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Bitcoin's blockchain is basicly just a mechanism to record timestamped entries. Bitcoin uses it as a transaction ledger, but it could be used for many other things.
For example, one could create a chain for a decentral chess-game, where each move is an entry in the blockchain. All nodes can verify if the players turns are by the book, and in the correct order, and decide who is the winner.
Can anyone think of other examples of alternative chains?
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Provably fair games can utilize it. For example, Satoshi dice published the hash of their daily secretkey hash list in the blockchain. In that way, they can prove that they haven't altered they secret key list. Or you could do a simple "guess a number" game that kept an encrypted copy of the result in the blockchain. Later reveal the secret key and
Essentially any kind of system that relies on the escrow of a small amount of information could use the blockchain for that escrow. You could even use it to as a communication channel if your messages were small enough.
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Just a note: You cannot really add arbitrary things like text into blockchain.
You can "comunicate" by exact values and by bitcoin addresses, yes, but there is only so much that you can encode in that.
Actually I'm fairly sure that there is field for an arbitrary string to be added to a transaction. Not all clients provide an interface, but I'm fairly sure it is there. I will add a link in a few minutes if I find it. – David Ogren – 2013-04-03T21:35:44.987
Where would it be stored? In the script? Miners reject transactions with non-standard script with additional information. – Karel Bílek – 2013-04-03T21:50:34.273
Yes, in the script. I was thinking of transaction with a message from bitcoin.it. If miners would reject it (despite being a valid transaction) that might change things. I'll take away my downvote just because I've never tried it and it might not be possible in the real world. But that would require an edit to the answer, I guess. (Side note: I had guessed that blockchain.info was implementing their "public note" feature with this, but I was wrong about that.)
– David Ogren – 2013-04-03T21:53:41.220I have tried it :D not only the miners will not accept it, but most importantly the regular clients will not even relay it and throw it away, so it never propagates through the P2P network. If you want to add random stuff to the blockchain, you will have to use Namecoin that is full of "rubbish" in the blockchain :D – Karel Bílek – 2013-04-03T21:59:41.033
see http://bitcoin.stackexchange.com/questions/4486/transaction-with-slightly-changed-script-is-never-relayed - my question that more or less dealt with that
– Karel Bílek – 2013-04-03T22:00:46.413@KarelBílek Im talking about a new chain. This new chain could have completely different limits on transaction size, so "not enough space" is not an issue. – Muis – 2013-04-04T08:33:58.247
Oh. In that case, again, just use Namecoin. You can save any arbitrary things into Namecoin blockchain if it is under I think 500 bytes (theoretically more, but there is bug in the client and nobody is developing Namecoin for about a year.) – Karel Bílek – 2013-04-04T10:58:08.867
Not sure why this is voted down. http://bitcoin.stackexchange.com/questions/4707/what-method-does-my-wallet-use-to-encode-messages-in-the-blockchain You can't embed a text message into the blockchain itself, although it will be received by the recipient and be stored on blockchain.info
– Simon Woodside – 2014-01-02T08:27:27.617