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

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

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 can do only traditional BIP32, p2pkh wallets (not bech32, nor segwit)
  • because their export/import of unsigned txn is limited
  • this will improve in time as they make updates
• with Bitcoin Core (version 0.17?), we can do PSBT transactions, which support everything
• we don't support coinbase transactions, so don't mine directly into a Coldcard wallet

Max Transaction Size

• we support transactions up to 384k-bytes in size when serialized into PSBT format
• 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

• each Coldcard can only be a single "leg" of the multisig
• we do not support PSBT combining of transactions involving P2SH signatures (but you can do your own combine step off-device) [This might be handled in future versions, but low priority for now.]
• Electrum plugin does not support multisig at this time
• no support for multisig signing yet, but will in a future version.

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 reject any txn that pays a fee of more than 10% of it's total value to miners. (Might become a setting someday.)

Developer / Source Code

• source code can probably only be compiled and developed on Mac OS
• we have very limited time support other devs geting 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