Internet Computer contracts can hold and spend Bitcoin without a bridge

What Chain Fusion Does

Smart contracts on @dfinity's Internet Computer, known as canisters, can now control Bitcoin, Ethereum, and Solana addresses outright, signing and submitting transactions with no bridge, wrapped asset, or oracle involved. The protocol calls this design Chain Fusion.

Chain Fusion on ICP refers to the ability of canister smart contracts to sign transactions and interact bi-directionally with other blockchains without relying on a single point of trust. ICP's Bitcoin integration lets canisters hold Bitcoin addresses, query balances and UTXOs, and sign and broadcast Bitcoin transactions, all without bridges or custodians.

Where previous cross-chain solutions have relied heavily on bridges, oracles, and custodial services, involving single points of failure, the Internet Computer enables blockchain integrations with the strongest possible trust properties thanks to the absence of additional trusted parties.

How the Key Never Gets Assembled

The core mechanism is threshold signing: a private key that is never fully assembled in one place. ICP features a suite of threshold signing protocols, secure mechanisms where private keys are generated in shares across nodes of a signing subset. Upon a canister transaction request, the nodes use their shares to sign Bitcoin transactions jointly rather than recreating the original private key. This signing protocol assumes that more than two thirds of the nodes are honest, and less than one third compromised. Canisters can request a signature but never access the underlying key material.

Using chain-key signatures, canister smart contracts obtain control over cryptographic keys: they have a public key, from which a blockchain address on another blockchain like Bitcoin or Ethereum can be derived, and they can sign transactions relative to that address.

Spending $BTC still follows Bitcoin's own rules. Bitcoin uses an unspent transaction output (UTXO) model, which requires one signature for every coin fragment used as an input. ICP supports signature schemes compatible with widely used blockchains, including Bitcoin, Ethereum, Solana and more than twenty other chains, meaning $ICP's derived keys conform to the same standards that existing Bitcoin and Ethereum wallets already use. The introduction of threshold Schnorr signing further enables support for Bitcoin Taproot transactions on ICP, as well as direct integration with other chains.

The practical upshot is that developers can write smart contract logic on the Internet Computer that moves real $BTC on the Bitcoin base layer, without wrapping it or routing it through a third-party custodian.

Sources
Internet Computer: Chain-key signatures
ICP Developer Docs: Chain Fusion
DFINITY: ICP Further Advances Chain Fusion With Latest Bitcoin Milestone