Firma

WARNING - Early-stage software, do not use with real bitcoins.

Firma is a tool to create bitcoin multisig wallets with private keys stored on offline devices.

The offline device could be a CLI terminal or a spare android phone.

Information is transferred between devices through QR codes. Since PSBT could become large some kB, more than 1 QR code could be needed, those QRs are chained with QR structured append

It is based on:

bitcoin core
PSBT (Partially Signed Bitcoin Transaction)
rust-bitcoin
and other libs

High-level process:

                                      +---------------------+
+---------------------+               |+---------------------+
|                     |     xpubs     ||                     |
| online machine      | <------------ ||  offline machines   |
|                     |               ||                     |
| * firma-online      |     PSBT      ||  * firma-offline    |
| * bitcoin node      | ------------> ||  * xprv             |
| * xpubs             | <------------ ||                     |
|                     |               +|                     |
+---------------------+                +---------------------+

Setup

Create one ore more extended private keys xprv on one or more offline devices.
Group together corresponding extended public keys xpub and import these on an on-line Bitcoin core node in watch-only mode.
Bring back the wallet descriptor with xpubs on offline machines. While not strictly necessary for signing, wallet on offline machine act as a backup and as added information (eg check if a change is owned by the wallet)

Usage

Receiving

The firma-online tool could create addresses to receive bitcoins.

Spending

Create the transaction from the firma-online tool and export it in PSBT format.
Bring PSBT to offline devices, check the transaction, if everything looks correct, sign the PSBT with the private key present on the device.
Bring all the PSBT back to the node which can combine and finalize these as complete transaction (this operation could occur in parallel or serially).

Requirements

You need Bitcoin core 0.20.1

To build executables you need rust (version >= 1.38.0).

git clone https://github.com/RCasatta/firma/
cd firma
cargo build --release

executables are target/release/firma-online and target/release/firma-offline

Tests

Integration tests require an env var pointing to bitcoin core executable (bitcoind).

For example:

BITCOIND_EXE=./bitcoin-0.20.1/bin/bitcoind cargo test

Example

Check the bin readme for an example with CLI

Faq

How Firma handle fee bug on segwit inputs?

Full previous tx is included in the PSBT to check the prevout hash match the previous transaction, causing an error if amounts are changed as the attack requires.

How Firma handle attacks on the online wallet generating receive addresses?

The offline app could generate addresses as well. The receive process should take into account both an online and an offline device, checking the receiving address generated matches.

How Firma offline know the change address is mine?

Firma online stores the full watch-only descriptor of the wallet thus could generate the address given the derivation present in the PSBT, if the address matches it is owned by the wallet.

How Firma tackle physical attacks on the device?

It doesn't. The security is given by the multi-signature scheme, the offline software doesn't use any secure element of the device.

Why do I need the wallet descriptor in the offline device?

While the wallet descriptor isn't strictly necessary in the offline, it allows some safety checks like the address checking. Most importantly the descriptor is absolutely necessary as a part of the backup, for example in 3of5 scheme, 3 master private keys are not enough to sign transactions because we need 5 master public keys. For this reason the flow requires every offline device store also the wallet descriptor containing all the master public keys.

Donations

I am the maintainer of one of the OpenTimestamps calendar, you can donate there (onchain or lightning) if you want to support this work.

lvaccaro / firma