Prologue
So, a month ago, I was trying to find out which hardware wallet should I buy. My conditions were simple; It has to be open-source and I have to make the purchase in the most private way possible. My only option was to buy BitBox 1, but it happens to be old, deprecated and their developers aren't known for being privacy seekers, which really underwhelmed me.

My only choice was to purchase a hardware wallet outside my country using a poste restante, which I didn't want to do for personal reasons. But, then dkbit98 suggested something I hadn't thought of; do the job with a Pi.

Quote from: dkbit98 on April 29, 2022, 09:47:23 AM

I don't know if you want to use hardware wallet just for Bitcoin or for other shitcoins, but if it's only for good old BTC than you can buy Raspberry Pi zero locally and make your own SeedSigner signing device.

And so I did. I bought a RPi Zero, a camera, a little screen and a few other stuff, and built a SeedSigner; an air-gapped ~~hardware wallet~~ signing device, which takes security into the next level. Wink

Disclaimer: There's no affiliation with SeedSigner and this thread isn't sponsored. I just bought it and share my thoughts.

Review

In summary:

Pros:

Cons:

Little hard setup
Experimental software; the project is, well from what I can judge, in an early stage
Works only with BlueWallet, Nunchuk, Sparrow, Specter Desktop
It's forcing you to generate the entropy yourself

Alright, let's begin.

SeedSigner aims to give a solution to one problem; the cost and complexity of multi-sig usage. However, at the same time, it can be used for single-sig setups, lowering the cost of your "hot" storage as well. There's nothing saved inside the SD card, besides your settings which is optional, therefore there's no danger for funds' loss. When you shut down SeedSigner, it erases the seeds; they're meant to be kept temporarily in memory and you have to import the seed on each startup. And that's basically one of the features that makes SeedSigner differentiate.

This has the following advantage: You can have the device on plain sight (don't, but you get the idea). As far as I understand, this is implemented to reduce the risk of money loss. For cold storage, create the QR code and find a good hiding spot. For daily transactions, you can just insert it into your drawer etc., without minding much.

For quick imports, use QR scanning:

Unfortunately, you can't create a new seed with an internal RNG. SeedSigner somewhat forces you to generate the entropy yourself. Either with a picture or dice rolls.

I get the spirit of "trust none!", but that's just wrong. It should allow you generate random entropy, even with a warning. Furthermore, it gives a false sense of security. If you don't test the dice is decently fair, then you shouldn't generate a wallet. Period. Quoting a forum legendary is needed here:

Quote from: nullius on March 26, 2021, 05:05:11 AM

Coders who make their own ad hoc randomness schemes are like kids playing with matches.

Hashing a picture can also be problematic, see thread: Turn photos into Bitcoin wallets. So, here's a feedback: Include /dev/urandom. Simple. Do it for those who want to avoid this fuss.

This is how they justify it:

One thing I also don't understand is how the rolls are 50/99 exactly. Doesn't each give 1.66 bits of entropy on average?

These are the features:

Quote from: https://github.com/SeedSigner/seedsigner

Feature Highlights:

Calculate word 12/24 of a BIP39 seed phrase
Create a 24-word BIP39 seed phrase with 99 dice rolls
Create a 24-word BIP39 seed phrase by taking a digital photo
Temporarily store up to 3 seed phrases while device is powered
Guided interface to manually create a SeedQR for instant input (demo video here)
BIP39 passphrase / word 25 support
Native Segwit Multisig XPUB generation w/ QR display
Scan and parse transaction data from animated QR codes
Sign transactions & transfer XPUB data using animated QR codes (demo video here)
Live preview during photo-to-seed and QR scanning UX
Optimized seed word entry interface
Support for Bitcoin Mainnet & Testnet
Support for custom user-defined derivation paths
On-demand receive address verification
User-configurable QR code display density
Responsive, event-driven user interface

One thing I've forgotten to say is that, besides open-source, the code is also easy to read. It's 100% written in Python, and there aren't many files to check. It doesn't take more than an hour. The src/seedsigner/models is what's all about.

How to use it

These are the steps to spend money:

Create a seed.
Export the master public key with a QR code.
Import the master public key to a wallet software. (From the available, I prefer Sparrow*)
Create a transaction.
Export the PSBT in QR code from your computer's screen.
Scan the QR code from SeedSigner.
Sign the transaction from Seed Signer.
Export the signed transaction in QR code.
Scan the QR code from your computer.
Broadcast the signed transaction

Ta-da! Transaction signed in the air! Shocked

*Sparrow is a wallet I'd never used, as I put Electrum above others, but I'll have to admit it's good. Perhaps even better than Electrum. The reason you can't use Electrum (at least not easily) is because it doesn't support animated QR codes, which is the way SeedSigner exports xpub keys and signed transactions. That's because the screen is small.

No, these cool, freshly ~~printed~~ mined bitcoins in the images above aren't included in the SeedSigner Kit. Tongue