Using a Raspberry Pi RP2040 microcontroller as a USB security device that provides:
- cryptographic hashing (SHA256)
- encryption and decryption (256 bit AES)
- public key cryptography (ECDSA - secp256k1, as Bitcoin)
I'm not a security expert and the device/software is almost certainly not hardened enough for serious use. I just did it cos it was there, and I was bored. Also, it's not fast, but that might be ok depending on your current lockdown status. Most importantly, it works. Here's some steps I took towards making it securer:
- the device is pin protected. Only the SHA256 hash of the (salted) pin is stored on the device.
- the private key is only initialised once a correct pin has been entered, and is a SHA256 hash of the (salted) unique device id. So no two devices should have the same key.
- the private key never leaves the device and is stored only in volatile memory.
Pico, Pico W and Tiny2040 boards are known to work. Other RP2040 boards have not been tested but are likely to (mostly) work. E.g. the Pico W requires the wifi driver purely for the LED (which is connected to the wifi chip) to function (though neither wifi nor bluetooth are enabled.)
- Adds support for Pimoroni Tiny2040.
- The webauthn-style workflow example has been improved.
Adds:
- Time synchronisation between host and device
- Device info: firmware version, board type, current time
- Generation of time-based authentication tokens (like Webauthn)
- Multiple build targets: Pico and Pico W. LED now works on Pico W.
- Switch to short-form public keys.
The device pin is now configurable. See PIN protection and the change pin example.
The device now uses USB CDC rather than serial to communicate with the host which allows much faster bitrates and avoids the need to encode binary data. Performance is improved, but varies considerably by task (results are for a 1000kB input):
task | CDC time(s) |
CDC bitrate(kbps) |
serial time(s) |
serial bitrate(kbps) |
Speedup(%) |
---|---|---|---|---|---|
hash | 2.6 | 3026.3 | 19.6 | 407.9 | 641.9 |
sign | 2.8 | 2904.1 | 19.6 | 408.3 | 611.3 |
verify | 0.4 | 0.5 | 16.0 | ||
encrypt | 23.9 | 334.2 | 43.5 | 183.8 | 81.9 |
decrypt | 23.8 | 336.0 | 43.1 | 185.7 | 81.0 |
pico-crypto-key
is a python (dev) package that provides:
- a simplified build process
- a python interface to the device.
First, clone/fork this repo and install the package in development (editable) mode:
pip install -e .[dev]
If this step fails, try upgrading to a more recent version of pip.
You will then need to:
-
install the compiler toolchain (arm cross-compiler) and cmake:
sudo apt install gcc-arm-none-eabi cmake
-
download pico-sdk, see here. NB This project uses a tagged release of pico-sdk, so download and extract e.g. 1.5.1
-
download and extract a release of tinyusb. Replace the empty
pico-sdk-1.5.1/lib/tinyusb
directory with a symlink to where you extracted it, e.g.cd pico-sdk-1.5.1/lib rmdir tinyusb ln -s ../../tinyusb-0.16.0 tinyusb
-
[pico_w only], repeat the above step for
cyw43-driver
, which can be found here -
download mbedtls: see also their repo. Currently using the 3.6.0 release/tag.
create symlinks in the project root to the pico SDK and mbedtls, e.g.:
ln -s ../pico-sdk-1.5.1 pico-sdk ln -s ../mbedtls-3.6.0 mbedtls
Not sure why, but I couldn't get it to work with the symlink inside pico-sdk like tinyusb.
You should now have a structure something like this:
.
├──mbedtls-3.6.0
├──pico-crypto-key
│ ├──examples
│ ├──mbedtls -> ../mbedtls-3.6.0
│ ├──pico_crypto_key
│ │ ├──build.py
│ │ ├──device.py
│ │ └──__init__.py
│ ├──pico-sdk -> ../pico-sdk-1.5.1
│ ├──pyproject.toml
│ ├──README.md
│ ├──src
│ └──test
├──pico-sdk-1.5.1
│ └──lib
│ ├──cyw43-driver -> ../../cyw43-driver-1.0.3 *
│ └──tinyusb -> ../../tinyusb-0.16.0
└──tinyusb-0.16.0
* required for pico_w only
If using a fresh download of mbedtls
- run the configuration script to customise the build for the Pico, e.g.:
./configure-mbedtls.sh
More info here
The target board can/should be specified using the --board
option when running check
, clean
, build
, install
or reset-pin
.
