This is a project to create a Hardware Security Module (HSM) with a Raspberry Pico. It converts your Pico board into a HSM which is able to generate and store private keys, encrypt or decrypt with AES or signing data without to disclose the private key. In detail, the private key never leaves the board and it cannot be retrieved as it is encrypted in the flash memory.
Private and secret keys are stored with a master AES 256 key (MKEK). The MKEK is, at the same time, encrypted with a hashed and salted version of the PIN. No private/secret keys, DKEK or PIN are stored in plain text ever. Never.
RSA key generation in place for 1024, 2048, 3072 and 4096 bits. Private keys never leave the device.
ECDSA key generation in place for different curves, from 192 to 521 bits.
It supports secp192r1, secp256r1, secp384r1, secp521r1, brainpoolP256r1, brainpoolP384r1, brainpoolP512r1, secp192k1 (insecure), secp256k1 curves. Also Curve25519 and Curve448.
ECDSA and RSA signature can be combined with SHA digest in place.
It supports RSA-PSS, RSA-PKCS and raw RSA signatures.
ECDSA signatures can be in raw or pre-hashed formats.
It supports the calculation of shared secrets with ECDH algorithm.
It allows ECDSA key derivation.1
It allows private decryption in place with RSA-OEP and RSA-X-509 algorithms.
It supports AES key generation in place with keys of 128, 192 and 256 bits.
Legacy AES encryption and decryption is performed in place.
Advanced AES encryption and decryption with multiples modes and customized IV/nonce and additional authenticated data (AAD).2
Besides 128, 192 and 256 bits, Pico HSM also supports key generation of 512 bits (64 bytes). These keys are specially indicated for running AES XTS, where two keys of 256 bits are concatenated.
It supports AES-CMAC authentication.1
It supports AES secret key derivation.1
Private and secret keys cannot be used without prior PIN authentication. It supports alphanumeric PIN.
The module can be interfaced with PKCS11 standard.
It contains a harware random number generator properly modeled to guarantee maximum entropy.
It supports DKEK share imports. DKEK are used to wrap, unwrap and encrypt private and secret keys in the device.
It supports a n-of-m threshold scheme to minimize outage when a DKEK custodian is not available during the import process.
Pico HSM has a full USB CCID stack to communicate with the host via OpenSC and PCSC. It allows the use of frontend applications such as OpenSSL via PKCS11 module.
It supports extended APDU packets, which allows up to 65535 bytes.
Pico HSM manipulates CVC certificates and requests to minimize the storage of internal certificates.
Every generated key is attached to a certificate, signed by an external PKI to ensure that a particular key is effectively generated by this specific device.
It allows private key and certificates import via WKY or PKCS#12 files.34
It allows transport PIN for provisioning and forcing to set a new PIN.3 It is a tampered mechanism that ensures the device has not been unsealed during the transportation from the issuer to the legitimate user.
It allows the use of BOOTSEL button to confirm operations with private/secret keys, such as signatures and decryption. When a private/secret key is loaded, the user has 15 seconds to press the button to confirm the operation. This feature protects the user from unwanted uses from background applications that may sign data without user notice.
It allows the storage of arbitrary files with binary data.
Pico HSM has a RTC with external datetime setting and getting.
Pico HSM supports secure channel, where the data packets between the host and device are encrypted to avoid man-in-the-middle attacks.
A specific session PIN can be set during the session opening to avoid the systemmatic use of PIN.
Secure channel messages are secured with a certificate issued by an external PKI.
Key domains are domains to store separate private/secret keys. Each domain is protected by a DKEK, independent from the other domains. Private/secret keys can be generated in different key domains to be used with separated DKEK. Therefore, a single device may contain different domains with independent keys.
A key usage counter is a counter that is reduced by 1 everytime that the private/secret key is used for signing, decrypting, derivation, etc. When it reaches 0, the key is disabled and cannot be used anymore.
Key usage can also be used to perform and auditory and track the usage of a particular key.
Public Key Authentication (PKA) allows to authenticate by using a secondary device with a private key and a registered public key in the primary device. A challenge is generated by the primary Pico HSM and given to the secondary for signature. The secondary device signs the challenge and returns the signature. Then, the primary device verifies the signature with the registered public key and if it is valid, it grants full access, as normal PIN authentication.
In PKA, the PIN is used for protecting the MKEK, as classic method with only PIN, and PKA is used for adding an extra security layer. Therefore, this mechanism provides a higher degree of security, since it needs a secondary Pico HSM to authenticate the primary one.
An extra layer can be added to the device by adding a private key stored on the computer to lock that Pico HSM to the specific computer. The content will be completely encrypted with a private key only available from a specific computer.
