Bitcoin: A Peer-to-Peer Electronic Cash System
When trying to understand what Bitcoin is and does, it's helpful to start with an understanding of the context in which it was build and the problem it was trying to solve. There were many digital currencies before Bitcoin, but Bitcoin was the first decentralized digital currency. Creating a digital currency without a central authority was the problem that was being solved for.
Bitcoin was first introduced to the world On October 31, 2008, with the publishing of the Bitcoin white paper Bitcoin: A Peer-to-Peer Electronic Cash System. The paper gives insight into the motivations and architecture of the system. Much of what is covered in the paper are topics that we will dive into in later units. So, we recommend reading through it briefly now and coming back to it often throughout your studies.
2. Transactions
We define an electronic coin as a chain of digital signatures. Each owner transfers the coin to the next by digitally signing a hash of the previous transaction and the public key of the next owner and adding these to the end of the coin. A payee can verify the signatures to verify the chain of ownership.
The problem of course is the payee can't verify that one of the owners did not double-spend the coin. A common solution is to introduce a trusted central authority, or mint, that checks every transaction for double spending. After each transaction, the coin must be returned to the mint to issue a new coin, and only coins issued directly from the mint are trusted not to be double-spent. The problem with this solution is that the fate of the entire money system depends on the company running the mint, with every transaction having to go through them, just like a bank.
We need a way for the payee to know that the previous owners did not sign any earlier transactions. For our purposes, the earliest transaction is the one that counts, so we don't care about later attempts to double-spend. The only way to confirm the absence of a transaction is to be aware of all transactions. In the mint based model, the mint was aware of all transactions and decided which arrived first. To accomplish this without a trusted party, transactions must be publicly announced, and we need a system for participants to agree on a single history of the order in which they were received. The payee needs proof that at the time of each transaction, the majority of nodes agreed it was the first received.