Boomerang: Bitcoin Cold Storage with Built-In Coercion Resistance

Boomerang is an innovative security protocol designed to protect large bitcoin holders from both cyber and physical threats, including the dire situation where someone might attempt to forcefully coerce the holder into transferring their bitcoins. This system is particularly aimed at individuals or businesses that possess significant amounts of bitcoin and seek to mitigate risks beyond those addressed by traditional cold storage solutions. Cold storage protects against online attacks by keeping keys offline but falls short when facing real-world threats such as kidnapping or extortion. Boomerang addresses this vulnerability by making the process of withdrawing bitcoins unpredictable and introducing mechanisms for securely signaling duress without alerting the attacker.

The essence of Boomerang lies in its ability to transform the withdrawal process into a non-deterministic affair over a specified period, thereby complicating any attempts by attackers to plan or execute forced transactions. The protocol involves splitting control among a small group of trusted individuals, each equipped with special hardware devices or applications to manage parts of the keys securely. Additionally, it incorporates neutral watchtowers and search and rescue services to coordinate actions and provide assistance if things go awry, further bolstered by encrypted personal information that can trigger alerts under specific conditions.

During the initial phase, known as the boomerang period, withdrawals are subject to a multi-step verification process, with each participant's device (Boomlet) randomly selecting a number of steps within a privately set range. This phase is characterized by its unpredictability in timing, designed to deter attackers by introducing uncertainty and allowing potential victims time to be rescued. A future milestone block on the Bitcoin blockchain is predetermined, marking the transition to a normal era where withdrawals revert to being fast and predictable.

Boomerang also features a unique duress protection mechanism whereby users memorize a consent set of five countries during setup. In a duress situation, the user can subtly signal for help by entering a different combination of countries during the withdrawal process. This signal is indistinguishable to the attacker but triggers the rescue service to initiate a search and rescue operation, leveraging encryption and anonymity networks to maintain secrecy.

Targeted at major bitcoin stakeholders and those in high-risk environments, Boomerang prioritizes ultimate protection over convenience. It is not suited for casual users or those looking for quick access to their funds. Instead, it offers a sophisticated solution for those seeking to safeguard their bitcoin against the most severe threats, transitioning from a complex, randomized access method to standard operations after reaching a significant milestone. The implementation of Boomerang involves coordinating with multiple parties, hardware investments, and potentially incurring fees for additional services, underscoring its focus on providing robust security measures for significant bitcoin reserves.