## Introduction
A bribery attack, also known as a short-range attack, is a method of bribing validators or miners to work on specific blocks or forks in blockchain systems. This type of attack allows attackers to present arbitrary transactions as valid, which dishonest nodes pay to verify. In this article, we will look at the mechanisms of a bribery attack, its consequences and defense methods, especially in the context of Proof of Stake (PoS) systems.
## Bribery attack mechanism
### Basic principles
A bribery attack relies on bribing network participants (validators or miners) to work on blocks or chains proposed by the attacker. The attacker pays an amount equal to or greater than the block reward to incentivize network participants to support his branch of the blockchain. If a block is successfully returned to the main network, miners or validators receive a reward, which creates a high incentive to participate in the attack.
### P+epsilon attack
In the context of a P+epsilon attack, an attacker can bribe users without paying them a bribe directly. The system itself rewards dishonest nodes, making the attacker’s branch the main chain. This creates additional motivation for network participants to support the malicious branch.
### Problems and risks
The main problem for the attacker is that if the malicious branch reverts for some reason (for example, the attacker cannot continue the bribe or the dishonest nodes stop working on that branch), he will have to pay a huge amount of bribes. For each maliciously created block, bribes will be accumulated, making the attack economically unprofitable in the long term.
## Bribery attacks in PoS systems
### The “nothing at stake” problem
In PoS systems, bribery attacks can be extended to the “nothing at stake” problem. In such systems, validators do not risk their funds when creating new blocks, which makes them more vulnerable to bribery.
### Protection methods
To protect against bribery attacks in PoS systems, the following methods are used:
1. Reduction Condition: If dishonest behavior is detected, validators may be punished by having their share of the network reduced.
2. Releasing offenders from their positions: Validators caught engaging in dishonest behavior can be kicked out of the network, reducing the likelihood of repeat attacks.
## Conclusion
Bribery attacks pose a serious threat to blockchain systems, especially in the context of PoS protocols. However, applying reduction conditions and releasing offenders from their positions allows these attacks to be effectively countered. It is important to continue researching and developing new security methods to ensure the security and resilience of blockchain networks.
## Links
Research on long-distance attacks for Proof of Stake protocols.