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BIP-39 Import

  • there must be 12, 18 or 24 words in your mnemonic
  • we have only the English word list
  • we support BIP-39 passwords during import

XPRV Import

  • we can import a BIP-32 HD-wallet root private key in XPRV format
  • it's assumed to be top-level, and we don't store the parent fingerprint or depth value
  • SLIP-132 format HD-wallet keys are also supported (xprv/yprv/zprv), but we strip the implied address format

PIN Codes

  • 2-2 through 6-6 in size, numeric digits only
  • pin code 999999-999999 is reserved (means 'clear pin')

Backup Files

  • we don't know what day it is, so meta data on files will not have correct date/time
  • encrypted files produced cannot be changed, and we don't support other tools making them

Micro SD

  • cards up to 32G are supported
  • we do not guarantee to support all cards ever made, or yet to be made
  • we recommend 8G cards or smaller, since our storage needs are very modest

Signing / Wallet Types

  • with Electrum, we support classic payment addresses (p2pkh), Bech32 Segwit and P2SH/Segwit
    • however, each wallet must be of a single address type; cannot be mixed (their limitation)
    • the same Coldcard could be used in each of the three modes (we don't care about address format)
  • with Bitcoin Core (version 0.17+), we can do PSBT transactions, which support all address types
  • we don't support signing coinbase transactions, so don't mine directly into a Coldcard wallet

Max Transaction Size

  • mk3:
    • we support transactions up to 384k-bytes in size when serialized into PSBT format
    • we can handle transactions with up to 20 inputs to be signed at one time.
    • a maximum of 250 outputs per transaction is supported (will attempt more if memory allows)
  • mk4/Q:
    • we support PSBT files up to 2M bytes in size.
    • any number of inputs and outputs are supported, limited only by final transaction size (100k)
    • tested with: 250 inputs, 2000 outputs
  • bitcoin limits transactions to 100k, but there could be large input transactions inside the PSBT. Reduce this by using segwit signatures and provide only the individual UTXO ("out points").
  • every transaction needs to have at least one output, or we reject it

P2SH / Multisig

  • only one signature will be added per input. However, if needed the partly-signed PSBT can be given again, and the "next" leg will be signed.
  • we do not support PSBT combining or finalizing of transactions involving P2SH signatures (so the combine step must be off-device)
  • we can sign for P2SH and P2WSH addresses that represent multisig (M of N) but we cannot sign for non-standard scripts because we don't know how to present that to the user for approval.
  • during USB "show address" for multisig, we limit subkey paths to 16 levels deep (including master fingerprint)
  • max of 15 co-signers due to 520 byte script limitation in consensus layer with classic P2SH
  • (mk3) we have space for up to 8 M-of-3 wallets, or a single M-of-15 wallet. YMMV
  • only a single multisig wallet can be involved in a PSBT; can't sign inputs from two different multisig wallets at the same time.
  • we always store xpubs in BIP-32 format, although we can read SLIP132 format (Ypub/Zpub/etc)
  • change outputs (indicated with paths, scripts in output section) must correspond to the active multisig wallet, and cannot be used to describe an unrelated (multisig) wallet.
  • derivation path for each cosigner must be known and consistent with PSBT
  • XFP values (fingerprints) MUST be unique for each of the co-signers
  • multisig wallet name can only contain printable ASCII characters range(32, 127)
  • we support only BIP-67 (sorted multisig) wallets.

SIGHASH types

  • all sighash flags are supported:
    • ALL
    • NONE
    • SINGLE
    • ALL|ANYONECANPAY
    • NONE|ANYONECANPAY
    • SINGLE|ANYONECANPAY
  • any value other than ALL will cause a warning to be shown to user
  • by default, we reject NONE and NONE|ANYONECANPAY but there is a setting to allow

U2F Protocol / Web Access to USB / WebUSB

  • we do not support U2F protocol, WebUSB or any other means for random websites to talk to us
  • only native desktop/mobile apps, or helpers for those, will be able to talk USB to Coldcard

Fee Limits / Warnings

  • Coldcard will, by default, reject any txn that pays a fee of more than 10% of its total value to miners. This limit is a setting: 10% (default), 25%, 50% or 'no limit'.
  • Fees over 5% (was 1%) are shown as warnings.

