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

Wallets data are saved in the local filesystem (see datadir) and optionally encrypted with an externally provided key (on Android everything is encrypted by the default with a key protected by native keystore).

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
• rust (MSRV >= 1.56.0).

Install from crates

cargo install firma-cli

Install from sources

git clone https://github.com/RCasatta/firma/
cd firma/cli
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.

Moreover, wallet signature is supported in the offline devices. This signature could be imported in the online device and verified, to ensure the watch-only wallet is not tampered.

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?

Anything that is persisted to disk can be optionally encrypted.

On cli, user could leverage their existing gpg infrastructure with:

# encryption key creation and storage in encrypted gpg
dd if=/dev/urandom bs=1 count=32 | gpg --encrypt >encryption_key.gpg

# bitcoin private key creation
gpg --decrypt encryption_key.gpg | firma-offline --read-stdin random --key-name bitcoin-key

On Android, system keystore is used to encrypt 32 random bytes, so that physical attacks need to break the secure element if the device has one.

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

While the wallet descriptor isn't strictly necessary in the offline signer, 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.

I've seen secret data in the logs even if I am using the `encryption_key`, what are you doing?

Logs output is disabled in release build, but you may see secrets in logs if you are using a debug builds. Actively redacting sensible data has been pursued, however, it could always be printed the json used to communicate with the lib that must containing this secret data to work, so the approach to disable logs in release build has been taken.

Sponsor

Project sponsored by Squarecrypto

Donations

I am the maintainer of one of the OpenTimestamps calendar, if you feel, you can donate there