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MCUboot comprises two packages:

The bootutil library performs most of the functions of a bootloader. 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. Bootloader functionality is separated in this manner to enable unit testing of the bootloader. A library can be unit tested, but an application can’t. Therefore, functionality is delegated to the bootutil library when possible.


The bootloader currently only supports images with the following characteristics:

Image format

The following definitions describe the image format.

#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 ih_protect_tlv_size;   /* Size of protected TLV area (bytes). */
    uint32_t ih_img_size;           /* Does not include header. */
    uint32_t ih_flags;              /* IMAGE_F_[...]. */
    struct image_version ih_ver;
    uint32_t _pad1;

#define IMAGE_TLV_INFO_MAGIC        0x6907
#define IMAGE_TLV_PROT_INFO_MAGIC   0x6908

/** 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_ENCRYPTED_AES128         0x00000004 /* Encrypted using AES128. */
#define IMAGE_F_ENCRYPTED_AES256         0x00000008 /* Encrypted using AES256. */
#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 - Not supported anymore */
#define IMAGE_TLV_ECDSA_SIG         0x22   /* ECDSA of hash output */
#define IMAGE_TLV_RSA3072_PSS       0x23   /* RSA3072 of hash output */
#define IMAGE_TLV_ED25519           0x24   /* ED25519 of hash output */
#define IMAGE_TLV_ENC_RSA2048       0x30   /* Key encrypted with RSA-OAEP-2048 */
#define IMAGE_TLV_ENC_KW            0x31   /* Key encrypted with AES-KW-128 or
                                              256 */
#define IMAGE_TLV_ENC_EC256         0x32   /* Key encrypted with ECIES-P256 */
#define IMAGE_TLV_ENC_X25519        0x33   /* Key encrypted with ECIES-X25519 */
#define IMAGE_TLV_DEPENDENCY        0x40   /* Image depends on other image */
#define IMAGE_TLV_SEC_CNT           0x50   /* security counter */

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

The ih_protect_tlv_size field indicates the length of the protected TLV area. If protected TLVs are present then a TLV info header with magic equal to IMAGE_TLV_PROT_INFO_MAGIC must be present and the protected TLVs (plus the info header itself) have to be included in the hash calculation. Otherwise the hash is only calculated over the image header and the image itself. In this case the value of the ih_protect_tlv_size field is 0.

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 bootloader uses the following flash area IDs:

/* Independent from multiple image boot */
#define FLASH_AREA_BOOTLOADER         0
/* If the bootloader is working with the first image */
/* If the bootloader is working with the second image */

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. Normally, the bootloader 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 (the exception to this is the direct-xip and the ram-load upgrade mode). If the bootloader 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.

Swap using scratch

When swap-using-scratch algorithm is used, in addition to the slots of image areas, the bootloader 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 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.

swap-using scratch algorithm assumes that the primary and the secondary image slot areas sizes are equal. The maximum image size available for the application will be:

maximum-image-size = image-slot-size - image-trailer-size

Where: image-slot-size is the size of the image slot. image-trailer-size is the size of the image trailer.

Swap without using scratch

This algorithm is an alternative to the swap-using-scratch algorithm. It uses an additional sector in the primary slot to make swap possible. The algorithm works as follows:

  1. Moves all sectors of the primary slot up by one sector. Beginning from N=0:
  2. Copies the N-th sector from the secondary slot to the N-th sector of the primary slot.
  3. Copies the (N+1)-th sector from the primary slot to the N-th sector of the secondary slot.
  4. Repeats steps 2. and 3. until all the slots’ sectors are swapped.

This algorithm is designed so that the higher sector of the primary slot is used only for allowing sectors to move up. Therefore the most memory-size-effective slot layout is when the primary slot is exactly one sector larger than the secondary slot, although same-sized slots are allowed as well. The algorithm is limited to support sectors of the same sector layout. All slot’s sectors should be of the same size.

When using this algorithm the maximum image size available for the application will be:

maximum-image-size = (N-1) * slot-sector-size - image-trailer-sectors-size

Where: N is the number of sectors in the primary slot. image-trailer-sectors-size is the size of the image trailer rounded up to the total size of sectors its occupied. For instance if the image-trailer-size is equal to 1056 B and the sector size is equal to 1024 B, then image-trailer-sectors-size will be equal to 2048 B.

