This project aims to transform a Raspberry Pi Pico or ESP32 microcontroller into a Hardware Security Module (HSM). The modified Pico or ESP32 board will be capable of generating and storing private keys, performing AES encryption or decryption, and signing data without exposing the private key. Specifically, the private key remains securely on the board and cannot be retrieved since it is encrypted within the flash memory.
Supports AES encryption and decryption in ECB, CBC, CFB, OFB, XTS, CTR, GCM, and CCM modes, with customizable IV/nonce and additional authenticated data (AAD).[^4]
Uses the BOOTSEL button to confirm operations with private/secret keys, providing a 15-second window to confirm the operation to protect against unauthorized use.
The OTP securely stores the MKEK (Master Key Encryption Key) and Device Key permanently, making it inaccessible from external interfaces. This ensures that the key is protected against unauthorized access and tampering.
Secure Boot ensures only authenticated firmware can run on the device, verifying each firmware’s digital signature to block unauthorized code.
### > Secure Lock
Secure Lock restricts the device to the manufacturer’s firmware only, locking out debug access and preventing any further boot key installations.
### > Rescue Interface
A built-in rescue interface allows recovery of the device if it becomes unresponsive or undetectable. This feature provides a way to restore the device to operational status without compromising security.
The LED can be customized to reflect device status and user preferences, offering flexible color and brightness options for an enhanced user experience.
[^1]: PKCS11 modules (`pkcs11-tool` and `sc-tool`) do not support CMAC and key derivation. It must be processed through raw APDU command (`opensc-tool -s`).
All secret keys (both asymmetric and symmetric) are encrypted and stored in the flash memory. The MKEK, a 256-bit AES key, is used to protect these private and secret keys. Keys are held in RAM only during signature and decryption operations, and are loaded and cleared each time to avoid potential security vulnerabilities.
The MKEK itself is encrypted using a doubly salted and hashed PIN, and the PIN is hashed in memory during sessions. This ensures that the PIN is never stored in plaintext, neither in flash memory nor in RAM. However, if no secure channel is used, the PIN may be transmitted in plaintext from the host to the HSM.
DKEKs are used during the export and import of private/secret keys and are part of a Key Domain. A Key Domain is a set of secret/private keys that share the same DKEK. These are also shared by the custodians and are not specific to Pico HSM. Therefore, if a key does not belong to a Key Domain (and thus lacks a DKEK), it cannot be exported.
RP2350 and ESP32-S3 microcontrollers are equipped with advanced security features, including Secure Boot and Secure Lock, ensuring that firmware integrity and authenticity are tightly controlled. Both devices support the storage of the Master Key Encryption Key (MKEK) in an OTP (One-Time Programmable) memory region, making it permanently inaccessible for external access or tampering. This secure, non-volatile region guarantees that critical security keys are embedded into the hardware, preventing unauthorized access and supporting robust defenses against code injection or firmware modification. Together, Secure Boot and Secure Lock enforce firmware authentication, while the MKEK in OTP memory solidifies the foundation for secure operations.
Secure Boot is a security feature that ensures that only trusted firmware, verified through digital signatures, can be loaded onto the device during the boot process. Once enabled, Secure Boot checks every piece of firmware against a cryptographic signature before execution, rejecting any unauthorized or modified code. This prevents malicious firmware from compromising the device’s operation and integrity. With Secure Boot activated, only firmware versions signed by a trusted authority, such as the device manufacturer, will be accepted, ensuring the device remains protected from unauthorized software modifications. **This is irreversible. Once enabled, it CANNOT be disabled.**
**IMPORTANT:** For users wishing to develop and compile custom firmware, a private-public key pair is essential. Activating Secure Boot requires users to generate and manage their own unique private-public key pair. The public key from this pair must be embedded into the device to validate all firmware. Firmware will not boot without a proper digital signature from this key pair. This means that users must sign all future firmware versions with their private key and embed the public key in the device to ensure compatibility.
