zAPPs
Apps that answer questions about Bitcoin from a proof. Each one hands you an answer and the proof behind it, and your browser checks the proof before it shows you anything. There is no server to trust, only the proof system and the published program.
What is a zAPP?
Today, to know something about Bitcoin for certain, you run a full node: download hundreds of gigabytes and check every block yourself. Anything less means asking someone else’s server, and taking its word.
A zAPP is a third way. It is a small app that answers one question about Bitcoin (is this the real chain of blocks? what does this address hold?) and gives you, with the answer, a proof that the answer follows from Bitcoin’s own rules. Your browser checks that proof in a fraction of a second. If the server lied, or made a mistake, the check fails and you are told so.
The proofs come from Hazync v2, which runs Bitcoin Core’s own consensus code inside a zero-knowledge virtual machine, so what a zAPP gives you is an answer you can use without trusting anyone.
The apps
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01
Live, follows the tip
Header chain
Every Bitcoin block header from the genesis block to the current tip, in one proof that they are valid under Bitcoin Core’s header rules. Your browser checks the whole chain in well under a second, without downloading the 78 MB of headers.
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02
Live, follows the board
Coin check
The balance and history of a Bitcoin script, answered with a short proof and checked against a proof of the chain from genesis. It reads the board’s latest such proof, so its height rises as the board proves more of the chain.
How they work, in more detail
Hazync’s engine, libzync, proves every block with Bitcoin Core’s own consensus code. Each proof also commits to a log of every coin the chain has created and a counting trie of what each script holds.
A zAPP asks one question of that record. The answer comes with a short path through the trie, and the page checks the path against the block proof on your device. A server that lies is caught by the check.