The algorithm does two erase cycles on the primary slot and one on the secondary slot during each swap. Assuming that receiving a new image by the DFU application requires 1 erase cycle on the secondary slot, this should result in leveling the flash wear between the slots.

The algorithm is enabled using the MCUBOOT_SWAP_USING_MOVE option.

Equal slots (direct-xip)

When the direct-xip mode is enabled the active image flag is “moved” between the slots during image upgrade and in contrast to the above, the bootloader can run an image directly from either the primary or the secondary slot (without having to move/copy it into the primary slot). Therefore the image update client, which downloads the new images must be aware, which slot contains the active image and which acts as a staging area and it is responsible for loading the proper images into the proper slot. All this requires that the images be built to be executed from the corresponding slot. At boot time the bootloader first looks for images in the slots and then inspects the version numbers in the image headers. It selects the newest image (with the highest version number) and then checks its validity (integrity check, signature verification etc.). If the image is invalid MCUboot erases its memory slot and starts to validate the other image. After a successful validation of the selected image the bootloader chain-loads it.

An additional “revert” mechanism is also supported. For more information, please read the corresponding section. Handling the primary and secondary slots as equals has its drawbacks. Since the images are not moved between the slots, the on-the-fly image encryption/decryption can’t be supported (it only applies to storing the image in an external flash on the device, the transport of encrypted image data is still feasible).

The overwrite and the direct-xip upgrade strategies are 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.

RAM loading

In ram-load mode the slots are equal. Like the direct-xip mode, this mode also selects the newest image by reading the image version numbers in the image headers. But instead of executing it in place, the newest image is copied to the RAM for execution. The load address, the location in RAM where the image is copied to, is stored in the image header. The ram-load upgrade mode can be useful when there is no internal flash in the SoC, but there is a big enough internal RAM to hold the images. Usually in this case the images are stored in an external storage device. Execution from external storage has some drawbacks (lower execution speed, image is exposed to attacks) therefore the image is always copied to the internal RAM before the authentication and execution. Ram-load mode requires the image to be built to be executed from the RAM address range instead of the storage device address range. If ram-load is enabled then platform must define the following parameters:

#define IMAGE_EXECUTABLE_RAM_START    <area_base_addr>
#define IMAGE_EXECUTABLE_RAM_SIZE     <area_size_in_bytes>

For multiple image load if multiple ram regions are used platform must define the MULTIPLE_EXECUTABLE_RAM_REGIONS flag instead and implement the following function:

int boot_get_image_exec_ram_info(uint32_t image_id,
                                 uint32_t *exec_ram_start,
                                 uint32_t *exec_ram_size)

When ram-load is enabled, the --load-addr <addr> option of the imgtool script must also be used when signing the images. This option set the RAM_LOAD flag in the image header which indicates that the image should be loaded to the RAM and also set the load address in the image header.

When the encryption option is enabled (MCUBOOT_ENC_IMAGES) along with ram-load the image is checked for encryption. If the image is not encrypted, RAM loading happens as described above. If the image is encrypted, it is copied in RAM at the provided address and then decrypted. Finally, the decrypted image is authenticated in RAM and executed.

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:

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.

Revert mechanism in direct-xip mode

The direct-xip mode also supports a “revert” mechanism which is the equivalent of the swap mode’s “revert” swap. When the direct-xip mode is selected it can be enabled with the MCUBOOT_DIRECT_XIP_REVERT config option and an image trailer must also be added to the signed images (the “–pad” option of the imgtool script must be used). For more information on this please read the Image Trailer section and the imgtool documentation. Making the images permanent (marking them as confirmed in advance) is also supported just like in swap mode. The individual steps of the direct-xip mode’s “revert” mechanism are the following:

