Multisig Features

(new in v2.1.0, major changes in v3.2.1)

What is Multisig?

Normal Bitcoin transactions presume a single "owner" of the coins. With Multisig transactions, there are up to 15 possible owners (signers) and between 1 to 15 of them are needed to approve any spending.

This is called an M-of-N wallet or "Multisig P2SH" (pay to script hash) wallet.

Coldcard supports M-of-N wallets with up 15 co-signers. This is an optional, advanced, feature and does not affect normal "single signer" operations.

The transaction approval and signing process is not significantly different from single signer mode, but in multisig cases the "wallet" needs to be defined before use. A typical PSBT file does not carry enough information to encode all the details of the signatures required, and to properly secure change outputs the parameters of the multisig wallet should be established before the PSBT is examined.

Setup Your Multisig Wallet

Before the Coldcard can sign transactions involving multiple signatures, it must learn about the configuration of the wallet and the keys of other co-signers. The following details are required:

These details can be imported either from a simple text file, or thanks to recent changes to BIP-174, the PSBT file can carry all these details (except the wallet name).

Multisig Settings Menu

multisig menu

There is a dedicated menu, "Settings > Multisig Wallets", which lists all established multisig wallets and contains related functions and settings.

"2/4: MeMyself"
Your multisig wallets will be listed here by name at the top of the menu. This example is a 2-of-4 wallet entitled "MeMyself". Choose the name of your wallet to see more details about it, and perform a number of operations. (See next section.)
Import from SD
Read a new wallet setup file from MicroSD card and import it (details are confirmed).
Export XPUB
Creates a file on the MicroSD file that has a few possible XPUB keys for this Coldcard. The resulting file is used in the next command:
Create Airgapped
Creates a new multisig wallet based on the contents of the MicroSD. You have a chance to pick M value, and see details of the wallet.
Trust PSBT?
A setting related to how we should treat the data relating to Multisig wallets in PSBT files.
Skip Checks?
Temporarily disables some more rigerous checks during PSBT signing.

Setting: Trust PSBT?

This setting controls what the Coldcard does with the co-signer public keys (XPUB) that may be provided inside a PSBT file. There are three choices:

Verify Only
Do not import the xpubs found, but do verify the correct wallet already exists on the Coldcard.
Offer Import
If it's a new multisig wallet, offer to import the details and store them as a new wallet in the Coldcard.
Trust PSBT
Use the wallet data in the PSBT as a temporary, multisig wallet, and do not import it. This permits some deniability and additional privacy.

When the XPUB data is not provided in the PSBT, regardless of the above, we require the appropriate multisig wallet to already exist on the Coldcard. The default is to 'Offer' unless at least one multisig wallet already exists, in which case the default becomes 'Verify'.

Setting: Skip Checks?

With this settings, after a warning screen is shown, you may disable some of the checks involved in multisig PSBT processing. This permits PSBTs to be signed that may be generated with less-than-perfect software but which you know are not security hazards. This settings is not preserved between power cycles, and must be re-confirmed each time.

Here is the text of the warning:

With many different wallet vendors and implementors involved, it
can be hard to create a PSBT consistent with the many keys involved.
With this setting, you can disable the more stringent verification
checks your Coldcard normally provides.

USE AT YOUR OWN RISK. These checks exist for good reason! Signed
txn may not be accepted by network.

This settings lasts only until power down.

The additional checks introduced in version 3.2.1 of the firmware are mostly disabled by this check so if you have a setup that worked in earlier Coldcard versions, this setting should allow you to continue until the PSBT generating software is updated.

Multisig Wallet Detail View

Choose the existing wallet, by name, from the top of the Multisig menu and you can view the details about your wallet. Here is an example of the sub-menu shown:

multisig menu

"MeMyself"
The first line is the name: "MeMyself".
View Details
Select this to the signing policy, XPUBs, and all other details. It will look something like this:

multisig detail view

Delete
Forget this wallet.
Coldcard Export
Create the text file other Coldcards would need to work with this wallet.
Electrum Wallet
Export a skeleton Electrum wallet file to support this wallet.

Configuration text file for Multisig

The details needed to define a multisig wallet on the Coldcard can be provided as a simple human-readable text file. The Coldcard can export this file and you edit it to update values, or it can be exported from Electrum or another Coldcard involved in the wallet.

