Bitcoin Network's Garbage Transaction Attack: Lessons from 2015

In the summer of 2015, the Bitcoin network faced a series of spam transaction attacks, which were an early skirmish in the block size debate. The attackers aimed to demonstrate the necessity of increasing the block size limit through this method. They believed that the 1MB limit was too small, easily filled with spam transactions, and relatively inexpensive.

Proponents of large blocks believe that filled blocks can make Bitcoin payments unreliable. They wish to increase the block size limit to raise the cost of spam transactions. Supporters of small blocks, on the other hand, argue that allowing spam transactions to quickly go on-chain does not stop attackers, but rather enables them to win.

The first round of attacks occurred on June 22, 2015, initiated by a certain wallet provider. They planned to generate 1MB of transaction data every 5 minutes, aiming to create a backlog of 241MB of transactions. However, due to technical reasons, this attack did not succeed as expected.

The second round of attacks took place on June 29 and seemed to be more effective. Some users complained about transactions taking a long time to confirm. However, certain mining pools successfully filtered out junk transactions, which sparked controversy over transaction fungibility.

The third wave of attacks occurred on July 7, on a larger scale, causing more chaos. The attackers employed diverse strategies, including sending dust transactions to public wallets and exploiting addresses with known private keys. Some developers believe that increasing the block size is the best defense measure.

The fourth round, which is also the final round of attacks, took place in September. The attackers used different methods, directly publishing private keys to gift Bitcoin. This led to a large number of conflicting transactions, but the impact was not as severe as the third round.

These attacks have had a significant impact on Bitcoin, not only changing relay strategies at a technical level but also affecting people's perceptions of junk transactions. It has led to a series of changes, including miners adjusting block size strategies, increasing minimum relay fees, and introducing memory pool limits.

This attack has also intensified the divisions in the debate over block size limits. Supporters of large blocks cite the decline in user experience as a reason to increase the limits, while supporters of small blocks stand their ground. Ultimately, the viewpoint of the small block supporters prevailed.

Looking back at this history, we can see that garbage trading attacks are not new. Compared to today, the motives of attackers in 2015 may have been clearer. Another interesting comparison is the cost of attacks - in 2015, about $10,000 could cause a significant impact, while recent "garbage" trading has cost hundreds of millions.