  1. Select the slot which holds the newest potential image.
  2. Was the image previously selected to run (during a previous boot)?
    • Yes: Did the image mark itself “OK” (was the self-test successful)?
      • Yes.
        • Proceed to step 3.
      • No.
        • Erase the image from the slot to prevent it from being selected again during the next boot.
        • Return to step 1 (the bootloader will attempt to select and possibly boot the previous image if there is one).
    • No.
      • Mark the image as “selected” (set the copy_done flag in the trailer).
      • Proceed to step 3.
  3. Proceed to image validation …

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) [*]              |
    |                                                               |
    |                    0xff padding as needed                     |
    |  (BOOT_MAX_ALIGN minus 16 octets from Encryption key 0) [*]   |
    |                 Encryption key 1 (16 octets) [*]              |
    |                                                               |
    |                    0xff padding as needed                     |
    |  (BOOT_MAX_ALIGN minus 16 octets from Encryption key 1) [*]   |
    |                      Swap size (4 octets)                     |
    |                    0xff padding as needed                     |
    |        (BOOT_MAX_ALIGN minus 4 octets from Swap size)         |
    |   Swap info   |  0xff padding (BOOT_MAX_ALIGN minus 1 octet)  |
    |   Copy done   |  0xff padding (BOOT_MAX_ALIGN minus 1 octet)  |
    |   Image OK    |  0xff padding (BOOT_MAX_ALIGN minus 1 octet)  |
    |                    0xff padding as needed                     |
    |         (BOOT_MAX_ALIGN minus 16 octets from MAGIC)           |
    |                       MAGIC (16 octets)                       |
    |                                                               |

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


“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-size is due to the behavior of flash hardware. The factor of 3 is explained below.

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

  2. 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.

  3. Swap info: A single byte which encodes the following information:

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

  2. 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).

  3. MAGIC: A 16-byte field identifying the image trailer layout. It may assume distinct values depending on the maximum supported write alignment (BOOT_MAX_ALIGN) of the image, as defined by the following construct:

union boot_img_magic_t
    struct {
        uint16_t align;
        uint8_t magic[14];
    uint8_t val[16];

If BOOT_MAX_ALIGN is 8 bytes, then MAGIC contains the following 16 bytes:

const union boot_img_magic_t boot_img_magic = {
    .val = {
        0x77, 0xc2, 0x95, 0xf3,
        0x60, 0xd2, 0xef, 0x7f,
        0x35, 0x52, 0x50, 0x0f,
        0x2c, 0xb6, 0x79, 0x80

In case BOOT_MAX_ALIGN is defined to any value different than 8, then the maximum supported write alignment value is encoded in the MAGIC field, followed by a fixed 14-byte pattern:

const union boot_img_magic_t boot_img_magic = {
    .align = BOOT_MAX_ALIGN,
    .magic = {
        0x2d, 0xe1,
        0x5d, 0x29, 0x41, 0x0b,
        0x8d, 0x77, 0x67, 0x9c,
        0x11, 0x0f, 0x1f, 0x8a

Note Be aware that the image trailers make the ending area of the image slot unavailable for carrying the image data. In particular, the swap status size could be huge. For example, for 128 slot sectors with a 4-byte alignment, it would become 1536 B.

Image trailers

At startup, the bootloader 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.


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.


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 bootloader’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.


  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 bootloader’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            |

MCUboot is also capable of handling dependencies between images. For example if an image needs to be reverted it might be necessary to revert another one too (e.g. due to API incompatibilities) or simply to prevent from being updated because of an unsatisfied dependency. Therefore all aborted swaps have to be completed and all the swap types have to be determined for each image before the dependency checks. Dependency handling is described in more detail in a following section. 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.

Multiple image boot for RAM loading and direct-xip

The operation of the bootloader is different when the ram-load or the direct-xip strategy is chosen. The flash map is very similar to the swap strategy but there is no need for Scratch area.

