Log Encryption

PX4 v1.13

The System Logger can be used to create encrypted logs, which may then be decrypted manually before analysis.

The default encryption algorithm is XChaCha20, and the default wrapping algorithm is RSA2048-OAEP.

WARNING

Log encryption is not enabled by default in PX4 firmware builds. To use it you will need to build firmware with this feature enabled and then upload it to the flight controller (see instructions below).

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Log encryption was has been improved in PX4 main (v1.16+) to generate a single encrypted log file that contains both encrypted log data, and an encrypted symmetric key that you can use to decrypt it (provided you can decrypt the symmetric key).

In earlier versions the encrypted symmetric key was stored in a separate file. For more information see the Log Encryption (PX4 v1.15).

How ULog Encryption Works

INFO

The encryption algorithm used is set in SDLOG_ALGORITHM. At time of writing, only XChaCha20 is supported (AES can be selected, but there is no implementation).

If another algorithm is supported in future, the process is likely to remain the same as documented here.

The encryption process for each new ULog is:

1. A XChaCha20 symmetric key is generated and encrypted using an RSA2048 public key. This wrapped (encrypted) key is stored on the SD card in the beginning of a file that has the suffix .ulge ("ulog encrypted").
2. When a log is captured, the ULog data is encrypted with the unwrapped symmetric key and the resulting data is appended into the end of the .ulge file immediately after the wrapped key data.

After the flight, the .ulge file containing both the wrapped symmetric key and the encrypted log data can be found on the SD card.

In order to extract the log file, a user must first decrypt the wrapped symmetric key, which can then be used to decrypt the log. Decrypting the wrapped symmetric key file is only possible if the user has the corresponding RSA private key for the public key that was used to wrap it.

This process is covered in more detail in Download & Decrypt Log Files below.

File Structure

Encrypted .ulge file contains following sections:

-------------------------
| Header                |
-------------------------
| Wrapped symmetric key |
-------------------------
| Encrypted ulog data   |
-------------------------

Header section (22 bytes) contains following fields:

Bytes	Field
0..6	File magic identifier
7	Header version
8..15	Timestamp
16	exchange algorithm
17	exchange key index
18..19	key size
20..21	nonce size

The header part begins with magic string: "ULogEnc", which identifies this is encrypted ulog file. The file offset of the symmetric key section is 22 and the file offset of the log data section is 22 + key_size + nonce_size (key_size and nonce_size are taken from the header section).

Custom PX4 Firmware with Log Encryption

You will need to build custom firmware that contains your own public RSA key and the required Crypto API modules to support log encryption. This section shows how to do this using the px4-fmu-v5 board as an example.

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We show you how to generate your own keys in the Generate RSA Public & Private Keys section below.

INFO

The modules in a PX4 build are defined in configuration files, which may be modified either manually or using the menuconfig tool. For more information see: PX4 Board Configuration (Kconfig).

Cryptotest Make Target

Crypto uses large amounts of flash memory, and is therefore not included in the default PX4 make targets for each board (such as make px4-fmu-v5). The easiest way to add support for encrypted logs is to define a custom make target that includes the required modules and your public RSA keys.

WARNING

Many builds are close to their maximum capacity. If you run into a build error telling you that you have gone above the maximum flash memory, you will need to disable other features in the .px4board file you are working on, or in the default.px4board file. Be careful not to disable something you need.

For example, if you found you were running out of memory on FMUv4 boards you could disable SIH mode by setting CONFIG_MODULES_SIMULATION_SIMULATOR_SIH=n in boards/px4/fmu-v4/default.px4board, which may free up enough flash memory to allow crypto to be added.

Pixhawk FMUv5 boards

The FMUv5 board already has a custom make target px4-fmu-v5_cryptotest that you can use to build custom firmware with the required modules and "test" RSA keys. The configuration file that enables the above make target is cryptotest.px4board file in boards/px4/fmu-v5. The relevant keys in that file are reproduced below:

CONFIG_BOARD_CRYPTO=y
CONFIG_DRIVERS_STUB_KEYSTORE=y
CONFIG_DRIVERS_SW_CRYPTO=y
CONFIG_PUBLIC_KEY1="../../../Tools/test_keys/rsa2048.pub"

INFO

The file also sets CONFIG_PUBLIC_KEY0 to a key named key0.pub. This is not used in the current PX4 implementation and can be ignored.