Developer / Source Code

  • source code can probably only be compiled and developed on Mac OS and Linux
  • we have very limited time to support other devs getting their setups working

Change Outputs

We will summarize transaction outputs as "change" back into same wallet, however:

  • PSBT must specify BIP-32 path in corresponding output section for us to treat as change
  • for p2sh-wrapped segwit outputs, redeem script must be provided when needed
  • any incorrect values here are assumed to be fraud attempts, and are highlighted to user
  • the redeemScript for p2wsh-p2sh is optional, but if provided must be correct, ie: 0x00 + 0x20 + sha256(witnessScript)
  • the witnessScript in a p2wsh-p2sh is not optional.
  • depending on the address type of the output, different values are required in the corresponding output section, as follows
    • p2pkh: no redeemScript, no witnessScript
    • p2wpkh-p2sh: only redeemScript (which will be: 0x00 + 0x14 + HASH160(key))
    • p2wpkh: no redeemScript, no witnessScript
    • p2sh: redeemScript that contains the a multisig script, ending in 0xAE
    • p2wsh-p2sh: redeemScript (which is: 0x00 + 0x20 + sha256(witnessScript)), and witnessScript (which contains the multisig script)
    • p2wsh: only witnessScript (which contains the actual multisig script)

Derivation Paths

  • key derivatation paths must be 12 or less in depth (MAX_PATH_DEPTH)

NFC Feature

  • can share up to 8000 bytes of PSBT or signed transaction data.
  • NFC-V (ISO-15693) radio/modulation is common on mobile phones but very rare on desktops

Fast Wipe

  • each use of "fast wipe" feature consumes a MCU key slot, of which there are 256.
  • use Advanced > Danger Zone > MCU Key Slots to view usage

Trick Pins

  • "deltamode" PIN must be same length as true pin, and differ only in final 4 positions.
  • there are 14 trick "slots", but on mk4, we avoid slot 10, so 13 available. (Q: 14)
  • duress wallets consume 2 slots (or 3 slots for legacy duress wallet) which must be contiguous
  • when restoring trick pins from backup files, "forgotten" pins are not restored, and any trick pin which matches the true PIN of the restored system will be dropped
  • deltamode PIN requirements are checked during wallet restore, and if the new true PIN is not compatible, the deltamode trick PIN is dropped and not restored
  • duress wallets are supported when derived from 24- or 12-word seed phrases

Debug Serial Port

  • virtual USB serial port disabled completely by default, and even if enabled in Danger Zone, only echos output, and does not accept any input
  • use hardware serial port for interactive REPL access (3.3v TTL levels)

BBQr Scanning (Q)

  • files up to 2MiB in size can be accepted
  • when 14 or less parts, we display status of each part, if more, just percent complete

QR Scanning (Q)

  • if not BBQr (or sent as unicode inside BBQr) we auto detect these data types:
    • PSBT in Base64 or hex
    • Wire Transaction in Base64 or hex
    • XPRV, XPUB, bare payment addresses, BIP-21 invoices
    • seed words (truncated, or full)
    • SeedQR (but not Compact SeedQR)
  • for Base58, Bech32 encoded values, if checksum is wrong then it is shown as text
  • binary QR codes are not supported
  • when pasting into a secure note, if the QR's data is > 60 chars long, we assume done

Secure Notes & Passwords (Q)

  • when Q picks a password for you (using F1 thru F5), the entropy is 126 bits or more, except F3 which makes an easier to type password of around 48 bits entropy.
  • Title, Sitename and Username fields are limited to 32 characters in length.
  • Passwords are limited to 128 characters.
  • Note text is unlimited, but storing very large notes may affect performance system-wide.
  • Q Backup files restored onto Mk4 will lose their notes, since notes feature is not supported.

Address Ownership

  • only the first 1528 addresses (764 each from internal and external (change/not) paths) are considered
  • if you have used an sub-account, without ever exploring it in the Address Explorer, nor signing a PSBT with those account numbers, we do not search it.
  • does not search Seed Vault, you'll need to load each of those and re-search
  • if you have an XFP collision between multiple wallets in SeedVault (ie. two wallets with same descriptors, but different seeds) you will get false negatives