Why Blockchain Exists

Digital cash was not a new idea in 2008. David Chaum had working systems in the 1980s. Companies shipped them in the 1990s. The cryptography was solid, the mathematics held up, and the products failed anyway.

They all failed at the same place, and understanding that place is the shortest route to understanding what blockchain is for.

The problem was copying#

A digital coin is a number. Numbers copy perfectly and for free. If a coin is just a signed message saying "this is worth one unit", you can send that same message to two merchants at the same time. Both signatures verify. Both merchants see valid money. One of them is about to lose.

This is double-spending, and it is not a cryptography problem. Signatures already prove authenticity and integrity. What they cannot prove is that a coin has not already been spent somewhere else.

The only way to know that is to know the order of events. Somebody has to be able to say: this spend came first.

Every earlier answer used an issuer#

The pre-2008 systems all solved it the same way. A central issuer kept the ledger. Before accepting a coin, the merchant checked with the issuer, who confirmed the coin was unspent and marked it spent.

It works. It also means the issuer is the system. They can freeze accounts, reverse settlements, go bankrupt, or be shut down by whoever has jurisdiction over them — and several were. The cryptography protected the coins; nothing protected the ledger.

Why ordering is genuinely hard#

It is worth being precise about the difficulty, because it is not obvious.

There is no global clock. Two machines on opposite sides of a network cannot agree on which of two events happened first — timestamps are self-reported and trivially forged, and message delays are unbounded. So you cannot order events by time.

You could vote. But on an open network anyone can create unlimited identities for free, so a vote count means nothing. That is the Sybil problem, and it defeats every naive voting scheme.

So: no clock, no membership list, no way to count votes. And yet a shared order is required.

The 2008 answer#

The insight in Bitcoin's design was to stop trying to determine what happened first, and instead make one particular history expensive to produce.

Transactions are batched into blocks. To publish a block you must find a value that makes its hash fall below a target — work that cannot be shortcut, only attempted. When two competing histories exist, the network follows the one with more accumulated work behind it.

This does not identify the true order. It makes an alternative order costly. To rewrite history you must redo all the work in it and outpace everyone extending the current one. Past a certain depth that stops being economically sensible.

Notice what this bought and what it cost. Ordering no longer needs an authority or a membership list. In exchange, ordering now costs electricity, permanently and by design. Bitcoin did not eliminate the cost of trust. It converted it into a cost that can be paid by anyone, without permission.

Why this history is worth carrying#

Because it tells you exactly when the design applies.

If your system has an acceptable authority, the problem blockchain solves is not present. You can keep an ordered log, sign it, replicate it, and audit it — cheaper, faster, simpler.

Nearly every unconvincing blockchain proposal is a case where an authority already exists and is doing fine. The technology was built for the situation where appointing one is not available to you. Outside that situation, you are paying for a solution to a problem you do not have.

Inside it, there is nothing else that does the job.