StarkWare's transaction is nonstandard under Bitcoin Core's relay rules, so MARA had to mine it directly. Hours of computation and up to $200 went into moving 10,000 satoshis.
StarkWare said on Wednesday that a Bitcoin transaction built to survive a quantum computer had been confirmed on mainnet, in block 964,199. It moved a 10,000-satoshi output, worth roughly $8 with bitcoin trading near $79,000. No ordinary Bitcoin node would relay it, so the transaction was handed directly to a mining pool.
The vulnerability the work addresses is narrow and well understood. Bitcoin addresses are protected by elliptic-curve cryptography, and the public key stays hidden until the moment funds are spent. Once a transaction is broadcast, that key is exposed and the coins sit unconfirmed in the mempool for as long as it takes a miner to include them. In March, Google researchers estimated that a sufficiently capable quantum computer could derive a private key nine to 12 minutes after the public key became visible, which is comfortably inside the window in which a pending transaction can be replaced by an attacker's own.
Quantum Safe Bitcoin, the scheme devised by StarkWare researcher Avihu Levy, closes that window without changing any consensus rule. According to Levy's paper and code repository, QSB combines hash-based one-time signatures with computational searches that bind an authorisation to one specific transaction. The construction is meant to hold even if elliptic-curve cryptography falls, because the security rests on hash functions, which the algorithm that breaks elliptic curves does not break.
It is deliberately, structurally expensive. The signature is not simply computed; it is searched for, with software grinding through candidates until one turns up in a form that does not leak the material an attacker would need. Levy put the cost at $75 to $150 of GPU computation when he published the method in April. StarkWare spokesperson Nathan Jeffay told Cointelegraph the completed transaction ran to "low hundreds of dollars," around $150 to $200, and the company said the process took hours.
The harder problem turned out to be getting the thing onto the chain at all. Levy's repository classifies QSB transactions as nonstandard under Bitcoin Core's default relay policies, which are separate from consensus rules and considerably stricter. Consensus decides what a block may legally contain. Relay policy decides what a node will pass to its peers, and nodes running default settings simply drop anything they do not recognise.
StarkWare put it plainly. "Bitcoin's network only passes along transactions written in the standard formats it recognizes, and this one is unusual enough that ordinary nodes would simply ignore it," the company said. "So instead of going into the public queue, it had to be handed straight to a miner willing to accept it." MARA Pool mined the block after receiving the transaction through Slipstream, the service MARA operates for transactions the public network will not carry.
Quantum-resistant spending is therefore available on Bitcoin today, to anyone who can afford hours of computation, a few hundred dollars per transaction, and a working relationship with a mining pool prepared to take the submission. For a custodian moving a large cold-storage balance under threat, that is a usable escape hatch. For everyone else it is a demonstration, and one whose safety depends on the continued cooperation of a small number of pools. A defence that routes through private channels to a handful of operators is not a property of the network. It is a favour.
Levy has always framed it that way, calling QSB a last-resort measure rather than a substitute for protocol-level protection, and StarkWare CEO Eli Ben-Sasson said the same on Wednesday. "A soft fork should happen, and I believe it will," Ben-Sasson said, describing the scheme as a safety net while permanent protections are developed. He has used a blunter image before. "I've said before that crypto is behaving like passengers on the Titanic. What Avihu has illustrated is that there are lifeboats. That is not a reason to relax. It is a reason to build more of them, and to build them now."
The protocol-level answer under discussion is BIP-360, a proposed soft fork that would add a Pay-to-Merkle-Root output type and strip out the quantum-vulnerable key-path spend that Taproot allows. The cryptography there is not the obstacle. Activation is. Bitcoin's recent record on coordinated upgrades is not encouraging: the enforcing chain behind BIP-110 produced two blocks in eight hours when it went live, a reminder that a soft fork with rough consensus on paper can still find almost no hashpower behind it in practice.
Money is arriving faster than agreement. Galaxy committed up to $5 million to bitcoin's quantum defence work in July, and the research pipeline has widened considerably this year. What has not moved is the part that requires miners, exchanges, wallet developers and node operators to change the same rules at the same time.
Levy's demonstration sits in block 964,199. It moved about $8, took hours to build, cost up to $200, and had to be delivered by hand to a mining pool because the network it was designed to protect would not pass it along.
