(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:
- How many co-signers are there? This is the N value.
- How many are required to approve spends? This is M.
- The Coldcard needs to know the XPUB of all co-signers, to be able display addresses and to verify change outputs.
- The XFP (extended fingerprint) of all cosigner keys. In some limited cases this can be determined from the cosigner's XPUB, but in general it cannot.
- The address format: traditional P2SH, P2WSH (segwit) or P2WSH wrapped in P2SH (transitional)
- The key derivation path is needed. In versions before 3.2.1 this was optional.
- A wallet name, limited to 20 characters.
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
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:
- 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:
- 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
- Comments start with
#and go the end of the line. Blank lines are okay.
- Values and labels are case insensitive.
- All lines are optional, but most users will want to specify the "Policy" and "Name" lines, and the derivation value is required in almost all cases.
- The "derivation" value can be repeated and will apply to all XPUB's lower in the file.
- The colon is required.
Here are the values and the expected values:
- Up to 20 characters. Shown on menus and on-screen during transaction approval.
- Defines M and N values. Can be written "M of N", or just "M/N".
- 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).
- Address format. Must be one of these values:
- 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:
- Set to "M-of-N" with M and N values filled-in.
- 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.
- Default: P2SH
- Missing XFP
- Calculated when possible, for example if XPUB is derived based on BIP-45 as
- 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:
- Choose: "File > New/Restore" from menu.
- Pick a new file.
- Under "Create new wallet", choose: "Multi-signature wallet"
- Use sliders to pick M and N values.
- Under "add cosigner" choose "Use a hardware device" for each co-signer which will be a Coldcard.
- Under "Hardware Keystore" pick your Coldcard.
- Pick desired address format.
- The "Master Public Key" can be ignored.
- Repeat for all co-signers.
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:
- Start with an empty MicroSD card.
- Use that same card on each Coldcard involved in the wallet, and choose: Settings > Multisig Wallets > Export XPUB. This creates an "ccxp" file which contains various public keys and XFP values.
- On the final Coldcard there is no need to do the export step (but harmless). Insert the MicroSD card and choose "Create Airgapped"
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:
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
BIP39 passwords can be used to create multiple co-signers from a single Coldcard.
Details of the multisig wallets are saved up as part of the normal encrypted backup. Multisig wallets can be reconstructed from the seed words, but the M-of-N policy, co-signer xfp/xpubs, derivation path and address type must be known.
BIP67 is used to define the ordering of public keys in all redeem scripts. It is an error to provide a redeem script in another order. Therefore, it doesn't matter what order the co-signers are listed when importing a wallet.
Due to limited encrypted memory, the Coldcard can only store the details of up to eight M-of-3 wallets, or a single M-of-15 wallet. If this is a problem, consider using "Trust PSBT" setting.