Keystone 3 Pro Review: The Air-Gapped Signer Built for MetaMask

A hardware wallet that talks to your phone over Bluetooth or plugs in over USB is convenient right up until that connection becomes the attack surface. Keystone built the 3 Pro around the opposite bet: strip every wireless and data cable out, and make the resulting inconvenience small enough that people actually use it.

Why the QR code matters more than the touchscreen

Most air-gapped wallets that claim "no internet connection" still accept data over USB, which some security researchers treat as an attack surface worth worrying about even when the wallet firmware is careful. The 3 Pro removes that path entirely: the charging port carries power only, there's no Bluetooth radio and no NFC chip, and every transaction crosses the gap as a QR code shown on its 4-inch touchscreen and read by your phone's camera. That single design choice is why Keystone gets called a genuine air-gapped wallet rather than a marketing one.

Three chips, not one

Coin Bureau's teardown names the exact parts: a Microchip ATECC608B alongside two Maxim chips, the DS28S60 and the MAX32520, working together to isolate your recovery phrase and fingerprint template from each other. Two of the three reportedly carry CC EAL5+ certification, the same tier used in passport chips and payment cards, which is a meaningfully higher bar than a single secure element design. If the device is opened, Keystone's anti-tamper circuitry is built to wipe stored data rather than let an attacker probe the chips directly.

Open source, with an asterisk

Keystone markets itself on transparency, and a lot of that claim holds up: the QR transmission protocol and much of the firmware sit on GitHub for anyone to read or fork. Read Keystone's own blog post on the subject, though, and the caveat is stated plainly: the core MCU library and the secure element firmware itself are not published. That's a common compromise among hardware wallet makers protecting anti-cloning logic, but it means "open source" here covers most of the stack, not literally every line running on the device.

Pairing with MetaMask

Keystone positions itself as MetaMask's official partner cold wallet, and the pairing flow backs that up: open MetaMask, choose "Connect Hardware Wallet," and scan the QR code the 3 Pro displays. From there, every transaction MetaMask wants signed shows up on the Keystone screen in full, not as a truncated hash, so you can actually read the contract call or the recipient address before approving it. With 45 or more supported wallets and coverage across more than 200 chains, the same flow extends to Solflare, Keplr and OKX Wallet for anyone spreading assets across ecosystems MetaMask doesn't reach.