Supercharge your SeedSigner with Javacards

What's SeedSigner

SeedSigner is an open-source, do-it-yourself (DIY) Bitcoin signing device designed with security in mind. Unlike traditional hardware wallets, it doesn't store your private keys permanently. Instead, you generate or scan them—often via QR codes—only when you need to sign a transaction. Because the device is stateless and holds no long-term secrets, the risk of theft if it's lost or compromised is significantly reduced.

This stateless approach is powerful, but it also has drawbacks. Each time you want to sign a transaction, you must load your seed onto the device. For users managing multiple wallets—and therefore multiple seeds—this quickly becomes a challenge, making secure and convenient seed management difficult.

The seed storage dilemma

The broader problem remains: how can private keys be stored in a way that is secure, resilient, and easy to use? Different wallets have tried different solutions. SeedQR, for instance, has gained popularity thanks to its speed and convenience compared to traditional seedphrases. Yet, depending solely on SeedQR brings its own concerns around security, durability, and privacy.

A more balanced approach is to store secrets on a PIN-protected Javacard such as Seedkeeper. This method combines strong security with practicality, offering a solution that is affordable, reliable, and accessible for everyday users—without the headaches of juggling multiple seeds or compromising on safety.

Supercharge your SeedSigner with smartcards

By supercharging your SeedSigner with smartcard support, you can manage all your seeds with Seedkeeper and even sign your Bitcoin transactions using Satochip.

With this setup, you can:

-Sign transactions and verify PSBTs directly with your Satochip card; -Sign messages and export xpubs (for both single and multisig setups) with full address explorer integration; -Manage all your seeds, wallet descriptors and secrets with Seedkeeper; -Use Seedkeeper with Electrum seeds, including support for split passphrases and encrypted QR codes.

Using smartcards with SeedSigner requires just a few adjustments: you'll need a smartcard reader and a dedicated SeedSigner firmware that integrates all Satochip smartcard features.

Easy to use & assemble

Once you get the kit, follow these 3 simple steps to unlock the full smartcard power on your SeedSigner:

1. Disassemble your existing SeedSigner and insert the smartcard hat between your Raspberry Pi Zero and the screen.
1. Download and flash the compatible SeedSigner firmware (with smartcard support) onto your existing SD card.
1. Reconnect everything and plug in a USB-C power source to your SeedSigner. You're now ready to use your smartcard.

Download SeedSigner fork with smartcard support: https://github.com/3rdIteration/seedsigner/releases

Credits

This project is the result of a collaborative effort, made possible thanks to:

GoBrrr for printing the case
CryptoGuide — also known as 3rdIteration — for the outstanding work on the smartcard hat
CryptoGuide / 3rdIteration again, for integrating Satochip's functionalities (and more) into the SeedSigner firmware
The SeedSigner Project for providing the foundation that makes all of this possible