Bitcoin address scheme

Every Sui address has its own unique Hashi Bitcoin deposit address, this allows hashi a lightweight way to know which address on sui to credit for a deposit. All Hashi deposit addresses are P2TR (Pay-to-Taproot) where the 2-of-2 multisig script between Hashi and the Guardian is encoded as the sole leaf in the Taproot tree.

The exact descriptor is:

tr({i}, multi_a(2, {g}, {h}))

where:

• H is the base Hashi MPC public key and can be found on-chain.
• h = derive(H, d) – the child public key derived from H using derivation path d (the depositor’s Sui address)
• g is the guardian’s fixed public key
• i is the NUMS (nothing-up-my-sleeve) internal key defined in BIP-341 (50929b74c1a04954b78b4b6035e97a5e078a5a0f28ec96d547bfee9ace803ac0) with no known private key, ensuring all spends occur via the script path

The key derivation is not BIP-32. It is a purpose-built unhardened derivation over secp256k1, keyed by the Sui address, giving each depositor a unique Bitcoin address while the master signing key remains shared across the MPC committee.

Note: for devnet the deposit address omits the guardian key and uses a single-key script path:

tr({i}, pk({h}))