bitcoin-hardware-coldcard
Coldcard hardware wallets (Coinkite): Mk4, Q. Bitcoin-only firmware, airgap support via SD card / NFC / QR, BIP85, MuSig2 (experimental), Trick PINs. USE WHEN: integrating with Coldcard, supporting Mk4 / Q, designing airgap signing flows.
What this skill does
# Coldcard (Coinkite)
Bitcoin-only hardware wallet. Industry favorite for **airgapped**
signing. Models: Mk4, Q.
## Models
| Model | Year | Notes |
|-------|------|-------|
| Mk4 | 2022 | USB-C + microSD + NFC, no battery |
| Q | 2024 | Color touchscreen, USB-C, microSD, NFC, full keyboard, battery |
## Airgap support
Multiple transport options:
- **microSD card**: write PSBT to card, sign on device, write back.
- **NFC**: tap-to-transfer.
- **QR codes** (Q only): scan + display.
- **USB**: direct (less airgap).
## Bitcoin-only firmware
Coldcard intentionally supports ONLY Bitcoin. No altcoins. Smaller
attack surface, simpler mental model.
## Trick PINs
Defense against coercion:
- Set up multiple PINs.
- "Real" PIN unlocks main wallet.
- "Duress" PIN unlocks decoy wallet (real-looking but with different
seed).
- "Login countdown" — N attempts before wallet wipe.
## BIP85
Native support for deriving sub-seeds via BIP85. UI lets you generate:
- BIP39 mnemonic (any word count).
- HEX bytes.
- WIF private key.
- xprv.
## MuSig2 (experimental)
Mk4 has experimental MuSig2 support for cooperative Taproot signing.
Not yet standard across HW wallets.
## Backup
- Standard 24-word seed.
- "Tarot card" backup grids for resilient recovery.
- BIP85-derived sub-seeds.
- Encrypted backups to microSD with a separate backup password.
## API
`ckcc-protocol` Python:
```python
from ckcc_protocol.client import ColdcardDevice
dev = ColdcardDevice()
pk = dev.get_xpub("m/84'/0'/0'")
dev.sign_psbt(psbt_bytes)
```
## Connection
- USB-C.
- microSD (airgap).
- NFC (Mk4, Q).
- Q only: QR code.
Third-party: HWI, Sparrow, Specter, Electrum.
## Common issues
- microSD format must be FAT32 (max 32 GB).
- USB driver issues on some Linux distros.
- NFC on iPhone limited to read-only — use Android for tap-to-sign.
## See also
- [trezor/SKILL.md](../trezor/SKILL.md)
- [ledger/SKILL.md](../ledger/SKILL.md)
- [psbt-flows/SKILL.md](../psbt-flows/SKILL.md)
- [hwi/SKILL.md](../hwi/SKILL.md)
- [../wallets/entropy/SKILL.md](../../wallets/entropy/SKILL.md)
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