Introduction

Punchboot is a secure and fast bootloader for embedded systems. It is designed to:

Boot as fast as possible
Integrate with the SoC's secure boot functionality
Authenticate the next piece of software in the boot chain
Support A/B system partitions for atomic updates
Support automatic rollbacks
Minimize software download time in production
Be useful for day-to-day development

Punchboot is designed for embedded systems and therefore it has a minimalistic apporach. There is no run-time configuration, everything is configured in the board files.

Punchboot could be useful if you care about the following:

Boot speed
Secure boot
Downloading software quickly in production

Design

Punchboot is written in C and some assembler. Currently armv7a and armv8 is supported.

The directory layout is as follows:

Folder	Description
/doc	Documentation
/pki	Crypto keys for testing
/	Bootloader source
/src/board	Board support
/src/arch	Architecture support
/src/plat	Platform support
/src/drivers	Drivers
/tools	Tools

Supported architectures:

Architecture	Supported
armv7a	Yes
armv8a	Yes
armv7m	Yes

Supported platforms:

Platform	Supported	USB	EMMC	HW Crypto	Secure Boot	Fusebox
NXP imx6ul	Yes	Yes	Yes	Yes	Yes	Yes
NXP imx8m	Yes	Yes	Yes	Yes	No	Yes
NXP imx8x	Yes	Yes	Yes	Yes	Yes	Yes

Supported boards:

Board	Supported	More info
Jiffy	Fully supported	https://github.com/jonasblixt/jiffy
Bebop	Fully supported	https://github.com/jonasblixt/bebop
Technexion PICO-IMX8M	Partial support	https://www.technexion.com/products/system-on-modules/pico/pico-compute-modules/detail/PICO-IMX8M
NXP IMX8QXP MEK	Fully supported	https://www.nxp.com/products/processors-and-microcontrollers/arm-based-processors-and-mcus/i.mx-applications-processors/i.mx-8-processors/i.mx-8-multisensory-enablement-kit:i.MX8-MEK

Hardware accelerated signature verification

Platform	RSA4096	EC secp256r1	EC secp384r1	EC secp521
NXP imx6ul	Yes	Yes	No	No
NXP imx8m	Yes	Yes	No	No
nxp imx8x	yes	yes	yes	yes
nxp imx RT	no	no	no	no

Hardware accelerated hash algorithms

Platform	MD5	SHA256	SHA384	SHA512
NXP imx6ul	Yes	Yes	No	No
NXP imx8m	Yes	Yes	No	No
NXP imx8x	Yes	Yes	Yes	Yes
NXP imx RT	No	No	No	No

Secure Boot

Typical and simplified secure boot flow

ROM loads a set of public keys, calculates the checksum of the keys and compares the result to a fused checksum
ROM loads punchboot, calculates checksum and verifies signature using key's in step one
Run punchboot
Punchboot loads a PBI bundle, calculates the checksum and verifies the signature using built in keys
Run next step in boot chain

Most SoC:s have a boot rom that includes meachanisms for calculating a checksum of the bootloader and cryptographically verifying a signature using a public key fused to the device.

Normally fuses are a limited resource and therefor a common way is to calculate a sha256 checksum of the public key(s) and then store this checksum in fuses, this way many different public keys can be stored in a flash memory and every time the device boots it will compute a sha256 checksum and compare it to the fused checksum.

Punchboot is designed to be a part of a secure boot chain. This means that the bootloader is cryptographically signed, the ROM code of the SoC must support a mechanism to validate this signature, otherwise there is no root of trust.

When punchboot has been verified it, in turn, will load and verify the next software component in the boot chain. The bootloader only supports signed binaries.

Testing and integration tests

Punchboot uses QEMU for all module and integration tests. The 'test' platform and board target relies on virtio serial ports and block devices. The punchboot cli can be built with a domain socket transport instead of USB for communicating with an QEMU environment.

The test platform code includes gcov code that calls the QEMU semihosting API for storing test coverage data on the host.

Building and running tests:

$ cp configs/test_defconfig .config
$ make
$ make check

Device identity

Most modern SoC's provide some kind of unique identity, that is guaranteed to be unique for that particular type of SoC / Vendor etc but can not be guarateed to be globally unique.

Punchboot provides a UUID3 device identity based on a combination of the unique data from the SoC and an allocated, random, namspace UUID per platform.

When booting a linux system this information is relayed to linux through in-line patching of the device-tree. The device identity can be found in '/proc/device-tree/chosen/device-uuid'

Command mode

Command mode is entered when the system can't boot or if the bootloader is forced by a configurable, external event to do so.

In the command mode it is possible to update the bootloader, write data to partitions and install default settings. From v0.3 and forward an 'authentication cookie' must be used to interact with the bootloader to prevent malicious activity. The only command that can be executed without authentication is listing the device information (including the device UUID)

The authentication cookie consists of the device UUID encrypted with one of the active key pair's private key.

punchboot tool

The punchboot CLI is used for interacting with the command mode. A summary of the features available:

Update the bootloader it self
Manually start system A or B
Activate boot partitions
Load image to ram and execute it
Display basic device info
Configure fuses and GPT parition tables
Call board specific functions

The tool is written in Python and some parts in C to allow bindings for other languages. The tool is built on top of a library to make it possible to integrate with other tooling and environments.

The tool is distributed through PyPi. Binary wheels are available for Windows, Linux and macos (x86_64 and arm64).

Install using pypi:

$ pip install punchboot

Image format

Punchboot uses the bitpacker file format (https://github.com/jonasblixt/bpak)

Authentication token

Punchboot enforces authentication when the SLC (Security Life Cycle) is locked. To interact with Punchboot the session must be authenticated by using a password or a signed token.

The authentication token is generated by hashing the device UUID and signing it with one of the active key pairs.

Example:

$ punchboot dev show
Bootloader version: v0.6.1-40-ga47f-dirty
Device UUID:        0b177094-6b62-3572-902e-c1de339ecb01
Board name:         pico8ml

Creating the authentication token using the 'createtoken.sh' script located in the tools folder. In this example the private key is stored on a yubikey 5 HSM.

$ ./createtoken.sh 0B177094-6B62-3572-902E-C1DE339ECB01 pkcs11 -sha256 "pkcs11:id=%02;type=private"
engine "pkcs11" set.
Enter PKCS#11 token PIN for PIV Card Holder pin (PIV_II):
Enter PKCS#11 key PIN for SIGN key:

Authenticating the session:

$ punchboot auth token ./0B177094-6B62-3572-902E-C1DE339ECB01.token <name of a key>
Signature format: secp256r1
Hash: sha256
Authenticating using key index 0 and './0B177094-6B62-3572-902E-C1DE339ECB01.token'
Read 103 bytes
Authentication successful

Now the command mode is fully unlocked. The token is of course only valid for the individual unit with that perticular UUID.

Metrics

Measurements taken on IMX6UL, running at 528 MHz loading a 400kByte binary.

Using hardware accelerators for SHA and RSA signatures:

Parameter	Value	Unit
Power On Reset	28	ms
Bootloader init	7	ms
Blockdev read	13	ms
SHA256 Hash	4	ms
RSA 4096 Signaure	5	ms

Total	57	ms

Using libtomcrypt for SHA and RSA:

Parameter	Value	Unit
Power On Reset	28	ms
Bootloader init	7	ms
Blockdev read	13	ms
SHA256 Hash	431	ms
RSA 4096 Signaure	567	ms

Total	1046	ms

Measurements taken on IMX8QXP, loading a 14296kByte binary.