Secure Boot

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Important

All references in this document are related to Secure Boot V1, i.e., the AES-based Secure Boot Scheme. For ESP32 v3.0 onwards, the preferred secure boot scheme is Secure Boot V2.

Please refer to Secure Boot V2 for ESP32 v3.0 or ESP32-S2.

Secure boot is a feature for ensuring only your code can run on the chip. Data loaded from flash is verified on each reset.

Secure boot is separate from the Flash Encryption feature, and you can use secure boot without encrypting the flash contents. However, for a secure environment, both should be used simultaneously. See Secure Boot & Flash Encryption for more details.

Important

Enabling secure boot limits your options for further updates of your ESP32. Make sure to read this document thoroughly and understand the implications of enabling secure boot.

Background

  • Most data is stored in flash. Flash access does not need to be protected from physical access for secure boot to function, because critical data is stored in eFuses internal to the chip and is non-software-accessible.

  • eFuses are used to store the secure boot information permanently. The secure boot key is stored in eFuse BLOCK2, and also, a single eFuse bit ABS_DONE_0 is burned, or written to 1, to permanently enable secure boot on the chip. For more details on eFuses, see ESP32 Technical Reference Manual > eFuse Controller (EFUSE) [PDF].

  • To understand the secure boot process, please familiarize yourself with the standard Application Startup Flow.

  • Both stages of the boot process, including initial software bootloader load and subsequent partition & app loading, are verified by the secure boot process, in a chain of trust relationship.

Secure Boot Process Overview

This is a high-level overview of the secure boot process. Step-by-step instructions are supplied under How to Enable Secure Boot. Further in-depth details are supplied under Technical Details:

  1. The options to enable secure boot are provided in the Project Configuration Menu, under Secure Boot Configuration.

  2. Secure boot defaults to signing images and partition table data during the build process. The Secure boot private signing key config item is a file path to an ECDSA public/private key pair in a PEM format file.

  3. The software bootloader image is built by ESP-IDF with secure boot support enabled, and the public key for signature verification is integrated into the bootloader image. This software bootloader image is flashed at offset 0x1000.

  4. On the first boot, the software bootloader follows the following process to enable a secure boot:

    • Hardware secure boot support generates a device-secure bootloader key and a secure digest. The secure bootloader key is generated with the help of the hardware RNG, and then stored in eFuse with read and write protection enabled. The digest is derived from the key, an initialization vector (IV), and the bootloader image contents.

    • The secure digest is flashed at offset 0x0 in the flash.

    • Depending on Secure Boot Configuration, eFuses are burned to disable JTAG and the ROM BASIC interpreter. It is strongly recommended that these options are turned on.

    • Bootloader permanently enables secure boot by burning the ABS_DONE_0 eFuse. The software bootloader then becomes protected. After this point, the chip will only boot a bootloader image if the digest matches.

  5. On subsequent boots, the ROM bootloader sees that the secure boot eFuse is burned, reads the saved digest at 0x0, and uses hardware secure boot support to compare it with a newly calculated digest. If the digest does not match then booting will not continue. The digest and comparison are performed entirely by hardware, and the calculated digest is not readable by software. For technical details see Secure Boot Hardware Support.

  6. When running in secure boot mode, the software bootloader uses the secure boot signing key, the public key of which is embedded in the bootloader itself and therefore validated as part of the bootloader, to verify the signature appended to all subsequent partition tables and app images before they are booted.

Keys

The following keys are used by the secure boot process:

  • The secure bootloader key is a 256-bit AES key that is stored in eFuse block 2. The bootloader can generate this key itself from the internal hardware random number generator, and you do not need to supply it. It is optionally possible to supply this key, see Reflashable Software Bootloader. The eFuse holding this key is read and write protected before the secure boot is enabled to prevent further software access of the key.

    • By default, the eFuse Block 2 Coding Scheme is None and a 256-bit key is stored in this block. On some versions of ESP32s, the Coding Scheme is set to 3/4 Encoding with CODING_SCHEME eFuse has value 1, and a 192-bit key must be stored in this block.

    For more details, see ESP32 Technical Reference Manual > eFuse Controller (EFUSE) > System Parameter coding_scheme [PDF].

    The algorithm operates on a 256-bit key in all cases. 192-bit keys are extended by repeating some bits, see Secure Bootloader Digest Algorithm.

  • secure boot signing key is a standard ECDSA public/private key pair in PEM format, see Image Signing Algorithm.

