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Electrum 2/3 PSBT Multi-sig

Keystone firmware version V1.1.1 (BTC-Only) or newer supports PSBT Muti-signature function. This instruction will show how to create a 2/3 PSBT Multi-signature with 2*Keystone, 1*Coldcard and Electrum V4.1.2 in details.
I. Preparation
1. Firmware version V1.1.1 (BTC-Only) or newer running on Keystone Essential or Pro.
   (Please read How to upgrade firmware for more details.)
2. Coldcard upgraded to firmware version V2.1.0 or newer.
3. MicroSD card (≤512GB; Default FAT32 format).
How to Format a MicroSD Card to FAT32?
4. Electrum Wallet (V4.1.2 or newer) running on PC/Mac.
Tips: If you want to switch to [Test Net], please go to "Keystone > Settings > Blockchain > Test Net" and "Coldcard > Settings > Blockchain > Test Net". Then continue with the following steps.
II. Create a 2/3 multisig wallet with 2*Keystone and 1*Coldcard
- Create multisig wallet on Coldcard
1. Export Keystone’s Xpub to microSD card
1) Insert the microSD card into Keystone.
2) Touch [Menu] > [Multisig Wallet] > Press the [•••] button on the top right > [Show/Export XPUB].
3) Touch [Export All] > [Export].
Tips:
① Please repeat this step on 2*Keystone.
② If you were using 1*Keystone and 2*Coldcard to create a 2/3 PSBT Muti-signature wallet, please export Xpubs from 1 Keystone and 2 Coldcard ([Settings] > [Multisig Wallets] > [Export XPUB]).
2. Create multisig wallet on Coldcard
1) Take out the microSD card from Keystone and insert it into Coldcard.
2) [Settings] > [Multisig Wallets] > [Create Airgapped].
3) Set address type (Default-P2WSH) and the number of cosigner (2 of 3).
4) Confirm (√) and complete creating a 2/3 multisig wallet.
3. After the previous steps, you will get 2 files on your microSD card
1) “export-CC-2-of-3.txt” - this is for importing multi-sig wallet to Keystone - allows Keystone to check the genuinity of multisig when signing.
2) “el-CC-2-of-3.json” - this is for creating watch-only wallet on Electrum.
Tips: If you didn’t get these two files, please export manually by following instructions:
1) Export the multisig wallet file (for importing multisig wallet to Keystone) to the microSD card
     ① Insert the microSD card into Coldcard.
     ② [Settings] > [Multisig Wallets] > [2/3: CC-2-of-3] > [Coldcard Export] > Export the “export-CC-2-of-3.txt” file to the microSD card.
2) Export the multisig wallet file (for creating watch-only wallet on Electrum) to the microSD card
     ① Insert the microSD card into Coldcard.
     ② [Settings] > [Multisig Wallets] > [2/3: CC-2-of-3 > [Electrum Export] > Export the “el-CC-2-of-3.json” file to the microSD card.
- Import multisig wallet file to Keystone
1. Take out the microSD card from Coldcard and insert it into Keystone.
2. Touch [Menu] > [Multisig Wallets] > [Import Multisig Wallet].
3. Select file “export-CC-2-of-3.txt” > Touch [Confirm].
- Import multisig wallet file to Electrum and create watch-only wallet
1. Take out the microSD card from Keystone and insert the it into PC/Mac.
2. Run Electrum and click on [Choose...] > Select file “el-CC-2-of-3.json” > [Open] > [Next] > [No].
Note: As Electrum doesn’t have a Keystone plugin (we are working on that), Keystone can’t generate a Electrum skeleton json file. So you have to export the Electrum skeleton json file from Coldcard to set up the watch-only wallet on Electrum.
III. Sending bitcoin from the 2/3 multisig address (2*Keystone & 1*Coldcard)
- Generate multisig transaction on watch-only wallet(Electrum) and export unsigned multisig transaction file to MicroSD card.
1. Click [Send] > Enter destination address and amount > [Pay] > [Send].
2. Click [Export] > [For hardware device; include Xpubs] > [Export for file] > [Save].
- Partially sign multisig transaction on Coldcard
1. Take out the microSD card from PC/Mac and insert it into Coldcard.
2. [Ready To Sign] > Confirm and export partially signed multisig transaction file to microSD card.
- Complete sign multisig transation on Keystone 
1. Take out the microSD card from Coldcard and insert it into Keystone.
2. Enter Multisig Wallet > Press "microSD card" icon on the top right > Select the partially signed transaction file > Confirm transaction details and press [Sign].
3. Password Verification > Press [Export] to export a fully signed multisig transaction file to the microSD card.
- Broadcast multisig transaction with Electrum
1. Click [Tools] > [Load transaction] > [From file] > Select fully signed multisig transaction file > [Open] > [Broadcast].
Instructions recommended: Delete Multisig Wallet on Keystone