This is a novel fast and efficient symmetric encryption algorithm. Similarly to AES, it can be used to cipher your private data.2
Both cruves Curve25519 and Curve448 are supported for doing DH X25519 and X448. Remember that cannot be used for signing.
It supports symmetric key derivations from different standards and RFC.
It supports performing HMAC from a secret key on an arbitrary data with SHA digest algorithm.
Similarly to HMAC, Pico HSM also supports CMAC with AES algorithm for keys of 128, 192 and 256 bits.
Besides DKEK, it supports a more advanced scheme to share keys. Based on private key domains, it is possible to wrap and unwrap private and secret keys inside the domain to only authorized devices. If a device outside the domain tries to unwrap a key, it will fail.
A Master Key Encryption Key is used to store safely all the keys. This key is also ciphered with an ephemereal key derived from the hashed PIN. Therefore, we can ensure all the keys are encrypted and stored.
It supports BIP32 for asymmetric deterministic key derivation and SLIP10 for symmetric key derivation. With it, crypto wallets can be deployed with Pico HSM, as infinite keys can be derived for signature and symmetric encryption. Curves NIST 256 and Koblitz 256 are supported for master key generation.2
All secret keys (asymmetric and symmetric) are stored encrypted in the flash memory of the Raspberry Pico. DKEK is used as a 256 bit AES key to protect private and secret keys. Keys are never stored in RAM except for signature and decryption operations and only during the process. All keys (including DKEK) are loaded and cleared every time to avoid potential security flaws.
At the same time, DKEK is encrypted with doubled salted and hashed PIN. Also, the PIN is hashed in memory during the session. Hence, PIN is never stored in plain text neither in flash nor in memory. Note that PIN is conveyed from the host to the HSM in plain text if no secure channel is provided.
If the Pico is stolen the contents of private and secret keys cannot be read without the PIN, even if the flash memory is dumped.
Please, go to the Release page and download the UF2 file for your board.
Note that UF2 files are shiped with a dummy VID/PID to avoid license issues (FEFF:FCFD). If you are planning to use it with OpenSC or similar, you should modify Info.plist of CCID driver to add these VID/PID or use the Pico Patcher tool.
Alternatively you can use the legacy VID/PID patcher as follows:
./patch_vidpid.sh VID:PID input_hsm_file.uf2 output_hsm_file.uf2
You can use whatever VID/PID (i.e., 234b:0000 from FISJ), but remember that you are not authorized to distribute the binary with a VID/PID that you do not own.
Note that the pure-browser option Pico Patcher tool is the most recommended.
Before building, ensure you have installed the toolchain for the Pico and the Pico SDK is properly located in your drive.
git clone https://github.com/polhenarejos/pico-hsm
git submodule update --init --recursive
cd pico-hsm
mkdir build
cd build
PICO_SDK_PATH=/path/to/pico-sdk cmake .. -DPICO_BOARD=board_type -DUSB_VID=0x1234 -DUSB_PID=0x5678
make
Note that PICO_BOARD, USB_VID and USB_PID are optional. If not provided, pico board and VID/PID FEFF:FCFD will be used.
After make ends, the binary file pico_hsm.uf2 will be generated. Put your pico board into loading mode, by pushing BOOTSEL button while pluging on, and copy the UF2 to the new fresh usb mass storage Pico device. Once copied, the pico mass storage will be disconnected automatically and the pico board will reset with the new firmware. A blinking led will indicate the device is ready to work.
Independent from your Linux distribution or when using another OS that supports Docker, you could build a specific pico-hsm version in a Linux container.
sudo docker build \
--build-arg VERSION_PICO_SDK=1.5.0 \
--build-arg VERSION_MAJOR=3 \
--build-arg VERSION_MINOR=4 \
--build-arg PICO_BOARD=waveshare_rp2040_zero \
--build-arg USB_VID=0xfeff \
--build-arg USB_PID=0xfcfd \
-t pico-hsm-builder .
sudo docker run \
--name mybuild \
-it pico-hsm-builder \
ls -l /home/builduser/pico-hsm/build_release/pico_hsm.uf2
sudo docker cp mybuild:/home/builduser/pico-hsm/build_release/pico_hsm.uf2 .
sudo docker rm mybuild
The firmware uploaded to the Pico contains a reader and a virtual smart card. It is like having a physical reader with an inserted SIM card. We recommend the use of OpenSC to communicate with the reader. If it is not installed, you can download and build it or install the binaries for your system. The first command is to ensure that the Pico is detected as a HSM:
opensc-tool -an
It should return a text like the following:
Using reader with a card: Free Software Initiative of Japan Gnuk
3b:fe:18:00:00:81:31:fe:45:80:31:81:54:48:53:4d:31:73:80:21:40:81:07:fa
SmartCard-HSM
The name of the reader may vary if you modified the VID/PID.
