firmware/docs/limitations.md

BIP39 Import

• there must be 12, 16 or 24 words in your mnemonic
• we have only the English word list
• we do not support BIP39 passwords during import (full support added in version 2.0.0)

XPRV Import

• we can import a BIP32 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:
  • 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").

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 BIP32 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
• fixed: XFP values (fingerprints) for each of the co-signers must be unique (limitation removed)

SIGHASH types

• only SIGHASH_ALL is supported at this time
• in time, we will add support for others, especially to support Coinjoin usage

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

Policy Stuff

• 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 hide transaction outputs if they are "change" back into same wallet, however:

• PSBT must specify BIP32 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 (Mk4)

• 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 (Mk4)

• 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 (Mk4)

• "deltamode" PIN must be same length as true pin, and differ only in final 4 positions.
• there are 14 trick "slots"
• duress wallets consume 2 slots (or 3 slots for legacy duress wallet)
• 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 (Mk4)

• 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)