A Review of the 2015 Bitcoin Spam Transaction Attack: Insights into Current Blockchain Controversies

Recently, a proposal was made in the Bitcoin Core software library to remove the policy limit on the size of OP_Return outputs, sparking a debate about what constitutes spam transactions on the Bitcoin blockchain and how to handle them. This article reviews the incident in the summer of 2015 when the Bitcoin network was subjected to spam transaction attacks, aiming to compare the situation then with now and discuss the lessons learned.

The summer 2015 garbage trade attack was an early skirmish in the block size debate. The attackers were "big block supporters" who wanted to raise the block size limit. Big block supporters believed the 1MB limit was too small, and blocks could easily be filled with garbage transactions, making Bitcoin payments unreliable. They advocated for increasing the block size limit to raise the cost of filling blocks with garbage transactions.

"Small block supporters" believe that allowing garbage transactions to quickly and cheaply go on-chain does not prevent attackers; rather, it enables them to win. If the block size limit increases, fees will decrease, making garbage transactions cheaper.

On June 20, 2015, a Bitcoin wallet provider named CoinWallet.eu announced that it would conduct a "Bitcoin stress test". They claimed this was to demonstrate the necessity of increasing the block size limit. The first round of attacks took place on June 22, but failed to achieve the desired effect due to technical issues.

On June 24, CoinWallet.EU announced that the second round of attacks would take place on June 29. This attack seems to be more effective, with some users reporting that Bitcoin has become difficult to use. However, some mining pools, such as Eligius, successfully filtered out junk transactions, mitigating the impact of the attacks.

On July 7, the third round of attacks occurred. This was the most intense one so far, causing significant chaos. The attackers employed a diversified strategy, such as sending dust transactions to public wallets and using addresses with known private keys to generate more spam transactions.

F2Pool cleaned up these junk outputs by creating large consolidation transactions, helping to alleviate the situation. Bitcoin developer Gregory Maxwell later assisted in optimizing this process, making consolidation transactions easier to verify.

In September 2015, CoinWallet conducted its fourth and final round of stress testing. This time they took a different approach by publicly releasing private keys containing Bitcoin balances. This led to a large number of conflicting transactions, but since they could be discarded using the "first-seen-safe" principle, the impact was not as severe as in the third round.

An academic paper analyzed the spam trading attack of 2015, finding that 23.41% of transactions were spam transactions during the peak 10 days. The attack increased the average fee of non-spam transactions by 51% and increased processing delays by 7 times.

These attacks had a significant impact on Bitcoin, leading to multiple technical changes:

1. Miners increased the block size limit policy from 250KB or 750KB to 1MB.
2. The minimum relay fee in Bitcoin Core has increased by 5 times.
3. Introduced memory pool limits and a default memory pool size limit of 300MB.
4. Intensified the tension and polarization in the debate over block size limits.

In the long run, supporters of small blocks have won this debate. It has become the norm for blocks to be filled, and the idea of increasing block size limits to allow more junk transactions on-chain is considered undesirable. However, the debate over the definition and handling of junk transactions continues.

Comparing the situation in 2015 to now, one obvious difference is the amount of fees. In 2015, about 10,000 dollars could have a significant impact, whereas since 2023, several hundred million dollars have been spent on so-called "junk" transaction fees. This reflects the growth of the Bitcoin network and the improvement in its ability to combat junk transactions.