Resources

People often ask me "How did you learn how to hack?" The answer: by reading. This page is a collection of the blog posts and other articles that I have accumulated over the years of my journey. Enjoy!

In March of 2013, an unexpected Bitcoin fork occurred, as documented in BIP 50. This was because a block with many transactions was mined. Bitcoin 0.8 nodes could process it, but pre-0.8 nodes could not. This caused a fork because pre-0.8 Bitcoin nodes accounted for about 60% of the mining power.

When switching to version 0.8, the upgrade now uses LevelDB instead of BerkeleyDB. BerkeleyDB had a limit on the number of transactions that could be in a block due to DB locks; this unintentionally became the new rule on the network. This limitation was removed on BerkeleyDB.

A user deposited $10K in BTC to OKPAY, which was included in the 0.8 fork. After some analysis, they realized that the TX was never confirmed on the 0.7 fork. They then created two transactions from the OKPAY transaction and broadcast them on the pre-0.8 fork block.

It's a double spend because one fork was actually being used by the payment provider, who used a different one. In reality, once the fork was detected, the payment processor should have stopped accepting Bitcoin transactions until the issue was resolved. Overall, a really interesting case of a double spend leading to stolen funds.