Image swapping

The bootloader swaps the contents of the two image slots for two reasons:

If the image trailers indicates that the image in the secondary slot should be run, the bootloader 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 bootloader resets in the middle of the swap operation. The two images are swapped according to the following procedure:

  1. Determine if both slots are compatible enough to have their images swapped. To be compatible, both have to have only sectors that can fit into the scratch area and if one of them has larger sectors than the other, it must be able to entirely fit some rounded number of sectors from the other slot. In the next steps we’ll use the terminology “region” for the total amount of data copied/erased because this can be any amount of sectors depending on how many the scratch is able to fit for some swap operation.
  2. Iterate the list of region indices in descending order (i.e., starting with the greatest index); only regions that are predetermined to be part of the image are copied; current element = “index”.
    • a. Erase scratch area.
    • b. Copy secondary_slot[index] to scratch area.
      • If this is the last region in the slot, scratch area has a temporary status area initialized to store the initial state, because the primary slot’s last region will have to be erased. In this case, only the data that was calculated to amount to the image is copied.
      • Else if this is the first swapped region but not the last region in the slot, initialize the status area in primary slot and copy the full region contents.
      • Else, copy entire region contents.
    • c. Write updated swap status (i).
    • d. Erase secondary_slot[index]
    • e. Copy primary_slot[index] to secondary_slot[index] according to amount previosly copied at step b.
      • If this is not the last region in the slot, erase the trailer in the secondary slot, to always use the one in the primary slot.
    • f. Write updated swap status (ii).
    • g. Erase primary_slot[index].
    • h. Copy scratch area to primary_slot[index] according to amount previously copied at step b.
      • If this is the last region in the slot, the status is read from scratch (where it was stored temporarily) and written anew in the primary slot.
    • i. 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 I).


If the region being copied contains 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.


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
        secondary_slot.magic = UNSET
        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
        secondary_slot.magic = UNSET
        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 bootloader 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 bootloader writes a record to the swap status region. Logically, the bootloader 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 bootloader 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.


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 bootloader 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 extracting 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 bootloader 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 bootloader 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 bootloader verifies the following aspects of an image:

For low performance MCU’s where the validation is a heavy process at boot (~1-2 seconds on a arm-cortex-M0), the MCUBOOT_VALIDATE_PRIMARY_SLOT_ONCE could be used. This option will cache the validation result as described above into the magic area of the primary slot. The next boot, the validation will be skipped if the previous validation was succesfull. This option is reducing the security level since if an attacker could modify the contents of the flash after a good image has been validated, the attacker could run his own image without running validation again. Enabling this option should be done with care.


As indicated above, the final step of the integrity check is signature verification. The bootloader can have one or more public keys embedded in it at build time. During signature verification, the bootloader 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 bootloader, as well as producing signed images, see: signed_images.

If you want to enable and use encrypted images, see: encrypted_images.


Image encryption is not supported when the direct-xip upgrade strategy is selected.

Using hardware keys for verification

By default, the whole public key is embedded in the bootloader code and its hash is added to the image manifest as a KEYHASH TLV entry. As an alternative the bootloader can be made independent of the keys (avoiding the incorporation of the public key into the code) by using one of the following options: MCUBOOT_HW_KEY or MCUBOOT_BUILTIN_KEY.

Using any of these options makes MCUboot independent from the public key(s). The key(s) can be provisioned any time and by different parties.

Hardware KEYs support options details:

Protected TLVs

If the TLV area contains protected TLV entries, by beginning with a struct image_tlv_info with a magic value of IMAGE_TLV_PROT_INFO_MAGIC then the data of those TLVs must also be integrity and authenticity protected. Beyond the full size of the protected TLVs being stored in the image_tlv_info, the size of the protected TLVs together with the size of the image_tlv_info struct itself are also saved in the ih_protected_size field inside the header.

Whenever an image has protected TLVs the SHA256 has to be calculated over not just the image header and the image but also the TLV info header and the protected TLVs.

A +---------------------+
  | Header              | <- struct image_header
  | Payload             |
  | TLV area            |
  | +-----------------+ |    struct image_tlv_info with
  | | TLV area header | | <- IMAGE_TLV_PROT_INFO_MAGIC (optional)
  | +-----------------+ |
  | | Protected TLVs  | | <- Protected TLVs (struct image_tlv)
B | +-----------------+ |
  | | TLV area header | | <- struct image_tlv_info with IMAGE_TLV_INFO_MAGIC
C | +-----------------+ |
  | | SHA256 hash     | | <- hash from A - B (struct image_tlv)
D | +-----------------+ |
  | | Keyhash         | | <- indicates which pub. key for sig (struct image_tlv)
  | +-----------------+ |
  | | Signature       | | <- signature from C - D (struct image_tlv), only hash
  | +-----------------+ |

Dependency check

MCUboot can handle multiple firmware images. It is possible to update them independently but in many cases it can be desired to be able to describe dependencies between the images (e.g. to ensure API compliance and avoid interoperability issues).

