Introduction to Hashi

Bitcoin holds more value than any other crypto asset, and almost none of it does anything. Bitcoin script cannot express lending, order books, or collateral. So BTC either sits still or leaves for a chain that can — historically through a **custodian** who holds the real coins and issues a receipt. Hashi is Mysten Labs' answer: put BTC to work on Sui while the coins stay on Bitcoin.

wBTC and tBTC are **IOUs** — a custodian or a signer set holds your BTC and mints a token that represents a claim on it. Hashi is different in one decisive way: your BTC never leaves Bitcoin. It sits at a Bitcoin address that only a Sui validator committee plus an independent Guardian can jointly spend, and Sui mints hBTC against it 1:1.

hBTC is a normal Sui coin with **8 decimals** — the same precision as Bitcoin, so one unit is one satoshi. Every hBTC in existence is backed by a confirmed Bitcoin UTXO the protocol controls. The SDK speaks in `bigint` satoshis end to end and never in floats, because a rounding error here is lost money.

Hashi does not use one shared vault address. Every Sui address gets its **own** Bitcoin deposit address, derived deterministically from the committee's MPC key and the Guardian key. On testnet it is a bech32m Taproot address starting `tb1p`. Derivation is pure math — the SDK computes it locally and it matches what the validators compute.

The first signer is not a company — it is the Sui validator set. Validators register into a committee whose voting weight *is* their Sui stake weight, and they act only by aggregate BLS certificate carrying more than **two thirds** of that weight. Approving a deposit or a withdrawal is the same kind of quorum act that finalizes a Sui block.

The second Taproot leaf belongs to an independent **Guardian** whose Bitcoin key is written on-chain once and is immutable after that. Moving BTC out needs the committee *and* the Guardian: a real 2-of-2, not a rubber stamp. And nowhere in the protocol is there an owner capability — pauses, config changes, even package upgrades all go through committee-weighted voting.

Getting hBTC takes four moves. You send BTC to your Taproot address on Bitcoin. Someone records that UTXO on Sui with `deposit` — permissionless, so an app can relay it for you. The committee certifies it with `approve_deposit`. Then `confirm_deposit`, also permissionless, mints hBTC to the recipient. No protocol deposit fee: the full UTXO amount is minted.

Two timers gate the mint. Bitcoin must bury the funding transaction under **6 confirmations**, roughly an hour. Then a **10-minute delay window** runs after committee approval before `confirm_deposit` can mint. That window is the fraud-catch: if a bad approval ever landed, there is time to notice and pause before hBTC exists. Roughly seventy minutes end to end.

The way out mirrors the way in. `request_withdrawal` escrows your hBTC and names a Bitcoin address. The committee approves, the commit step burns the hBTC, MPC and Guardian sign the Bitcoin transaction, and finalize broadcasts it. Same 30,000-sat minimum. You pay no protocol fee, but the Bitcoin miner fee comes out of your output — and that one can never be sponsored.