For initialization and asymmetric operations, check doc/usage.md.
For signing and verification operations, check doc/sign-verify.md.
For asymmetric encryption and decryption, check doc/asymmetric-ciphering.md.
For backup, restore and DKEK share management, check doc/backup-and-restore.md.
For AES key generation, encryption and decryption, check doc/aes.md.
For 4096 bits RSA support, check doc/scs3.md.
For storing and retrieving arbitrary data, check doc/store_data.md.
For extra options, such as set/get real datetime or enable/disable press-to-confirm button, check doc/extra_command.md.
For Public Key Authentication, check doc/public_key_authentication.md.
Generating EC keys is almost instant. RSA keypair generation takes some time, specially for 3072 and 4096 bits.
| RSA key length (bits) | Average time (seconds) |
|---|---|
| 1024 | 16 |
| 2048 | 124 |
| 3072 | 600 |
| 4096 | ~1000 |
| RSA key length (bits) | Average time (seconds) |
|---|---|
| 1024 | 1 |
| 2048 | 3 |
| 3072 | 7 |
| 4096 | 15 |
Raspberry Pico comes with the BOOTSEL button to load the firmware. When this firmware is running, the button can be used for other purposes. Pico HSM uses this button to confirm private/secret operations. This feature is optional and it shall be enabled. For more information, see doc/extra_command.md.
With this feature enabled, everytime that a private/secret key is loaded, the Pico HSM awaits for the user confirmation by pressing the BOOTSEL button. The Led of the Pico HSM will remain almost illuminated, turning off quickly once a second, indicating that the user must press the button to confirm the operation. Otherwise, the Pico HSM waits indefinitely. See Led blink for a picture of the blinking sequence. When in this mode, the Pico HSM sends periodic timeout commands to the host to do not trigger the timeout operation.
This feature is an extra layer of security, as it requires the user intervention to sign or decrypt and it ensures that any application will use the Pico HSM without user awareness. However, it is not recommended for servers or other environments where operations are authomatized, since it requires a physical access to the Pico HSM to push the button.
Pico HSM uses the led to indicate the current status. Four states are available:
The Led is almost on all the time. It goes off for 100 miliseconds every second.
In idle mode, the Pico HSM goes to sleep. It waits for a command and it is awaken by the driver. The Led is almost off all the time. It goes on for 500 milliseconds every second.
In active mode, the Pico HSM is awaken and ready to receive a command. It blinks four times in a second.
While processing, the Pico HSM is busy and cannot receive additional commands until the current is processed. In this state, the Led blinks 20 times in a second.
Pico HSM uses the sc-hsm driver provided by OpenSC or the sc-hsm-embedded driver provided by CardContact. This driver utilizes the standardized PKCS#11 interface to communicate with the user and it can be used with many engines that accept PKCS#11 interface, such as OpenSSL, P11 library or pkcs11-tool.
Pico HSM relies on PKCS#15 structure to store and manipulate the internal files (PINs, private keys, certificates, etc.) and directories. Therefore, it accepts the commands from pkcs15-tool. For instance, pkcs15-tool -D will list all elements stored in the Pico HSM.
The way to communicate is exactly the same as with other cards, such as OpenPGP or similar.
For an advanced usage, see the docs and examples.
Pico HSM also supports SCS3 tool for advanced use and multiple key domain. See SCS3 for more information.
OpenSC relies on PCSC driver, which reads a list (Info.plist) that contains a pair of VID/PID of supported readers. In order to be detectable, you must patch the UF2 binary (if you just downloaded from the Release section) or configure the project with the proper VID/PID with USB_VID and USB_PID parameters in CMake (see Build section). Note that you cannot distribute the patched/compiled binary if you do not own the VID/PID or have an explicit authorization.
Pico HSM uses the following libraries or portion of code:
- mbedTLS for cryptographic operations.
- TinyUSB for low level USB procedures.
Footnotes
-
PKCS11 modules (
pkcs11-toolandsc-tool) do not support CMAC and key derivation. It must be processed through raw APDU command (opensc-tool -s). ↩ ↩2 ↩3 -
Available via SCS3 tool. See SCS3 for more information. ↩ ↩2
-
Imports are available only if the Pico HSM is previously initialized with a DKEK and DKEK shares are available during the import process. ↩