The dependencies between images can be described with additional TLV entries in the protected TLV area after the end of an image. There can be more than one dependency entry, but in practice if the platform only supports two individual images then there can be maximum one entry which reflects to the other image.

At the phase of dependency check all aborted swaps are finalized if there were any. During the dependency check the bootloader verifies whether the image dependencies are all satisfied. If at least one of the dependencies of an image is not fulfilled then the swap type of that image has to be modified accordingly and the dependency check needs to be restarted. This way the number of unsatisfied dependencies will decrease or remain the same. There is always at least 1 valid configuration. In worst case, the system returns to the initial state after dependency check.

For more information on adding dependency entries to an image, see: imgtool.

Downgrade prevention

Downgrade prevention is a feature which enforces that the new image must have a higher version/security counter number than the image it is replacing, thus preventing the malicious downgrading of the device to an older and possibly vulnerable version of its firmware.

Software-based downgrade prevention

During the software based downgrade prevention the image version numbers are compared. This feature is enabled with the MCUBOOT_DOWNGRADE_PREVENTION option. In this case downgrade prevention is only available when the overwrite-based image update strategy is used (i.e. MCUBOOT_OVERWRITE_ONLY is set).

Hardware-based downgrade prevention

Each signed image can contain a security counter in its protected TLV area, which can be added to the image using the -s option of the imgtool script. During the hardware based downgrade prevention (alias rollback protection) the new image’s security counter will be compared with the currently active security counter value which must be stored in a non-volatile and trusted component of the device. It is beneficial to handle this counter independently from image version number:

It is an optional step of the image validation process and can be enabled with the MCUBOOT_HW_ROLLBACK_PROT config option. When enabled, the target must provide an implementation of the security counter interface defined in boot/bootutil/include/security_cnt.h.

Measured boot and data sharing

MCUboot defines a mechanism for sharing boot status information (also known as measured boot) and an interface for sharing application specific information with the runtime software. If any of these are enabled the target must provide a shared data area between the bootloader and runtime firmware and define the following parameters:

#define MCUBOOT_SHARED_DATA_BASE    <area_base_addr>
#define MCUBOOT_SHARED_DATA_SIZE    <area_size_in_bytes>

In the shared memory area all data entries are stored in a type-length-value (TLV) format. Before adding the first data entry, the whole area is overwritten with zeros and a TLV header is added at the beginning of the area during an initialization phase. This TLV header contains a tlv_magic field with a value of SHARED_DATA_TLV_INFO_MAGIC and a tlv_tot_len field which is indicating the total length of shared TLV area including this header. The header is followed by the the data TLV entries which are composed from a shared_data_tlv_entry header and the data itself. In the data header there is a tlv_type field which identifies the consumer of the entry (in the runtime software) and specifies the subtype of that data item. More information about the tlv_type field and data types can be found in the boot/bootutil/include/bootutil/boot_status.h file. The type is followed by a tlv_len field which indicates the size of the data entry in bytes, not including the entry header. After this header structure comes the actual data.

/** Shared data TLV header.  All fields in little endian. */
struct shared_data_tlv_header {
    uint16_t tlv_magic;
    uint16_t tlv_tot_len; /* size of whole TLV area (including this header) */

/** Shared data TLV entry header format. All fields in little endian. */
struct shared_data_tlv_entry {
    uint16_t tlv_type;
    uint16_t tlv_len; /* TLV data length (not including this header). */

The measured boot can be enabled with the MCUBOOT_MEASURED_BOOT config option. When enabled, the --boot_record argument of the imgtool script must also be used during the image signing process to add a BOOT_RECORD TLV to the image manifest. This TLV contains the following attributes/measurements of the image in CBOR encoded format:

The sw_type string that is passed as the --boot_record option’s parameter will be the value of the “Software type” attribute in the generated BOOT_RECORD TLV. The target must also define the MAX_BOOT_RECORD_SZ macro which indicates the maximum size of the CBOR encoded boot record in bytes. During boot, MCUboot will look for these TLVs (in case of multiple images) in the manifests of the active images (the latest and validated) and copy the CBOR encoded binary data to the shared data area. Preserving all these image attributes from the boot stage for use by later runtime services (such as an attestation service) is known as a measured boot.