Secure Lock builds on Secure Boot by imposing an even stricter security model. Once activated, Secure Lock prevents any further installation of new boot keys, effectively locking the device to only run firmware that is authorized by the device's primary vendor—in this case, Pico Keys. In addition to preventing additional keys, Secure Lock disables debugging interfaces and puts additional safeguards in place to resist tampering and intrusion attempts. This ensures that the device operates exclusively with the original vendor’s firmware and resists unauthorized access, making it highly secure against external threats. **This is irreversible. Once enabled, it CANNOT be disabled.**
**IMPORTANT:** Activating Secure Lock not only enables Secure Boot but also invalidates all keys except the official Pico Key. This means that only firmware signed by Pico Key will be recognized, and custom code will no longer be allowed. Once enabled, the Pico Key device will run solely on the official firmware available on the website, with no option for generating or compiling new code for the device.
If you own a Raspberry Pico (RP2040 or RP2350), go to [Download page](https://www.picokeys.com/getting-started/), select your vendor and model and download the proper firmware; or go to [Release page](https://www.github.com/polhenarejos/pico-hsm/releases/) 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 plan to use it with OpenSC or similar tools, you should modify Info.plist of CCID driver to add these VID/PID or use the [Pico Commissioner](https://www.picokeys.com/pico-commissioner/ "Pico Commissioner").
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.
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.
The firmware uploaded to the Pico contains a reader and a virtual smart card, similar to having a physical reader with an inserted SIM card. We recommend using [OpenSC](http://github.com/opensc/opensc/ "OpenSC") to communicate with the reader. If OpenSC is not installed, you can download and build it or install the binaries for your system.
To ensure that the Pico is detected as an HSM, use the following command:
The Raspberry Pico includes a BOOTSEL button used for loading firmware initially. Once the Pico HSM firmware is running, this button can be repurposed for additional functionalities. Specifically, the Pico HSM utilizes this button to confirm private and secret operations, a feature that is optional but highly recommended for enhanced security.
When enabled, each time a private or secret key operation is initiated, the Pico HSM enters a waiting state where it awaits user confirmation by pressing the BOOTSEL button. During this waiting period, the Pico HSM's LED remains mostly illuminated but blinks off briefly every second, signaling to the user to press the button for confirmation. If no action is taken, the Pico HSM will continue to wait indefinitely. This operation mode includes periodic timeout commands sent to the host to prevent the session from timing out prematurely.
This feature adds an additional layer of security by requiring physical user intervention for sensitive operations such as signing or decrypting data. It mitigates risks associated with unauthorized applications or scripts using the Pico HSM without user awareness. However, it is not recommended for server environments or other automated settings where physical access to press the button may not be practical.
For more details on configuring and using this feature, refer to the [doc/extra_command.md](/doc/extra_command.md) document.
The Led is almost on all the time. It goes off for 100 miliseconds every second.
### Idle mode
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.
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.
The Pico HSM uses either the `sc-hsm` driver from [OpenSC](https://github.com/OpenSC/OpenSC/) or the `sc-hsm-embedded` driver from [CardContact](https://github.com/CardContact/sc-hsm-embedded/) to interface with external applications. These drivers employ the standardized PKCS#11 interface, making it compatible with various cryptographic engines that support PKCS#11, such as OpenSSL, P11 library, or pkcs11-tool.
Internally, the Pico HSM organizes and manages its data using the PKCS#15 structure, which includes elements like PINs, private keys, and certificates. Commands can be issued to interact with these stored elements using tools such as `pkcs15-tool`. For example, `pkcs15-tool -D` lists all elements stored within the Pico HSM.
For advanced usage scenarios, refer to the documentation and examples provided. Additionally, the Pico HSM supports the SCS3 tool for more sophisticated operations and includes features like multiple key domains. For detailed information on SCS3 usage, refer to [SCS3 documentation](/doc/scs3.md).
- Released under the GNU Affero General Public License v3 (AGPLv3).
- You are free to study, modify, and run the code, including for internal evaluation.
- If you distribute modified binaries/firmware, OR if you run a modified version of this project as a network-accessible service, you must provide the corresponding source code to the users of that binary or service, as required by AGPLv3.
- No warranty. No SLA. No guaranteed support.
**Enterprise / Commercial Edition**
- Proprietary license for organizations that want to:
- run this in production with multiple users/devices,
