A white-hat hacker who moved about 4,000 BTC from Liquid Network (a Bitcoin sidechain) has communicated with Blockstream through OP_RETURN (embedded blockchain data) messages and PGP-encrypted messages. The hacker has promised to return most of the Bitcoin after the vulnerability is fixed, according to on-chain analyst Alex Thorn. The exchanges mark an unusual effort to negotiate the return of funds while a security flaw remains unresolved.