Boomerang: Bitcoin Cold Storage with Built-In Coercion Resistance

Traditional cold storage methods, while effective against online attacks by keeping keys offline, do not address real-world threats like kidnapping or extortion. Boomerang enhances these security measures by making the withdrawal process unpredictable and allowing secure, discreet signaling of duress without alerting potential attackers.

At its core, Boomerang involves a sophisticated duress mechanism where users during setup memorize a consent set of five countries. This set plays a crucial role in the duress checks performed at critical moments, such as withdrawals. If a user is under duress and selects a different combination of countries from a randomly generated list, the system discreetly signals distress. This triggers encrypted data transmission to Search and Rescue (SAR) teams, enabling them to initiate a rescue operation using both static and dynamic user information. The design also includes mechanisms to counter various forms of attack, ensuring the user's actions remain indistinguishable to the observer, thereby maintaining their safety even under coercion.

During the initial phase known as the boomerang period, withdrawals involve a multi-step verification process managed through special hardware devices or applications controlled by a small group of trusted individuals. This phase introduces timing unpredictability, complicating any attempts by attackers. A predetermined future milestone on the Bitcoin blockchain marks the transition to a normal era where withdrawals revert to being fast and predictable. The protocol not only splits control among parties but also incorporates neutral watchtowers and SARs to coordinate actions and provide assistance if needed.

The implementation of Boomerang requires coordination with multiple parties, investment in specific hardware, and potentially additional fees for enhanced services. It is targeted at major bitcoin stakeholders and those in high-risk environments, prioritizing ultimate protection over convenience. This protocol is not ideal for casual users or those requiring immediate access to their funds.

Additionally, there are considerations for long-term sustainability and recovery strategies. The protocol includes backup mechanisms for critical devices like the hardware wallet, known as Boomlet, though activation mechanisms for backups are still under development. Plans to introduce redundancy for watchtowers and SAR services are underway. Moreover, incorporating a recovery checklist and practicing it with smaller amounts is suggested to ensure robustness over time, addressing concerns about device loss or service unavailability in the future.