To initiate bridging from Bitcoin to Avalanche, a transaction is sent on Bitcoin from the new Core Wallet that 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
For a more in-depth read on how the Bitcoin Bridge works, please read the following articles.