[Paper Review] Custody Protocols Using Bitcoin Vaults

Jacob Swambo, Spencer Hommel+2 more|arXiv (Cornell University)|May 24, 2020

Blockchain Technology Applications and Security12 references4 citations

TL;DR

This paper proposes a Bitcoin custody protocol using time-locked vault covenants and push-to-recovery-wallet transactions to enhance security and control over funds. By combining pre-signed transactions with secure key deletion and a watchtower system, it enables a tunable trade-off between accessibility and theft resistance through adjustable time-locks and fund compartmentalization.

ABSTRACT

A bitcoin extit{covenant} is a mechanism to enforce conditions on future bitcoin transactions. A bitcoin extit{vault} is a specific type of covenant transaction that enforces a time-lock on the transfer of control of funds to a hot wallet, but enables an immediate transfer of funds into a deep cold recovery wallet. This paper demonstrates how to integrate a bitcoin vault into a custody protocol and demonstrates the security properties of that protocol. The vault is implemented using pre-signed transactions with secure key deletion (as proposed in \cite{Swambo2020cov}). It is shown that vault-custody protocols enable the wallet owner to specify their desired balance for an inherent trade-off between the security of and accessibility of bitcoin holdings by adjusting the length of time-locks used. It is also demonstrated that wallet owners have increased control of risk-management by compartmentalizing funds across numerous vault transactions. While it isn't realistic to completely prevent theft, the most likely theft scenarios (compromising the hot wallet) have severely limited profitability for an attacker, deterring attempts at theft from the beginning. The proposed architecture was designed to offer defence-in-depth through redundancy and fault-tolerant functionality as well as countermeasures for class breaks through diversity across hardware and software layers. Finally, the architecture employs a detection (a watchtower) and response system that enables fail-safe recovery from attempted or partial thefts through a second type of covenant transaction, a push-to-recovery-wallet transaction.

Motivation & Objective

• To design a practical custody protocol that improves security and control for Bitcoin holdings beyond traditional multi-signature schemes.
• To address the inherent trade-off between fund accessibility and protection against theft by introducing adjustable time-locks and compartmentalized vaults.
• To reduce the attack surface by replacing persistent private keys with pre-signed transactions and secure key deletion.
• To enable rapid recovery from suspected breaches using a second covenant transaction (push-to-recovery-wallet) triggered by a watchtower.
• To demonstrate how covenant-based custody can support defense-in-depth through redundancy, fault tolerance, and layered security across hardware and software

Proposed method

• Implementing a Bitcoin vault using pre-signed transactions with secure key deletion to enforce time-locked spending paths.
• Using a relative time-lock in a covenant to delay active wallet access until a specified duration after an un-vaulting attempt.
• Introducing a push-to-recovery-wallet (P2RW) covenant transaction that immediately disables active wallet access and redirects funds to a deep cold recovery wallet.
• Employing a watchtower to monitor the blockchain and peer-to-peer network for unauthorized spending attempts and trigger P2RW transactions upon detection.
• Compartmentalizing funds across multiple vault transactions to limit the financial loss per successful attack and rate-limit un-vaulting to reduce risk exposure.
• Designing a multi-component architecture with active, vault, and recovery wallets, along with dedicated hardware modules for key management and transaction signing.

Experimental results

Research questions

• RQ1 How can Bitcoin covenants be used to create a custody protocol that balances security and accessibility for high-value holdings?
• RQ2 What is the impact of time-locked spending paths and pre-signed transactions on reducing the attack surface compared to traditional key-based custody?
• RQ3 How does the integration of a watchtower and push-to-recovery-wallet covenant improve resilience against partial compromises?
• RQ4 In what ways does fund compartmentalization and rate-limiting reduce the profitability of attacks targeting the active wallet?
• RQ5 What are the trade-offs between security, operational complexity, and usability in a covenant-based custody protocol?

Key findings

• The vault-custody protocol enables a tunable security-accessibility trade-off by adjusting the length of time-locks and the rate of fund un-vaulting.
• Compartmentalizing funds across multiple vault transactions limits the maximum loss per successful attack to a pre-defined, small portion of total holdings.
• The use of pre-signed transactions with secure key deletion reduces the sensitivity of funds compared to traditional private key control, limiting the attack surface.
• The protocol deters theft by making attacks on the active wallet unprofitable, as attackers cannot immediately access funds and must wait for time-lock expiration.
• The watchtower mechanism enables detection of unauthorized spending attempts and triggers immediate fail-safe recovery via the P2RW covenant.
• The architecture supports defense-in-depth through redundancy, fault tolerance, and diversity across hardware and software layers, improving resilience against class breaks.