How a bug in Coldcard’s code went unnoticed for years, leading to $100 million in hacked funds

Hardware wallets are somewhere in between a paper wallet and a browser-based hot wallet. They’re harder to hack than software, harder to lose than paper, but they’re not infallible. They can be lost or stolen, and users need to be able to trust the device to create their keys properly in the first place.

“Air-gapped systems help, but they are not a perfect fix,” Bobby Gray, founder of TEXITcoin, told CoinDesk. “Security has to begin with how the keys are generated and continue through every part of the custody process.”

This is, unfortunately, where things went wrong for Coinkite, the maker of the Coldcard wallet.

A bug in the system

In March 2016, the Toronto-based bitcoin company told customers it was sunsetting its hosted hot wallet. Running an online financial services company had brought persistent floods of junk internet traffic aimed at knocking their services offline, along with mounting legal costs and regulatory complications.

Instead, Coinkite said it wanted to try something different. It wanted to build decentralized hardware and "software-not-as-a-service.” That was early in crypto’s history, before Bitcoin’s second halving, when one entire bitcoin was trading slightly above the $400 mark.

Coinkite’s pivot first produced Opendime in April 2016. The small USB stick generated and concealed a private key, allowing bitcoin to be passed from one person to another like a physical bearer instrument. Physically breaking the device's seal revealed the key and allowed the funds to be spent.