To initiate bridging from Bitcoin to Avalanche®, a transaction is sent on Bitcoin from Core transfers BTC to the bridge address controlled only by the SGX enclave. The bridge is designed to only support Pay-To-Witness-Public-Key-Hash output types on Bitcoin. In order to ensure that the BTC transfer has this output type and meets all of the bridging requirements, users should always use the bridge UI in the Core Wallet extension that is built to provide a quick and easy bridging experience. Once this transaction is accepted into a block on Bitcoin and receives the necessary number of confirmations, it will be indexed by the Bridge Nodes and reported to the Intel SGX application for processing. All transfers across the Bridge must be approved by Bridge Nodes that secure and back-up an encrypted piece of the Bridge’s keys. At this point, the SGX application mints (creates) the equivalent amount of BTC.b to the user wallet that sent the initiating Bitcoin transaction.
Similarly, when moving BTC.b from Avalanche back to Bitcoin, the user sends a transaction on Avalanche that calls the “unwrap” method of the BTC.b contract, which burns (destroys) the BTC.b tokens. The Bridge Nodes will index this transaction, and it will be processed by the SGX enclave by sending the equivalent amount of native BTC back to the user’s wallet on Bitcoin
When moving assets from Bitcoin to Avalanche, the bridge fee is currently $3 worth of Bitcoin at the time the transfer is processed. When moving assets from Avalanche to Bitcoin, the bridge fee is $20 plus the expected Bitcoin transaction fee. The expected transaction fee varies with the current Bitcoin fee rate and the size of the transaction but is typically less than $2. You can find more information regarding the expected Bitcoin transaction fee in the "Bridge Fees" section here.
When Bitcoin is bridged to Avalanche, it is represented by an ERC20 token on the Avalanche C-chain with the symbol BTC.b. The “.b” indicates that the token was bridged from the Bitcoin network, just as Avalanche Bridge™ tokens bridged from the Ethereum end in “.e”. Each BTC.b token is fully backed by a Bitcoin locked by the bridge enclave application. The BTC.b token contract can be found on Snowtrace here, and the bridge enclave application Bitcoin address can be viewed here.
For a more in-depth read on how the Avalanche Bridge works, please read the following articles.
For any additional questions, please view our other knowledge base articles or contact a support team member via the chat button.