    • The public key from this key pair is compiled into the software bootloader. It is only used for signature verification purposes and not signature creation. This public key is used to verify the second stage of booting, including the partition table and app image, before booting continues. The public key can be freely distributed; it does not need to be kept secret.

    • The private key from this key pair must be securely kept private, as anyone who has this key can authenticate to any bootloader that is configured with a secure boot and the matching public key.

Bootloader Size

Enabling secure boot and/or flash encryption will increase the size of the bootloader, which might require updating the partition table offset. See Bootloader Size.

How to Enable Secure Boot

  1. Open the Project Configuration Menu, navigate to Secure Boot Configuration and select the option One-time Flash. To understand the alternative Reflashable option, see Reflashable Software Bootloader.

  2. Select a name for the secure boot signing key. This option will appear after secure boot is enabled, and the key file can be anywhere on your system. If you opt for a relative path, it will be evaluated based on the project directory. Importantly, there's no need for the file to exist when making this selection.

  3. Set other menuconfig options as desired. Pay particular attention to the Bootloader Config options, as you can only flash the bootloader once. Then exit menuconfig and save your configuration.

  4. When you run idf.py build for the first time, if the signing key is not found, then an error message will be printed with a command to generate a signing key via espsecure.py generate_signing_key.

Important

A signing key generated this way will use the best random number source available to the OS and its Python installation, i.e., /dev/urandom on OSX/Linux and CryptGenRandom() on Windows. If this random number source is weak, then the private key will be weak.

Important

For production environments, we recommend generating the key pair using OpenSSL or another industry-standard encryption program. See Generating Secure Boot Signing Key for more details.

  1. Run idf.py bootloader to build a secure boot-enabled bootloader. The build output will include a prompt for a flashing command, using esptool.py write_flash.

  1. When you are ready to flash the bootloader, run the specified command. You have to enter it yourself, this step is not performed by make. And then wait for flashing to complete. Remember this is a one-time flash, you can not change the bootloader after this!

  2. Run idf.py flash to build and flash the partition table and the just-built app image. The app image will be signed using the signing key you generated in step 4.

Note

idf.py flash does not flash the bootloader if secure boot is enabled.

  1. Reset the ESP32 and it will boot the software bootloader you flashed. The software bootloader will enable secure boot on the chip, and then it verifies the app image signature and boots the app. You should watch the serial console output from the ESP32 to verify that secure boot is enabled and no errors have occurred due to the build configuration.

Note

The secure boot will not be enabled until after a valid partition table and app image have been flashed. This is to prevent accidents before the system is fully configured.

Note

If ESP32 is reset or powered down during the first boot, it will start the process again on the next boot.

  1. On subsequent boots, the secure boot hardware will verify the software bootloader has not changed using the secure bootloader key, and then the software bootloader will verify the signed partition table and app image using the public key portion of the secure boot signing key.

Reflashable Software Bootloader

Configuration Secure Boot: One-Time Flash is the recommended configuration for production devices. In this mode, each device gets a unique key that is never stored outside the device.

However, an alternative mode CONFIG_SECURE_BOOTLOADER_MODE is also available. This mode allows you to supply a binary key file that is used for the secure bootloader key. As you have the key file, you can generate new bootloader images and secure boot digests for them.

In the ESP-IDF build process, this 256-bit key file is derived from the ECDSA app signing key generated by the user, see the Generating Secure Boot Signing Key step below. This private key's SHA-256 digest is used as the secure bootloader key in eFuse, as-is for Coding Scheme None, or truncated to 192 bytes for 3/4 Encoding. This is a convenience so you only need to generate or protect a single private key.

Note

Although it is possible, we strongly recommend not generating one secure boot key and flashing it to every device in a production environment. The One-Time Flash option is recommended for production environments.

To enable a reflashable bootloader:

  1. In the Project Configuration Menu, select Bootloader Config > CONFIG_SECURE_BOOT > CONFIG_SECURE_BOOT_V1_ENABLED > CONFIG_SECURE_BOOTLOADER_MODE > Reflashable.

  2. If necessary, set the CONFIG_SECURE_BOOTLOADER_KEY_ENCODING based on the coding scheme used by the device. The coding scheme is shown in the Features line when esptool.py connects to the chip, or in the espefuse.py summary output.

  3. Please follow the steps shown in Generating Secure Boot Signing Key to generate the signing key. The path of the generated key file must be specified in the Secure Boot Configuration menu.