Here is an example file:

# Coldcard Multisig setup file (exported from 4369050F)
#
Name: MeMyself
Policy: 2 of 4
Derivation: m/45'
Format: P2WSH

D0CFA66B: tpubD9429UXFGCTKJ9NdiNK4rC5...DdP9
8E697B74: tpubD97nVL37v5tWyMf9ofh5rzn...XgSc
BE26B07B: tpubD9ArfXowvGHnuECKdGXVKDM...FxPa
4369050F: tpubD8NXmKsmWp3a3DXhbihAYbY...9C8n

Here are the values and the expected values:

Name
Up to 20 characters. Shown on menus and on-screen during transaction approval.
Policy
Defines M and N values. Can be written "M of N", or just "M/N".
Derivation
Specify the derivation of the XPUBs. Applies to all subsequent XPUBs and may be repeated. Must use single quote for hardened keys, and start with "m/" (master).
Format
Address format. Must be one of these values: p2sh, p2wsh, or p2sh-p2wsh.
XFP (8 digits of hex)
The fingerprint of the XPUB is used to label it in this file. The rest of the line, after the colon is the XPUB for that signer. It should be the XPUB at the derivation path, not the master key. SLIP-132 format keys can be used, but they will be converted and stored in BIP-32 format internally (and in future exports).

All details that can be verified by the Coldcard are checked. For example, if the XPUB reports a child depth (aka. tree depth) of one, the XFP provided and the parent key fingerprint need to match. Generally, it is important the tree depth of the XPUB is accurate because the derivation paths provided in PSBT files will be masked-out to that depth. One of the keys provided must have a XFP equal to the Coldcard's fingerprint, indicating that this Coldcard is one of the co-signers.

Default values are provided as follows:

Name
Set to "M-of-N" with M and N values filled-in.
Derivation
Optional, but no default can be provided unless it can be deduced from all XPUBs, which would require tree depth of one, which is very rare.
Format
Default: P2SH
Missing XFP
Calculated when possible, for example if XPUB is derived based on BIP-45 as m/45'.
Policy
Default is M==N. Number of keys as detected in file.

As a result of these defaults, in some very limited cases, it is possible to just provide the XPUB values. However, if the derivation is not BIP-45, that will not work as XFP values are required and there is no way to calculate them from the XPUB itself if it's child-depth is not one. Name and Policy cannot be edited on the Coldcard itself, so you will usually want to set those as well.

As the next two sections discuss, you may not need to create this config file, since you can create a wallet on the Coldcard itself, using just one MicroSD card, or by using the Electrum plugin to build the multisig wallet. It's also possible to import a wallet from a PSBT file.

Creating a Multisig Wallet using Electrum

To create a multisig wallet on Electrum, involving one or more Coldcards, proceed as follows:

At this point, the wallet should be functional. To sign transactions or to show addresses, however, the details must be imported into the Coldcard (or Coldcards), so The final step is to export the setup file. Use Electrum to create the file by choosing "Wallet > Information" and then "Export for Coldcard" button. You'll be prompted for a file name where the text file will be made.

Here is an example:

# Exported from Electrum
Name: wallet_3
Policy: 2 of 2
Format: P2SH-PW2SH

Derivation: m/48'/1'/0'/1'
4369050F: Upub5T4XUooQzDXL58NC...8B2fuBvtSa6

Derivation: m
EB5B9686: Upub5TJpKgtw4cBcaAom...GRSP43VHvGm

This wallet consists of a Coldcard and a key held by Electrum (a BIP39 Seed in this case) with 2-of-2 signing policy.

Import that file onto the Coldcard using "Settings > Multisig Wallets > Import from SD". You'll have a chance to view the details of the wallet before accepting it.

Air-Gapped Creation of a Multisig Wallet

We support air-gapped creation of multisig wallets, so that you never need to directly connect the Coldcard to a computer. The steps are as follows:

Insert SD card with exported XPUB files
from at least one other Coldcard. A multisig
wallet will be constructed using those
keys and this device.

Default is P2WSH addresses (segwit), but
press (1) for P2SH-P2WSH or (2) for P2SH
(legacy) instead.

Press either OK, (1), or (2) based on your preference for address formats, and the next screen will be similar to this:

multisig menu

Now you may press (7) or (9) to change the M value. The N value (number of keys) is determined by the number of "ccxp" files found on the MicroSD card.

Press OK, and you'll see the usual preview of the new wallet to be imported. Confirm that, and the Coldcard will record it and then export two files back onto the MicroSD: a coldcard multisig wallet config file (for importing into the other Coldcards involved), and also an electrum skeleton wallet file.

(Both those files can be re-exported later using the various menus on the Coldcard, if need be.)

At this point, you should return the MicroSD to the other cosigning Coldcards, and import the multisig wallet into each. You are now ready to sign transactions.

Notes and Comments