I set up a 2-of-3 multisig using Electrum, a Trezor, and a Coldcard. During testing, I ran into issues using the Coldcard to sign.
- (On Electrum) File -> New/Restore -> Multi-signature wallet -> Use a hardware device
- Plug in Trezor, unlock, and set up as p2wsh.
- Repeat steps 1 and 2 for second Trezor.
- Plug MicroSD card into Coldcard.
- (On Coldcard) Settings -> Multisig Wallets -> Export XPUB
- Plug MicroSD card into computer running Electrum.
- (On Electrum) Enter cosigner key
- Paste p2wsh xpub from MicroSD card into text box.
This worked and my wallet was set up. I sent a small amount of bitcoin into it to test that I could sign transactions with Electrum. I was expecting the UX to be similar to that of a singlesig airgapped Coldcard used with Electrum, meaning I would create a transaction and click the "Export PSBT" button. Bring that to my Coldcard and sign the transaction. However, the transaction popup only has an "Export" button and no "Export PSBT" button. The "Export" button gives me a .txn file that is json containing the tx hex. Coldcard does not recognize this file and therefore cannot sign it. I was able to find these docs, specifically the section titled "Air-Gapped Creation of a Multisig Wallet". However, those docs seem specific to Coldcard-only multisig setups.
Is it possible to export multisig PSBTs from Electrum for signing on an airgapped Coldcard or is this something that is simply not currently supported? If it's supported, what do I need to do?