Unstoppable JPGs: Bitcoin Private Keys as Image Storage

BitMEX Research revisits the debate over OP_RETURN policy limits after Bitcoin Core removed the size cap in June 2025, sparking concerns about onchain JPG spam. With around 17% of nodes running the filtering client Bitcoin Knots, the cat-and-mouse battle between spammers and developers continues under the lens of information theory. The report demonstrates how images can be encoded in fake Bitcoin addresses (outputs that can never be spent) and why filters cannot fully prevent this. Pushing the concept further, researchers show that private keys—256-bit random data—can carry Unstoppable JPGs without breaking Bitcoin’s protocol, by embedding tiny black-and-white images or even larger files split across multiple keys. They build a 15-of-15 P2SH transaction with weakly signed inputs, making 14 private keys publicly reconstructable to reveal a hidden JPEG logo. Despite theoretical proposals to ban fake addresses or require output signatures, such changes would demand a disruptive hard fork, break wallet compatibility, and still leave room for new spam techniques. The Unstoppable JPGs case underscores that fee markets, not filters, remain the most practical defense against blockchain spam.