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electrum-usage.md

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Using Coldcard with Electrum

(please use the latest version of our plugin)

Off-line signing (air gap)

Setup Time:

  • setup your Electrum wallet associated with the seed of the Coldcard
  • can be done over USB as a connected device... ie. like normal
  • might be possible to do this completely offline

To spend:

  • setup the details of the transaction (amount,destination)
  • press preview, or press send when the Coldcard not connected
  • you get a "transaction details" window, along bottom, new button is shown: "Save PSBT"
  • click that, and it saves a new file ... perhaps direct to SD card if you have it
  • make note of the filename; it has date and time and wallet name in it.
  • close things, move the MicroSD to Coldcard
  • pick "Ready to Sign" from main menu on Coldcard
  • select the correct transaction from list
  • observe and confirm details, press OK and the Coldcard signs it.
  • a new file will be created on SD card: blah-final.txn .. contains HEX of txn to send
  • use any tool to transmit that on Bitcoin P2P network... for Electrum:
    • import the final transaction (-final.txn), using the "Tools > Load Transaction > from File" menu item
    • press Broadcast
    • should show up in wallet immediately

Restoring Coldcard Seed into Electrum

Use this process to recover your funds if you loose your Coldcard and you still have the seed words.

Import from Seed Words

  • choose New/Restore => (filename) => Standard Wallet => I already have a Seed

  • type in 24 word phrase.

  • then click "Options", and enable checkbox labeled "BIP39 seed"

  • a warning is shown:

    BIP39 seeds can be imported in Electrum, so that users can access funds locked in other wallets. However, we do not generate BIP39 seeds, because they do not meet our safety standard. BIP39 seeds do not include a version number, which compromises compatibility with future software. We do not guarantee that BIP39 imports will always be supported in Electrum.

  • press Next (seed phrase checksum is verified, so you must get all the words right)

Import from Extended Public Key (XPUB)

  • You may also save the XPUB from the Coldcard and import that.
  • On the Coldcard, choose Dump Summary from the Advanced menu, and open the public.txt file that generated on your computer.
  • In public.txt, look for the section labeled "BIP44 / Electrum" and Copy the top xpub value, beside: m/44'/0'
  • In Electrum, choose New/Restore => (filename) => Standard Wallet => Use a master key
  • Paste in the xpub value into the text box and press Next.
  • Verify your work by selecting View -> Show Addresses and clicking on the Addresses tab.
  • The first few receive addresses should match the values shown in public.txt beside the text: m/44'/0'/0'/0/0, m/44'/0'/0'/0/1 and so on.

Note

You could also import the master xpub ('m') from that file, and use that key in Electrum, but that would not follow BIP44, which is the default when importing seed words.

Derivation Screen

  • default value is m/44'/0'/0' (legacy BIP44), but options include:
    • p2sh-segwit BIP49: m/49'/0'/0' [NOT YET SUPPORTED]
    • native-segwit BIP84: m/84'/0'/0' [should work]
  • for testnet, the first 0' will be 1'
  • choose appropriately... but if you imported the top-level (master) xpub/xpriv, then use just m/ which really is no derivation at all
  • Electrum always uses /{change}/{index} for individual addresses, where:
    • change is one if it's change back to the wallet, and zero for normal receive
    • index starts at zero and counts up based on usage and gap size.