- Pico:
--board pico
(default) - Pico W:
--board pico_w
- Pimoroni Tiny2040 2MB:
--board pimoroni_tiny2040_2mb
Using the correct board will ensure (amongst other things?) the LED will work. (NB Images built for one board may work on other boards, aside from the LED. YMMV...)
Board | Init | Ready* | Busy | Invalid | Fatal Error |
---|---|---|---|---|---|
Pico | Flash | - | On | - | Flashing |
Pico W | Flash | - | On | - | Flashing |
Tiny2040 | White flash | Green | Blue | Red | Flashing Red |
* "Ready" state required a valid pin to be supplied.
These steps use the picobuild
script. (See picobuild --help
.) Optionally check your configuration looks correct then build:
picobuild check
picobuild build
Ensure your device is connected and mounted ready to accept a new image (press BOOTSEL
when connecting), then:
picobuild install /path/to/RPI-RP2
picobuild test
The device is protected with a PIN, the salted hash of which is read from flash memory. Before first use (or a forgotten PIN), a hash must be written to flash (press BOOTSEL
when connecting):
picobuild reset-pin /path/to/RPI-RP2
If the device LED is flashing (red if supported by the board) after this, the reset failed - the flash memory may be worn. Otherwise now reinstall the crypto key image as above. The pin will then be "pico", and it can be changed - see the example.
The python driver will first check for an env var PICO_CRYPTO_KEY_PIN
and fall back to a prompt if this is not present.
(NB for the tests to run, the env var must be set)
On boards with multicoloured LEDs (e.g. tiny2040), initialisation is green, busy is blue and error states are red.
The CryptoKey
class provides the python interface and is context-managed to help ensure the device gets properly opened and closed. The correct pin must be provided to activate it. Methods available are:
pubkey
return the ECDSA public key (short-form, 33 bytes)hash
compute the SHA256 hash of the inputsign
compute the SHA256 hash and ECDSA signature of the inputverify
verify the given hash matches the signature and public keyencrypt
encrypts using AES256decrypt
decrypts using AES256register
dynamically create an ECDSA public key for verifying, along the lines of WebAuthnauth
generates a one-time ECDSA-based authentication string, along the lines of WebAuthnset_pin
set a new PINinfo
returns version, board type and device time
See the examples for more details.
If there are low-level errors with any of the crypto algorithms then the device may enter an unrecoverable error state where the LED will flash. The error codes can be interpreted like so:
Long flashes | Short flashes | Algorithm | mbedtls error code |
---|---|---|---|
1 | 0 | ECDSA | Unknown error |
1 | 1 | ECDSA | MBEDTLS_ERR_ECP_BAD_INPUT_DATA |
1 | 2 | ECDSA | MBEDTLS_ERR_ECP_BUFFER_TOO_SMALL |
1 | 3 | ECDSA | MBEDTLS_ERR_ECP_FEATURE_UNAVAILABLE |
1 | 4 | ECDSA | MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED |
1 | 5 | ECDSA | MBEDTLS_ERR_MPI_ALLOC_FAILED |
2 | 0 | AES | Unknown error |
2 | 1 | AES | MBEDTLS_ERR_AES_INVALID_KEY_LENGTH |
3 | 0 | SHA | Unknown error |
-
If you get
[Errno 13] Permission denied: '/dev/ttyACM0'
, adding yourself to thedialout
group and rebooting should fix. -
If you get
usb.core.USBError: [Errno 13] Access denied (insufficient permissions)
you'll need to add a udev rule for the device, see this stackoverflow post. This worked for me:SUBSYSTEMS=="usb", ENV{DEVTYPE}=="usb_device", ATTRS{idVendor}=="aafe", ATTRS{idProduct}=="c0ff", GROUP="plugdev", MODE="0777"
-
the device can get out of sync quite easily when something goes wrong. If so, turn it off and on again ;)
This script just prints the hash of itself.
$ python examples/hash_file.py examples/hash_file.py
PicoCryptoKey 1.3.0-pico
examples/hash_file.py: f99e202cdb1c7091f291a3361eac6b8d2230eef28bd165415e86ec235b03a938
This example will look for an encrypted version of the data (examples/dataframe.csv). If not found it will first encrypt the plaintext.
Then it decrypts the ciphertext and loads the data into a pandas dataframe (you may need to install pandas).
python examples/decrypt_data.py
If you are using the same device you used to encrypt the data, you should see something like this:
decryption took 2.56s
Area DC1117EW_C_SEX DC1117EW_C_AGE NewEthpop_ETH
0 E02001730 2 62 WBI
1 E02001713 2 60 WBI
2 E02001713 1 30 WBI
3 E02001736 1 41 OAS
4 E02001719 2 22 WBI
... ... ... ... ...
