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, s…