YubiShard — split a wallet seed across YubiKeys

I built an open source tool called YubiShard, for a problem in self-custody that has bothered me for a long time.

A recovery phrase is a single object, and that's the problem. Every copy you make so you don't *lose* it is another copy someone can *steal*.

YubiShard splits it instead. It turns a wallet recovery phrase into standard SLIP-39 shares and writes one share onto each YubiKey, using the WebAuthn largeBlob extension. Any threshold of the keys rebuilds the phrase — 3 of 5, say. Fewer reveal nothing at all.

Splitting a seed isn't new. Putting the shares somewhere unreadable is the part that was missing. A share on paper can be read by whoever finds it and photographed in a second. A share on a YubiKey sits behind the key's PIN, and eight wrong attempts wipe it.

Three things I'd defend:

→ The shares are ordinary SLIP-39. Trezor, Electrum, Sparrow and BlueWallet read them, so you're not locked into this tool.

→ No dependencies, no build step, no minification — three hand-written files. What ships is exactly what you read. And after the page loads it cannot make a network request at all: no fetch, no XHR, no WebSocket, no storage, no analytics. That's a one-minute check with grep.

→ The crypto runs against all 45 official SLIP-39 vectors on every push, cross-checked against Trezor's own decoder.

What it doesn't do: The PIN is the boundary. Anyone holding enough keys *and* their PINs has your wallet.

Open source, MIT. Chrome, YubiKey firmware 5.7+. No hardware? Chrome's virtual authenticator runs the whole flow for demo.

Online: https://yubishard.com/

Repo: https://github.com/hosseinpro/yubishard