Why Bitcoin Core Began Retiring Satoshi Nakamoto’s Alert System 10 Years Ago

Bitcoin Core contributors announced plans on Nov. 1, 2016, to retire an alert system created by Bitcoin creator Satoshi Nakamoto because its shared signing key could not identify who sent a warning.

Nakamoto introduced the feature in August 2010 with Bitcoin version 0.3.11. A holder of the private key could sign a message that compatible clients would display as a warning. The design could also put certain wallet commands into safe mode, which users could override.

The key did not let its holder stop nodes from processing blocks or mining. “Nodes keep operating and do not stop generating in response to an alert,” Nakamoto wrote when he introduced the feature. The system warned users about problems; it did not give a key holder control over Bitcoin’s operating rules.

A March 2013 chain split illustrated the different actions available during a software emergency. After Bitcoin versions 0.8 and 0.7 followed different chains, mining pools downgraded to 0.7 to restore a common chain. Developers also planned alerts urging users of older software to upgrade. The miners’ software change resolved the split; the planned alerts addressed users running older versions.

By 2016, the alert key had passed among multiple developers. Former holders could not have their access revoked, and a valid signature proved possession of the key without identifying the sender. That left compatible clients unable to distinguish an authorized warning from one sent by another person who still held the key.

Bitcoin Core had begun withdrawing the feature before the November announcement. Version 0.12.1 turned peer-to-peer alerts off by default on April 15, 2016. Version 0.13.0 removed the feature on Aug. 23. The retirement plan addressed older software that could still receive signed alerts.

Bitcoin Core broadcast a final alert on Jan. 19, 2017, and included it in version 0.14.0, released March 8, for older nodes. Developers then delayed publication of the private key after identifying vulnerabilities in older software that still handled alerts.

The separate Bitcoin SV (BSV) network uses a different alert system. A valid message requires signatures from three of five key holders, and its documented messages include directives to freeze or reassign specified unspent transaction outputs. Those functions extend beyond the warnings Nakamoto described for Bitcoin (BTC).

The mainnet and testnet private keys for Bitcoin’s original alert system were disclosed on July 3, 2018. From that point, a signature made with either disclosed key could no longer establish that a warning came from a trusted sender.