5630 E02001721 1 60 WBI
5631 E02001720 1 22 WBI
5632 E02001729 2 24 MIX
5633 E02006893 1 32 WBI
5634 E02001708 1 28 OAS
[5635 rows x 4 columns]
If you now switch to a different device, it won't be able to decrypt the ciphertext and will return garbage.
This example will compute a hash (SHA256) of a file and sign it. It outputs a json object containing the filename, the hash, the signature, and the device's public key.
python examples/sign_data.py
gives you something like
PicoCryptoKey 1.3.0-pico
signing took 0.46s
signature written to signature.json
where signature.json contains
{
"file": "examples/dataframe.csv",
"hash": "28d839df69762085f8ac7b360cd5ee0435030247143260cfaff0b313f99a251c",
"signature": "30460221009531919bd13f964544fb494393e1bea1cf5a04a53d572e914ea1cbc30657166c022100e1d1394240eb9a8269eafa8d06a82d1c087a0af260576577da5f45352ebb4162",
"pubkey": "020a7dfbd2272ad9e9d49dd11aec4743d10ba3d9e3affc3fa3a64c8a28fd78a212"
}
The signature data above should be verifiable by any ECDSA validation algorithm, but you can use the device for this. First it verifies the supplied hash corresponds to the file, then it verifies the signature against the hash and the given public key. It also prints whether the public key provided matches it's own public key.
python examples/verify_data.py
PicoCryptoKey 1.3.0-pico
file hash matches file
verifying device is the signing device
signature is valid
verifying took 0.79s
or, if you use a different board
PicoCryptoKey 1.3.0-pico
file hash matches file
verifying device is not the signing device
signature is valid
verifying took 0.79s
Step 1 generates registration keys for two relying parties - these are short-form ECDSA public keys.
Step 2 generates a time-based auth tokens for each receiving party from a challenge string. The tokens are base64-encoded ECDSA signatures of the SHA256 of the challenge appended with the timestamp rounded to the minute.
Third-party code (the ecdsa python package) is then used to verify the public key-auth token pairs.
python examples/auth.py
PicoCryptoKey 1.3.1-pimoroni_tiny2040_2mb 2024-07-09 18:00:41.382000+00:00
Host-device time diff: 0.002865s
registered auth_user@example.com: 0334195ea7cc307c5908bd5f80b5fd0513edf5e8bf0f49c544231e089b2ea6c682
registered auth_user@another.org: 024f4f8fc6a8fca7069ffeb7122f545833ea82727fd3a8286e13f77bdbf6214dc9
challenge is: b'testing time-based auth'
auth response auth_user@example.com: b'MEQCIG4Pp5o/wXMh6RY0Z2zvr1IOBWVhQcHoRyGeQQls8genAiBaJjKeM4R4kI3DD5s3xet4R/K/bQRncyWqBoO89QILkA=='
auth response auth_user@another.org: b'MEYCIQDezWSAyNwvioDXbsO/xDMnDJLhZWWVGLhMLFNoNezRUwIhANC8gdkUtcfDcciGw9J2hbB2NdoqFP+o5RuyYQms30xk'
example.com verified: True
another.org verified: True
example.com cannot verify b'MEYCIQDezWSAyNwvioDXbsO/xDMnDJLhZWWVGLhMLFNoNezRUwIhANC8gdkUtcfDcciGw9J2hbB2NdoqFP+o5RuyYQms30xk'
another.org cannot verify b'MEQCIG4Pp5o/wXMh6RY0Z2zvr1IOBWVhQcHoRyGeQQls8genAiBaJjKeM4R4kI3DD5s3xet4R/K/bQRncyWqBoO89QILkA=='
As above, but using a webauthn-style workflow using a local fastapi instance (would normally be a remote website).
First start the host:
fastapi run examples/webauthn_host.py
When up and running, the API endpoints should be documented at http://localhost:8000/docs
Then use the interactive client script to interact with the crypto key and allow you to register and authenticate with the host, like in the example above:
python examples/webauthn_user.py
Try playing around with different users and different keys...
python examples/change_pin.py
This just runs the PIN reset process:
- initialise device
- reset device (you'll need to enter the old PIN, even if this was set in the env)
- enter new PIN and repeat to confirm
- write new PIN to device
- reset device and initialise with new PIN