over looking for the matching key, which then
​      will then be used to verify the image contents.

## Security

As indicated above, the final step of the integrity check is signature
verification.  The boot loader can have one or more public keys embedded in it
at build time.  During signature verification, the boot loader verifies that an
image was signed with a private key that corresponds to the embedded keyhash
TLV.

For information on embedding public keys in the boot loader, as well as
producing signed images, see: [signed_images](signed_images.md).

If you want to enable and use encrypted images, see:
[encrypted_images](encrypted_images.md).