I built an offline BIP-39 seed generator using coin flips, dice and Von Neumann debiasing

I built NYENYC (Not Your Entropy Not Your Coins), a small open-source, completely offline device that generates a standard 24-word BIP-39 mnemonic from entropy supplied entirely by the user.

The device itself generates no entropy. It is basically a glorified BIP-39 calculator: you give it coin flips or dice rolls, it gives you the corresponding 24 words.

It just does all the heavy lifting for you: it collects the bits from coin flips or dice rolls, computes the checksum and converts the 11*24=264 bits into 24 BIP-39 words.

Why?

Because when a hardware wallet generates a seed internally, at some point you have to trust its entropy generation chain.

You can have a perfectly good hardware TRNG, but you still have to trust that the firmware actually uses it correctly when generating the seed. A bug anywhere in that path can make the hardware RNG irrelevant. The recent Coldcard entropy issue is a good example of this failure mode.

With NYENYC the entropy source is outside the device and visible to you.

The part I particularly like is the Von Neumann coin mode, in which you do not need a perfectly fair coin to get unbiased output bits.

Under the normal assumption that flips are independent and the coin bias stays the same, the output is mathematically unbiased even if the physical coin itself is biased.

https://en.wikipedia.org/wiki/Fair_coin

So you can take a completely ordinary coin and keep flipping until the device has collected 256 unbiased bits.

There are also plain coin-flip and dice modes.

The hardware is intentionally stupid: PIC18F47K42, 2×16 LCD and two buttons. No USB, no networking and no persistent storage. The generated seed only exists in RAM and disappears when the device is powered off.

Everything is open source, hardware and firmware:

https://github.com/gianlucag/NYENYC

Any feedback is very welcome! And please don't judge my soldering skills :-)