Raspberry Pico HSM

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.

Capabilities

• Key generation and encrypted storage.
• RSA key generation from 1024 to 4096 bits.
• ECDSA key generation from 192 to 521 bits.
• ECC curves secp192r1, secp256r1, secp384r1, secp521r1, brainpoolP256r1, brainpoolP384r1, brainpoolP512r1, secp192k1 (insecure), secp256k1.
• SHA1, SHA224, SHA256, SHA384, SHA256 digests.
• RSA-PSS, RSA-PKCS and raw RSA signature.
• ECDSA raw and hash signature.
• ECDH key derivation.
• EC private key derivation.¹
• RSA-OEP and RSA-X-509 decryption.
• AES key generation of 128, 192 and 256 bits.
• AES-CBC encryption/decryption.
• AES-CMAC authentication.¹
• AES secret key derivation.¹
• PIN authorization.
• PKCS11 compliant interface.
• HRNG (hardware random number generator).
• Device Key Encryption Key (DKEK) shares.
• DKEK n-of-m threshold scheme.
• USB/CCID support with OpenSC, openssl, etc.
• Extended APDU support.

Security considerations

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. All keys (including DKEK) are loaded and cleared every time to avoid potential 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.

Download

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 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.

Build

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
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.

Usage

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.

Key generation time

Generating EC keys is almost instant. RSA keypair generation takes some time, specially for 2048 and 4096 bits.

RSA key length (bits)	Average time (seconds)
1024	16
2048	124
3072	N/A
4096	N/A

Driver

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.

Important

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.

Credits

Pico HSM uses the following libraries or portion of code:

• OpenSC for ASN1 manipulation.
• mbedTLS for cryptographic operations.
• gnuk for low level CCID procedures and OpenPGP support.
• TinyUSB for low level USB procedures.

In the case of gnuk, it is intended to work with STM32 processor and its family. Part of the code of CCID procedures are ported and adapted to run with Pico.

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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). ↩︎