Remember those adverts in which someone shoots an automatic assault rifle at some brave old soul with only a pane of bulletproof glass to protect them? That’s how Robert M.C. Forster, creator of "blockchain hack intervention" service Blockd, is testing out his product. Forster staked one ether ($136) to show off a system that claims to prevent hackers from stealing crypto from wallets, even if they have access to a private key.

He posted the following on Reddit: “I'm publicly posting my Ethereum private key (holding 1 Ether) to demonstrate Blockd's security. Private key and information within.

“First to send away my 1 Ether gets to keep it.

The address is: 0xa5653e88D9c352387deDdC79bcf99f0ada62e9c6

The private key is: ca9a3a3d4026e6228713e683a9c45ef65a538b2f9336813bd597f5effa38668d

The Etherscan link is: https://etherscan.io/address/0xa5653e88D9c352387deDdC79bcf99f0ada62e9c6

The safety wallet that should receive the funds is: 0x25eE1E352892Bc4f036F25441E6CEE84f5E06729”

But when one user, u/gucards, tried to use the private key to drain the wallet of ETH, they were met with the following message: “Blockd". In other words, the system prevented the user from draining the funds. When u/gucards tried to drain the wallet, Blockd redirected the funds in the wallet to a safety wallet—an unused address that the owner of the original wallet has control over—and made u/gucards foot the bill.

The “Replace-By-Fee” protocol

Blockd makes use of a blockchain’s “Replace-By-Fee” protocol, which, as Forster wrote in a Medium post, lets a user replace an unconfirmed transaction with another as long as the replacement has a higher transaction fee. With Blockd, users generate and sign transactions, but let the service broadcast these transactions to the network.

Forster wrote on Reddit that the service could discourage hackers from stealing funds from unknown addresses they have private keys for:

“If the hacker doesn't know the wallet is protected, how much would they be willing to sacrifice to assume that it is? If they do know, do they even want to put in the effort to hack it in the first place? How high-priced do they think the top blocker transaction is for the account? While 10% is greater than 0%, it's a lot less than 100% so there's a big opportunity cost there.”

Bet Binance, Bithumb, Mt. Gox, and countless others, wished they thought of the idea first.