Comment by zkhalique | Hacker News Reader

Comment by zkhalique | Hacker News Reader

Yes they are Merkle trees. But the missing part of a blockchain is the incentive part. Obviously you don't even need git or anything so complicated for a blockchain. All you need is a network of computers that would periodically:

  * receive all the transactions (or their hashes) that occurred
  * sign this with some HMAC, and publish it
  * then we know that the transactions occurred by X time

of course, the question is how do we know transaction A really occurred after X time? Perhaps some will claim it occurred before X, but just wasn't included by anyone in the network for X because they are discriminating against that transaction's participants, or something.

And what if it was included by some in the network but not others? How do we know some are not cheating?

That's where proof-of-work comes in. You can also have proof-of-stake and other mechanisms, by which to solve the Byzantine Generals problem. How do we know that the signers aren't cheating? Who watches the watchers?

All of the complexity of bitcoin (including having and paying miners) comes from the requirement to be completely distributed, with no authority that could refuse to include "bad" transactions.

But if you're not signing any black- or grey-hat documents or transactions, then you can just pick a trustworthy central party to receive the hashes and publish the chain.

The startup GuardTime has been doing document timestamping and integrity checks with a blockchain for a bit while longer than bitcoin.

* You hash documents that you want to timestamp, and send the hash to them.

* They hash all received hashes and the previous head of the blockchain together, and publish the head hash in the paper copy of Financial Times: https://guardtime.com/photos/ft14-2.png

* The exact contents and "created before than" timestamp of the documents can now be fully verified, unless someone can manage to gather and destroy all paper copies of Financial Times published after it was sent to them.

The incentive for them is an usual freemium payment model.

This is a more straightforward description of what GuardTime does than anything the company themselves have managed to publish.

The problem is, as zkhalique said, that while it's easy to show a transaction happened before time X, it's hard to show it after. The single publisher (in this case the Financial Times or GuardTime) could have refused to include the transaction. Or the transaction might have not happened. How do you know which it is? What I pay you 10,000 bicoins and I promise GuardTime 3,000 to retract that fact later?

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