Depots: Theft-Proof, Self-Custodial Bitcoin For Billions Of Users

The concept of depots presents a scalable solution for Bitcoin users, aiming to address the limitations of current blockchain technology in managing vast numbers of transactions securely and efficiently. These depots allow users to own bitcoin in off-chain Lightning channels with significantly reduced on-chain transaction requirements. Key to the functionality of depots is their reliance on griefer penalization, ensuring secure cooperation between users and operators without the need for trust. This system operates effectively even when users do not possess sufficient bitcoins to cover on-chain transaction fees.

Depots employ several advanced Bitcoin protocol enhancements such as CheckTemplateVerify (CTV) [BIP-119], CheckSigFromStack (CSFS) [BIP-348][Roose25], PairCommit [BIP-PC][Black24], OP_MUL [Russell24], and OP_MOD [Russell24]. These features collectively facilitate the creation of probabilistic off-chain Lightning channels that maintain high-value transactions off-chain until absolutely necessary. The structure includes complex mechanisms like target value revelation and channel revocation, which are crucial for maintaining the integrity and fairness of the depot system.

The security model of depots builds upon the observed behaviors in standard Lightning protocols, where parties act in self-interest to prevent financial losses. Depots extend this model by quantifying the expected losses through griefing security parameters, thereby discouraging malicious behavior through economic disincentives. These parameters ensure that both users and operators engage in cooperative behavior, bolstered by the potential financial repercussions of any adversarial actions.

From a technical standpoint, depots minimize the blockchain space required per user, making it feasible to support billions of users with minimal on-chain footprint. This efficiency is critical in preventing the so-called "thundering herd" problem, where too many transactions attempt to go on-chain simultaneously at the expiry of depots, potentially leading to massive delays and network congestion.

Furthermore, the deployment of depots involves detailed transaction protocols for creating and revoking channels, as demonstrated through various scenarios in the provided diagrams. These include normal and default resolutions, which handle the outcomes based on whether targets are met or missed. The intricate design of these transactions ensures that all possible outcomes are accounted for, securing the user's and operator's funds against unauthorized access or loss.

In essence, depots represent a promising advancement in cryptocurrency technology, offering a robust framework for scaling Bitcoin usage to accommodate a global user base while retaining the secure, decentralized principles fundamental to blockchain technology. Their successful implementation could dramatically enhance the scalability and security of Bitcoin transactions, supporting more widespread adoption and use in everyday transactions.