Overview of crypto-relevant keys

Argument	Description
CONFIG_BOARD_CRYPTO	Include crypto module in firmware. = y: Enable log encryption. = n: Disable log encryption.
CONFIG_DRIVERS_SW_CRYPTO	Include the PX4 crypto backend library (used by above library). = y: Enable = n: Disable
CONFIG_DRIVERS_STUB_KEYSTORE	Includes the PX4 stub keystore driver. = y: Enable = n: Disable
CONFIG_PUBLIC_KEY0	Location of public key for keystore index 0.
CONFIG_PUBLIC_KEY1	Location of public key for keystore index 1. = {path to key1}
CONFIG_PUBLIC_KEY2	Location of public key for keystore index 2. = {path to key2}
CONFIG_PUBLIC_KEY3	Location of public key for keystore index 3. = {path to key3}

The stub keystore is a keystore implementation that can store up to four keys. The initial values of these keys are set in the locations defined by CONFIG_PUBLIC_KEY0 to CONFIG_PUBLIC_KEY3. The keys can be used for different cryptographic purposes, which are determined by parameters.

The exchange key, which is the public key used for encrypting the symmetric key stored in the beginning of the .ulge file, is specified using SDLOG_EXCH_KEY as an index value into the key store. The value is 1 by default, which maps to the key defined in CONFIG_PUBLIC_KEY1.

The logging key is the unencrypted symmetric key. This is specified using SDLOG_KEY as an index value into the key store, and default to 2. Note that the value is generated fresh for each log, and any value specified in CONFIG_PUBLIC_KEY2 would be overwritten.

You can use choose different locations for your keys as long as they aren't used by anything else.

The key in CONFIG_PUBLIC_KEY1 is the public key used to wrap the symmetric key in the the beginning of .ulge file (by default: see SDLOG_EXCH_KEY). You can use the rsa2048.pub key for testing, or replace it with the path to your own public key in the file (see Generate RSA Public & Private Keys).

Build the firmware like this:

make px4-fmu-v5_cryptotest

Other Boards

For other boards you will need to first copy cryptotest.px4board into the root of the target board directory. For example, for FMUv6 you would copy the board to /boards/px4/fmu-v6x.

Then you will need to add a few more configuration settings that are present in FMUv5 default configuration but not in the other boards. We do add these using the menuconfig tool.

To use menuconfig you will need to add these dependencies:

sudo apt-get install libncurses-dev flex bison openssl libssl-dev dkms libelf-dev libudev-dev libpci-dev libiberty-dev autoconf

Now, in PX4, run the normal make command you would use to build the board you are targeting, but add "menuconfig" at the end of it. Here we use px4_fmu-v5_cryptotest as an example, because that already has the settings that we want to copy:

make px4_fmu-v5_cryptotest menuconfig

Navigate to Crypto API and use the Y key to select it.

This will open the menu below. Enable the settings: Blake2s hash algorithm, Entropy pool and strong random number generator, and Use interrupts to feed timing randomness to entropy pool.

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Some of these options can be tweaked if desired.

After enabling encryption settings, exit menuconfig. You can now build and test.

Download & Decrypt Log Files

Encrypted log files are downloaded using the QGroundControl Log Download view (Analyze Tools > Log Download) just like ordinary log files.

Note that the encrypted files will be downloaded with the .ulg suffix, instead of .ulge.

Decrypt ULogs

Before you can analyze your encrypted logs, you will need to decrypt them. There is a Python script that can be used to decrypt logs in Tools/decrypt_ulog.py.

When decrypting a .ulge file the script takes 3 arguments:

1. The encrypted log file.
2. An empty string ''.
3. The decryption key (the RSA2048 .pem private key which is used to unwrap the symmetric key).

For example:

python3 decrypt_ulog.py \
/home/john/Downloads/log_24_2024-10-6-23-39-50.ulg '' \
new_keys/private_key.pem

On success the decrypted log file is created with the .ulog suffix.

INFO

The script can be used with both .ulge logs and the .ulgc/.ulgk files used in PX4 v1.15 Log Encryption. The full command line syntax is given below:

usage: decrypt_ulog.py [-h] [ulog_file] [ulog_key] [rsa_key]

CLI tool to decrypt an ulog file

positional arguments:
  ulog_file   .ulge/.ulgc, encrypted log file
  ulog_key    .ulgk, legacy encrypted key (give empty string '' to ignore for .ulge)
  rsa_key     .pem format key for decrypting the ulog key

optional arguments:
  -h, --help  show this help message and exit

Generate RSA Public & Private Keys

To generate a RSA2048 private and public key, you can use OpenSSL:

openssl genpkey -algorithm RSA -out private_key.pem -pkeyopt rsa_keygen_bits:2048

Then you can create a public key from this private key:

# Convert private_key.pem to a DER file
openssl rsa -pubout -in private_key.pem -outform DER -out public_key.der
# From the DER file, generate a public key in hex format, separated by commas
xxd -p public_key.der | tr -d '\n' | sed 's/\(..\)/0x\1, /g' > public_key.pub

To use this key you would modify your .px4board file to point CONFIG_PUBLIC_KEY1 to the file location of public_key.pub. The private key generated should be stored safely and used when you need to decrypt log files.