Secure
Your Bitcoin

Q Mk4

Dual Secure Element for Key Storage

We find it quite scary that some hardware wallets trust the main microprocessor with their most valuable secrets. Instead, COLDCARD uses two Secure Elements, from different vendors, to protect your Bitcoin.

Specifically, the COLDCARD (Mk4) uses Microchip's ATECC608B and Maxim's DS28C36B, to store the critical master secret: the 24-word seed phrase for your BIP39 wallet.

Q Mk4

Genuine vs. Caution Lights

To resist Evil Maids, and other sneaky people with physical access to your COLDCARD, we sign our firmware with a factory key. During boot-up, the firmware's signature, and nearly every byte of flash memory, will be verified and the appropriate Green/Red light set.

Changing that light's status is actually controlled by dedicated circuitry connected directly to a Secure Element, so a rogue bit of software cannot override it. The circuit for the lights is exposed on the top surface of the product, so any physical tampering by those maids will be visible as well.

Q Mk4

Anti-phishing Words

The PIN code on COLDCARD is divided into two parts, such as 1234-5678. You first enter 1234 and then you will be shown two words on-screen. Those words are unique for all PIN prefixes, and for each COLDCARD ever made. (The secrets used to enforce that come from inside the secure element, and are unknown to the rest of the world.)

Your job is to memorize those two words, keep them secret, and every time you use the COLDCARD, check them before entering the final 5678 part of your PIN. This protects you against a trojan-horse COLDCARD that might look like yours but it cannot know those two words.

Q Mk4

Physical Security

The carefully designed PCB increases the SE probing difficulty. Our clear case is part of our security model too, so you can look and see if a "hardware implant" has been inserted inside your device.

Because of the in-depth use of the secure elements, there is no "factory reset" for the COLDCARD. If you forget your COLDCARD PIN, there is nothing we can do except remind you to recycle your e-waste responsibly!

We've even put a label, "SHOOT THESE", for more effective device destruction... When the time comes.

Q Mk4

Air Gap Operation

COLDCARD never needs to touch a computer. It can work entirely from a USB power pack or AC power adapter. This includes everything you need to do in the whole life of the product:

• Initial PIN choosing and setup.
• Pick your 24-seed words using our TRNG, import existing secrets, or use your dice rolls.
• Export skeleton wallet files, for setup of Electrum or other desktop/mobile wallets.
• Export lists of payment (deposit) address, using the Address Explorer.
• Backup of seed and settings, which saves an encrypted 7z file.
• Sign transactions for spending your Bitcoin, using PSBT files (BIP174) from any standards-compliant wallet
• Firmware upgrades
• Advanced users can even setup a multisig wallet between multiple cosigners, entirely on-device, and air gapped

Using our industrial grade MicroSD Cards or any standard MicroSD card, for each of the above steps that require data to come in and out. Sneakernet for the win! If you want to reach the next level paranoia, you can use different cards for the data coming into versus out of the COLDCARD, and/or use cards a single time only.

Q Mk4

Dice Roll and Provable Bitcoin Seed Generation

If you don't trust our random number generator, you can generate the BIP39 seed phrase using dice rolls. We help with this process: you just have to press 1–6 for each roll (99 rolls recommended). At the end of that process, you'll have a properly-encoded seed phrase based solely on the dice rolls. Learn how to verify COLDCARD's dice-rolls math here.