Physical Security Notes

Defence in Depth

Defence in Depth means multi-layer security, many layers and many types of protection. We mention defence in depth and physical security frequently because it is as important to us as it is to you.

You're probably aware of COLDCARD®'s layers like encryption, air gapping, and verifiable software (if not, please browse our documentation). In this section, we focus on COLDCARD's physical security features:

Security is in the Bag

The first line of defence is the tamper-evident bag with its colourful and detailed printing, bar code, and water damage indicator. Our Quickstart Guide covers the bag-opening process for new customers.

Bag Number

One of the most important features is the bag number. The factory records this number in a secure location within the COLDCARD's flash memory. So, there is an actual link between a COLDCARD and its bag.

The bag number appears three times: ① in the blue border at the top of the bag, ② with a bar code under the orange Coinkite® text, and ③ inside the bag on a torn-off tab. All three instances of the bag number match.

Mk3 COLDCARD Bag Number Locations

Tear-Off Tab

The tear-off tab inside is another way to confirm factory freshness. The numbered tab gets removed from the top of the bag and sealed inside before shipping. Put the two together and inspect them. You should see the tear-off tab matches the top of the bag.

Mk3 COLDCARD Tear-Off Tab Matched with Bag


Pulling apart the bag reveals the VOID seal. If you open the bag and don't see the words VOID and OPEN (for example, you cut the bag instead of pulling it apart), you can still make sure the VOID seal is present and genuine by locating the layers and pulling them apart.

Mk3 Bag V2 VOID Seal

Mk3 Bag V2 VOID Seal Layers

NOTE: Tweezers for demonstration purposes only. Tweezers won't pull the layers apart very well.

Water Damage Indicator

If the bag has been compromised and exposed to water, the thin white stripe below the VOID seal will wash away, releasing a pale blue dye.

Compromised Mk3 Bag V2 Dipped in Water

The left side shows where the water has reached; the right side shows the white stripe intact:

Compromised Mk3 Bag V2 Water Damage Indicated

A Clear Case for Security

Malicious hardware has nowhere to hide. COLDCARDs have see-through cases making physical attempts to alter the hardware glaringly obvious. The case material is tough, impact-resistant plastic. Even if you accidentally drop your COLDCARD, you're still hodling tight and worry-free.




Attackers discouraged from implanting hardware may consider the firmware route to compromise a COLDCARD. Unfortunately for them, the factory signs every firmware release, and the device looks for this signature on startup.

COLDCARD Caution LED Glowing Red

Each time a COLDCARD connects to power, it verifies the firmware signature and checks all data stored in its flash memory. The red CAUTION LED is lit during this process while the screen displays the verification notification message. Once the signature and the flash memory are verified, the green GENUINE LED indicates it is safe to enter your PIN. If the firmware is not factory-signed, the red CAUTION LED stays lit as a warning not to enter your PIN.

These LEDs also have a role in the firmware upgrade process, which you can read about here.

Glorious Globs of Epoxy

The epoxy inside a COLDCARD isn't there just for looks. It defends your secrets in several ways by covering critical components like:

  • The MCU
  • The Secure Element
  • The LED control circuit

We've used a special case to give you a clearer look at our epoxy.

On the back of the case, you'll find our most apparent application of epoxy, which covers the MCU and bonds it to the case. This bond ensures splitting the case apart causes visible and functional damage.

Close-Up of Epoxy Covering COLDCARD MCU

Flip the COLDCARD over and between the GENUINE and CAUTION LEDs is the second most visible dab of epoxy, the one covering the Secure Element.

Close-Up of Epoxy Covering COLDCARD Secure Element

As for the LEDs, epoxy also covers their control circuit. Getting through the epoxy to tamper with the circuit is easily detected.

Each dab of epoxy is unique, like a fingerprint. A user familiar with the appearance of their COLDCARD's epoxy can treat the epoxy as yet another identifier if their device gets secretly swapped.

In short, the epoxy globs:

  • Protect the case from being opened without notice
  • Can act as an identifying trait of a particular COLDCARD
  • Prevent the LED circuit from being altered unseen
  • Make it exceptionally difficult to interfere with the connections between components
  • Complicate any attempts to modify, remove, or swap out the protected parts

One Tough Chip: The Secure Element

You might not think of physical security as part of a chip's job, but the Secure Element's contribution to the cause is much deeper than the epoxy covering it.

Secure Element defence highlights:

  • Provides dedicated, purpose-built secure storage for your seed words, physically separated from the MCU
  • Controls the LEDs so malicious software can't change their behaviour
  • Removes the possibility of resetting the device due to its heavy involvement in COLDCARD operations

It's even willing to take a bullet to protect your secrets. We've marked where to aim - just in case.

To learn more about secure elements, checkout our substack article: Understanding Mk4 Security Model

Tamper-Evidence Security Bag

Our white tamper-evident security bag (sold seperately) is designed to keep your ColdCard device and other items safe during storage or transportation. While the bag itself can't physically deter someone from tampering with the contents, it's shows clearly whether the contents have been exposed or not due to the VOID watermark. Once the bag is opened, there's no way to reseal it.

Bag Image 1

How To Use:

  • Gather up all the items you want to keep safe and proceed to put them inside the bag.
  • Rip the perforated tab at the top of the bag. Keep the tab with you in a safe and seperate location from the bag or put it in the bag for future refference. When opening the bag in the future, make sure to compare the number on the bag with the number on the tab same confirm they're identical.
  • Now it's time to seal the bag; pull the front flap down and peel the white plastic behind the blue tab to expose the adheasive. Push the blue tab to the back of the flap to seal the bag.

Bag Image 2

Opening The Bag:

  • Tear the bag open where the top blue strip is to expose the VOID watermark. You now know someone has tampered with your bag and it is unsealed.
  • Confirm the bag number is the same as the number written on the perforated tab.
  • If you want to reseal your items once more, make sure to use a new tamper-evident security bag.

Checkout our Tamper-Evident Security Bag Youtube tutorial here: Coinkite Security Bag Guide

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