  4. Run idf.py bootloader. A binary key file will be created, derived from the private key that is used for signing. Two sets of flashing steps will be printed. The first set of steps includes an espefuse.py burn_key secure_boot_v1 path_to/secure-bootloader-key-xxx.bin command which is used to write the bootloader key to eFuse. Flashing this key is a one-time-only process. The second set of steps can be used to reflash the bootloader with a pre-calculated digest, which is generated during the build process.

  5. Resume from Step 6 of the one-time flashing process, to flash the bootloader and enable secure boot. Watch the console log output closely to ensure there were no errors in the secure boot configuration.

Generating Secure Boot Signing Key

The build system will prompt you with a command to generate a new signing key via espsecure.py generate_signing_key. This uses the python-ecdsa library, which in turn uses Python's os.urandom() as a random number source.

The strength of the signing key is proportional to the random number source of the system, and the correctness of the algorithm used. For production devices, we recommend generating signing keys from a system with a quality entropy source and using the best available EC key generation utilities.

For example, to generate a signing key using the OpenSSL command line:

openssl ecparam -name prime256v1 -genkey -noout -out my_secure_boot_signing_key.pem

Remember that the strength of the secure boot system depends on keeping the signing key private.

Remote Signing of Images

For production builds, it can be good practice to use a remote signing server rather than have the signing key on the build machine, which is the default ESP-IDF secure boot configuration. The espsecure.py command line program can be used to sign app images & partition table data for secure boot, on a remote system.

To use remote signing, disable the option Sign binaries during build. The private signing key does not need to be present on the build system. However, the public signature verification key is required because it is compiled into the bootloader, and can be used to verify image signatures during OTA updates.

To extract the public key from the private key:

espsecure.py extract_public_key --keyfile PRIVATE_SIGNING_KEY PUBLIC_VERIFICATION_KEY

The path to the public signature verification key needs to be specified in the menuconfig under Secure boot public signature verification key in order to build the secure bootloader.

After the app image and partition table are built, the build system will print signing steps using espsecure.py:

espsecure.py sign_data --keyfile PRIVATE_SIGNING_KEY BINARY_FILE

The above command appends the image signature to the existing binary. You can use the --output argument to write the signed binary to a separate file:

espsecure.py sign_data --keyfile PRIVATE_SIGNING_KEY --output SIGNED_BINARY_FILE BINARY_FILE

Secure Boot Best Practices

  • Generate the signing key on a system with a quality source of entropy.

  • Keep the signing key private at all times. A leak of this key will compromise the secure boot system.

  • Do not allow any third party to observe any aspects of the key generation or signing process using espsecure.py. Both processes are vulnerable to timing or other side-channel attacks.

  • Enable all secure boot options in Secure Boot Configuration. These include flash encryption, disabling of JTAG, disabling BASIC ROM interpreter, and disabling the UART bootloader encrypted flash access.

  • Use secure boot in combination with Flash Encryption to prevent local readout of the flash contents.

Technical Details

The following sections contain low-level reference descriptions of various secure boot elements:

Secure Boot Hardware Support

The first stage of secure boot verification, i.e., checking the software bootloader, is done via hardware. The ESP32's secure boot support hardware can perform three basic operations:

  1. Generate a random sequence of bytes from a hardware random number generator.

  2. Generate a digest from data, usually the bootloader image from flash, using a key stored in eFuse block 2. The key in eFuse can and should be read/write protected, which prevents software access. For full details of this algorithm see Secure Bootloader Digest Algorithm. The digest can only be read back by software if eFuse ABS_DONE_0 is not burned, i.e., still 0.

  3. Generate a digest from data, usually the bootloader image from flash, using the same algorithm as step 2 and compare it to a pre-calculated digest supplied in a buffer, usually read from flash offset 0x0. The hardware returns a true/false comparison without making the digest available to the software. This function is available even when eFuse ABS_DONE_0 is burned.

Secure Bootloader Digest Algorithm

Starting with an "image" of binary data as input, this algorithm generates a digest as output. The digest is sometimes referred to as an "abstract" in hardware documentation.

For a Python version of this algorithm, see the espsecure.py tool in the /esptool_py directory. Specifically, the digest_secure_bootloader command.

Items marked with (^) are to fulfill hardware restrictions, as opposed to cryptographic restrictions.

  1. Read the AES key from eFuse block 2, in reversed byte order. If the Coding Scheme is set to 3/4 Encoding, extend the 192-bit key to 256 bits using the same algorithm described in Flash Encryption Algorithm.

