Mycelium Entropy
User Manual

Your cute little new device provides a very secure way to print paper wallets. However, the security of a system is—​at best—​only as good as its weakest link. So please observe the following words of caution before you begin:

If you wish to create a new paper wallet, push the button on your Entropy device, and wait a few seconds for your printer to detect a new image.

To create a 2-of-3 split key wallet, briefly press the button right after inserting the device into your printer. This will create a Bitcoin address with a private key that is split into 3 parts, requiring a combination of any 2 of them to be able to spend from it. To make another one, double-click the button. You can use Mycelium Wallet’s Cold Storage function to spend from these wallets.

A number of printers have been tested for compatibility. They are listed in Appendix A: Printers.

For technical information about Mycelium Entropy and why it is secure, please see Appendix B: How It Works and the source code.

Mycelium Entropy can split a generated private key into three parts in such a way that:

This method is commonly known as Shamir’s 2-of-3 Secret Sharing Scheme. It is implemented according to the specification at github.com/cetuscetus/btctool/blob/bip/bip-xxxx.mediawiki. Funds from a split wallet can be spent with Mycelium Bitcoin Wallet. The private key can also be recovered from shares with this tool: mycelium.com/assets/entropy/checksalt/sss.html.

Mycelium Entropy does not currently support this scheme with Hierarchical Deterministic wallets.

An ordinary application running on a regular computer can read files from a USB drive and transmit them over the Internet. Such an application need not look like a “virus”, and the computer need not be “infected” for that to happen. That’s why it is highly inadvisable to print Mycelium Entropy wallets intended for any serious amounts through a computer—​unless that computer will never be connected to the Internet, will always be physically secure, and you know what you are doing.

Please print your wallets directly on a printer from your Entropy.

It is, of course, perfectly safe to plug Mycelium Entropy into a computer. While the wallets generated this way should be considered compromised and should only be used for experiments, the device itself is not under any threat. Even a computer full of malware cannot compromise any previous or future wallets from this device. There are certain simple precautions that must be followed in the Configuration and Firmware Update mode; they are described in the corresponding section of this manual.

However, please keep in mind that some printers are more secure than others. The following advice is taken from a report by luckdragon69.

Pretty much any home/personal printer will not have a hard drive, but most will have some kind of memory installed. The type of printer, as well as the model, will determine how much, if any, memory is installed.

Most memory that is in home/personal printers only hold the data for the current print job from anywhere from a few lines to a few pages, as the job is being printed. Once the job is complete or the printer is turned off, any data that was in memory is erased and unrecoverable. Printers commonly use basic RAM memory, which is commonly referred to as volatile memory since it cannot store data once power is removed.

Large office printers usually have a hard disk that may keep either a full copy of everything they print, or metadata, containing the filename and the user who created the print job.

Certain features, such as other coin types or Hierarchical Deterministic wallets, can be enabled and configured by placing Mycelium Entropy into Configuration mode:

This procedure is also described in the readme.txt file on the ME-CONFIG disk. The settings.txt file is self-documented so you can tweak the configuration without access to this manual.

  Windows may occasionally suggest checking the disk for errors. This is normal, and does not mean there is a problem with the device. You may let it check it, or leave it as is.
If something more serious is reported, such as the disk being broken, a possible cause can be that the device was removed without clicking the button while it was in the Configuration mode. To fix this, either erase the first block of the disk, or format it using your operating system. The configuration will revert to factory defaults.

To update the device’s firmware, enter Configuration mode as described in Configuration section above. Instead of steps 4 and 5, copy the new firmware image to ME-CONFIG. Follow the rest of the procedure from step 6.

If the LED turns off after a few seconds of fast blinking, the update has been successful. If it indicates an error, the current firmware has remained intact.

For security reasons, Mycelium Entropy only accepts firmware images that have been cryptographically signed. By default, only Mycelium signatures are trusted. Should you desire to flash your own firmware, or third party firmware that you trust, you must add the corresponding public key to settings.txt. Even then, your device will request confirmation from you with a 4-blink warning code. Press the button on the device to authorise the update.

This extra confirmation is a necessary protection measure against malware. A malicious program can quietly copy rigged firmware to the device and sneak its signing key into settings.txt, but it cannot press the button for you.

Please feel free to move your mouse over or touch the table below to see how Mycelium Entropy displays these messages.

Always treat your Mycelium Entropy with care. It can be damaged if dropped, sat on, bent, crushed, chewed, punctured, or if it is exposed to liquid, corrosive chemicals, strong microwave radiation, or extreme temperatures, below −20ºC (−4ºF) or above +80ºC (176ºF).

Do not leave your device within reach of small children. It constitutes a choking hazard and has sharp edges and corners that may cause an injury.

Never force the device’s connector into a port or apply excessive pressure to the device’s button, as this may cause damage to the device and surrounding objects. Keep in mind that the quantum nature of USB devices requires you to spin a device three times before it will fit. Take particular care around the device when it is plugged in to avoid accidental mechanical damage.

This device is not intended for use in safety-critical environments, or where its failure could lead to death, personal injury, or collapse of the Universe.

Mycelium Entropy runs Free Software. Its source code together with build instructions is available here: github.com/mycelium-com/entropy.

Your device contains software distributed under the following conditions.

The following table lists printers that have been tested with Entropy. The prices were updated in February 2015. Please also see the Security section for general information about selecting a printer.

Mycelium Entropy uses one primary and two secondary sources of entropy. The raw sources are mixed and entropy is extracted using the SHA-512 cryptographic hash function. The results are rendered to a JPEG file, which is generated on demand. All sensitive information, such as keys and JPEG contents, is kept in volatile memory and is wiped out within seven seconds of the device’s removal from the USB port.

Copyright © 2015 Mycelium SA, Luxembourg. All rights reserved.