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Fidelity Bonds in Bitcoin: Enhancing Security and Privacy

Fidelity Bonds in Bitcoin

Fidelity bonds are a fascinating and crucial aspect of the Bitcoin ecosystem, especially when it comes to enhancing the security and privacy of transactions. This article aims to provide a beginner-friendly guide to understanding fidelity bonds and their role in the Bitcoin protocol.

What are Fidelity Bonds?

In traditional finance, a fidelity bond is a type of insurance contract designed to protect against losses caused by forgery, theft, and other dishonest activities. These bonds are typically used by businesses to safeguard against the dishonest actions of their employees.

In the context of Bitcoin, fidelity bonds take on a slightly different meaning but retain the core principle of ensuring trust and security. They are mechanisms where a certain amount of bitcoin value is deliberately sacrificed to make a cryptographic identity expensive to obtain. This sacrifice is done in a way that can be proven to a third party.

Why are Fidelity Bonds Important in Bitcoin?

The primary purpose of fidelity bonds in the Bitcoin ecosystem is to improve resistance to sybil attacks, thereby enhancing the overall privacy of the system. A sybil attack is where a single adversary controls multiple nodes in a network, primarily to subvert the network's functionality.

By introducing fidelity bonds, the system becomes much more expensive to attack. An attacker would need to sacrifice a significant amount of value to frequently get chosen by other participants in the network for creating transactions, such as coinjoins.

How do Fidelity Bonds Work in JoinMarket?

JoinMarket is a decentralized market for CoinJoins, a type of Bitcoin transaction where multiple participants combine their coins into a single transaction to improve privacy.

Fidelity bonds in JoinMarket serve to improve the system's resistance to sybil attacks. Takers in JoinMarket are more likely to create coinjoins with makers who publish more valuable fidelity bonds. This means that as a maker, if you sacrifice bitcoin value to create a fidelity bond, you can participate in many more coinjoins and earn more fees.

One practical way to create a fidelity bond is to send bitcoins to a time-locked address using the opcode OP_CHECKLOCKTIMEVERIFY. The valuable thing being sacrificed here is the time-value-of-money. For long-term holders of bitcoins, this essentially means they can buy time-locked fidelity bonds for free, assuming they never intended to transact with their coins.


Fidelity bonds offer an innovative solution to enhance the security and privacy of Bitcoin transactions. By requiring participants to make a provable sacrifice of bitcoin value, the system becomes more resilient against potential attacks, ensuring that users can transact with confidence.

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