  2. Prefix the image with a 128-byte randomly generated IV.

  3. If the image length is not modulo 128, pad the image to a 128-byte boundary with 0xFF. (^)

  4. For each 16-byte plaintext block of the input image:

    • Reverse the byte order of the plaintext input block. (^)

    • Apply AES256 in ECB mode to the plaintext block.

    • Reverse the byte order of the ciphertext output block. (^)

    • Append to the overall ciphertext output.

  5. Byte-swap each 4-byte word of the ciphertext. (^)

  6. Calculate SHA-512 of the ciphertext.

  7. Byte-swap each 4-byte word of the above-calculated digest. (^)

Output digest is 192 bytes of data: the 128-byte IV, followed by the 64-byte SHA-512 digest.

Image Signing Algorithm

Deterministic ECDSA as specified by RFC 6979.

  • Curve is NIST256p. OpenSSL calls this curve prime256v1, and it is also sometimes called secp256r1.

  • The hash function is SHA256.

  • The key format used for storage is PEM.

    • In the bootloader, the public key for signature verification is flashed as 64 raw bytes.

  • Image signature is 68 bytes: a 4-byte version word (currently zero), followed by 64 bytes of signature data. These 68 bytes are appended to an app image or partition table data.

Manual Commands

Secure boot is integrated into the ESP-IDF build system, so idf.py build will automatically sign an app image if secure boot is enabled. idf.py bootloader will produce a bootloader digest if menuconfig is configured for it.

However, it is possible to use the espsecure.py tool to make standalone signatures and digests.

To sign a binary image:

espsecure.py sign_data --keyfile ./my_signing_key.pem --output ./image_signed.bin image-unsigned.bin

The keyfile is the PEM file containing an ECDSA private signing key.

To generate a bootloader digest:

espsecure.py digest_secure_bootloader --keyfile ./securebootkey.bin --output ./bootloader-digest.bin build/bootloader/bootloader.bin

The keyfile is the 32-byte raw secure boot key for the device.

The output of the espsecure.py digest_secure_bootloader command is a single file that contains both the digest and the bootloader appended to it. To flash the combined digest plus bootloader to the device:

esptool.py write_flash 0x0 bootloader-digest.bin

Secure Boot & Flash Encryption

If secure boot is used without Flash Encryption, it is possible to launch a time-of-check to time-of-use attack, where flash contents are swapped after the image is verified and running. Therefore, it is recommended to use both features together.

Signed App Verification Without Hardware Secure Boot

The integrity of apps can be checked even without enabling the hardware secure boot option. This option uses the same app signature scheme as hardware secure boot, but unlike hardware secure boot, it does not prevent the bootloader from being physically updated. This means that the device can be secured against remote network access, but not physical access. Compared to using hardware secure boot, this option is much simpler to implement. See How To Enable Signed App Verification for step-by-step instructions.

An app can be verified on update and, optionally, be verified on boot.

  • Verification on update: When enabled, the signature is automatically checked whenever the esp_ota_ops.h APIs are used for OTA updates. If hardware secure boot is enabled, this option is always enabled and cannot be disabled. If hardware secure boot is not enabled, this option still adds significant security against network-based attackers by preventing spoofing of OTA updates.

  • Verification on boot: When enabled, the bootloader will be compiled with code to verify that an app is signed before booting it. If hardware secure boot is enabled, this option is always enabled and cannot be disabled. If hardware secure boot is not enabled, this option does not add significant security by itself so most users will want to leave it disabled.

How To Enable Signed App Verification

  1. Open Project Configuration Menu > Security features > enable CONFIG_SECURE_SIGNED_APPS_NO_SECURE_BOOT

  2. Bootloader verifies app signatures can be enabled, which verifies app on boot.

  3. By default, Sign binaries during build will be enabled by selecting the Require signed app images option, which will sign binary files as a part of the build process. The file named Secure boot private signing key will be used to sign the image.

  4. If you disable the Sign binaries during build option then you will have to enter the path of a public key file used to verify signed images in the Secure boot public signature verification key.

    In this case, the private signing key should be generated by following instructions in Generating Secure Boot Signing Key; the public verification key and signed image should be generated by following instructions in Remote Signing of Images.

Advanced Features

JTAG Debugging

By default, when secure boot is enabled, JTAG debugging is disabled via eFuse. The bootloader does this on the first boot, at the same time it enables secure boot.

See JTAG with Flash Encryption or Secure Boot for more information about using JTAG Debugging with either secure boot or signed app verification enabled.

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