Setting the MCUBOOT_DATA_SHARING option enables the sharing of application specific data using the same shared data area as for the measured boot. For this, the target must provide a definition for the boot_save_shared_data() function which is declared in boot/bootutil/include/bootutil/boot_record.h. The boot_add_data_to_shared_area() function can be used for adding new TLV entries to the shared data area. Alternatively, setting the MCUBOOT_DATA_SHARING_BOOTINFO option will provide a default function for this which saves information such as the maximum application size, bootloader version (if available), running slot number, if recovery is part of MCUboot and the signature type. Details of the TLVs for this information can be found in boot/bootutil/include/bootutil/boot_status.h with BLINFO_ prefixes.

Testing in CI

Testing Fault Injection Hardening (FIH)

The CI currently tests the Fault Injection Hardening feature of MCUboot by executing instruction skip during execution, and looking at whether a corrupted image was booted by the bootloader or not.

The main idea is that instruction skipping can be automated by scripting a debugger to automatically execute the following steps:

Whether or not the corrupted image was booted or not can be decided by looking for certain entries in the log.

As MCUboot is deployed on a microcontroller, testing FI would not make much sense in the simulator environment running on a host machine with different architecture than the MCU’s, as the degree of hardening depends on compiler behavior. For example, (a bit counterintuitively) the code produced by gcc with -O0 optimisation is more resilient against FI attacks than the code generated with -O3 or -Os optimizations.

To run on a desired architecture in the CI, the tests need to be executed on an emulator (as real devices are not available in the CI environment). For this implementation QEMU is selected.

For the tests MCUboot needs a set of drivers and an implementation of a main function. For the purpose of this test Trusted-Firmware-M has been selected as it supports Armv8-M platforms that are also emulated by QEMU.

The tests run in a docker container inside the CI VMs, to make it more easy to deploy build and test environment (QEMU, compilers, interpreters). The CI VMs seems to be using quite old Ubuntu (16.04).

The sequence of the testing is the following (pseudo code):

fn main()
  # Implemented in ci/

  # See details below. Implemented in ci/
  # Calling the function with different parameters is done by Travis CI based on
  # the values provided in the .travis.yaml
  start_docker_image(skip_sizes, build_type, damage_type, fih_level)

fn start_docker_image(skip_sizes, build_type, damage_type, fih_level)
  # implemented in ci/fih_test_docker/

  # implemented in ci/fih_test_docker/

  # implemented in ci/fih_test_docker/
  ranges = generate_address_ranges()
  for s in skip_sizes
    for r in ranges
      do_skip_in_qemu(s, r) # See details below

fn do_skip_in_qemu(size, range)
  for a in r
    run_qemu(a, size)  # See details below

# this part is implemented in ci/fih_test_docker/
fn run_qemu(a, size)
  script = create_debugger_script(a, size)
  start_qemu_in_bacground() # logs serial out to a file

  # This checks the debugger and the quemu logs, and decides whether the tets
  # was executed successfully, and whether the image is booted or not. Then
  # emits a yaml fragment on the standard out to be processed by the caller
  # script

Further notes:

An advantage of having the tests running in a docker image is that it is possible to run the tests on a local machine that has git and docker, without installing any additional software.

So, running the test on the host looks like the following (The commands below are issued from the MCUboot source directory):

$ mkdir docker
$ ./ci/

On the travis CI the environment variables in the last command are set based on the configs provided in the .travis.yaml

This starts the tests, however the shell that it is running in is not interactive, it is not possible to examine the results of the test run. To have an interactive shell where the results can be examined, the following can be done: