Coldcard

Coldcard is a Bitcoin-only hardware signing device manufactured by Coinkite, a company based in Toronto, Canada. It is designed to generate and store private keys and sign Bitcoin transactions while remaining disconnected from internet-connected computers, an approach commonly described as air-gapped operation. Across its model line, Coldcard can move signed transactions using microSD cards or, on the Coldcard Q, via QR codes and a built-in scanner, so the device never needs a live USB data connection to a host.

Open-source status

Coldcard's firmware is published under the MIT license and is intended to be reproducibly buildable, meaning users can compile the source themselves and compare the result against the released binary. Coinkite documents this verification process publicly. The device stores key material in two hardware secure elements sourced from different manufacturers (a Microchip ATECC608 and a Maxim DS28C36B), so that a flaw in any single chip does not, on its own, expose the seed.

Workflow and compatibility

Coldcard works with external coordinator software such as Sparrow and Electrum, exchanging unsigned and signed data as files or QR codes. It supports BIP-39 passphrases, multisig configurations, and PSBT-based signing, which lets it serve as one cosigner in a multi-key setup without ever holding the full wallet online.

For related concepts, see air-gapped signing and seed phrase.