Bitcoin overlay protocols
In discussion on the BitDNS thread I came up with an idea for overlaying other protocols onto Bitcoin. From one point of view, Bitcoin is a global, decentralized, yet consistent database. This DB is used to record transfers of coins, but it could potentially be used for more. There are many applications for a global consistent database.
Borrowing from my BitDNS description, the way this would work is we would use the mysterious and extravagant “scripting” system to add additional data to regular Bitcoin transactions. These would look like NOPs to current clients and be ignored, but overlay aware clients would look inside this NOP block and see the extra protocol-specific data, and interpret it according to the overlay protocol.
Specifically i could imagine using OP_NOP1 to signal overlay data, then OP_PUSHDATA to push the specific data, then OP_DROP to drop it from the stack, followed by the regular tx opcodes. This will have no effect on regular clients and look like a regular transaction (can be a dummy tx, 0.01 to yourself) but overlay aware code sees a protocol transaction.
As an example, Bitcoin could be used as an inexpensive timestamp service, allowing you to prove that a certain document existed on or before a certain date. All you need to do is create a dummy transaction to yourself, and hack the client to do an OP_PUSHDATA of the hash of the document, then OP_DROP it. The hash will be around for all time in the block chain and stand as proof that the document existed at that date.
That might be suitable in some cases. With Bitcoin, lightweight clients can download just the 80-byte block headers for each block and still securely verify transactions, but this is only possible because the network verifies transactions before including them in blocks. The network can’t verify overlay transactions, so overlay clients will have to download the entire block chain. This will become a problem when the block chain is 1,000,000 blocks long and you have to download 1+ TB of data to use BitDNS.
A more minor problem is that the bitcoins associated with these transactions become unusable. If the transaction is spent, the network is allowed to forget about it, so you must keep the transaction unspent for as long as you need the data within it to stay alive. Also, Bitcoin clients will not recognize non-standard transactions. If you send one of these transactions to someone not using a modified client, they will just ignore it.
It’s unnecessary to use OP_NOP1 as a flag. Just say:
“BitDNSv0001
Also, Bitcoin clients will not recognize non-standard transactions. If you send one of these transactions to someone not using a modified client, they will just ignore it.
Thanks for the comments. To clarify, do you mean that transactions with OP_PUSHDATA and OP_DROP, etc would be rejected by current clients?
Thanks for the comments. To clarify, do you mean that transactions with OP_PUSHDATA and OP_DROP, etc would be rejected by current clients?
They would not be rejected by the network, but transactions that differ at all from the two standard transactions can’t be received by standard clients.
So if I take a standard transaction to 179c7kVKJNxJN1TJ8EqcFhzXDAkrMqr2uB:
OP_DUP OP_HASH160 43716564ed4d679067da12bee139fd294c1f1b84 OP_EQUALVERIFY OP_CHECKSIGAnd I add some extra data to it:"BitDNS v0001 asdf" OP_DROP OP_DUP OP_HASH160 43716564ed4d679067da12bee139fd294c1f1b84 OP_EQUALVERIFY OP_CHECKSIG179c7kVKJNxJN1TJ8EqcFhzXDAkrMqr2uB will not even see my transaction in their client because they don't know how to read that modified transaction. It will still be in the block chain, though.179c7kVKJNxJN1TJ8EqcFhzXDAkrMqr2uB will not even see my transaction in their client because they don’t know how to read that modified transaction. It will still be in the block chain, though.
Ah, thanks. Is this just a UI issue with display of transactions, or do the clients not recognize the transfer of value at all? If that address 179c7kVKJNxJN1TJ8EqcFhzXDAkrMqr2uB goes on to spend its value (supposing its client was patched to allow it), would the transaction be accepted as valid?
I thought of a simple way to implement the timestamp concept I mentioned above. Run sha1sum on the file you want to timestamp. Convert the result to a Bitcoin address, such as via http://blockexplorer.com/q/hashtoaddress . Then send a small payment to that address.
The money will be lost forever, as there is no way to spend it further, but the timestamp Bitcoin address will remain in the block chain as a record of the file’s existence.
I understand that this is arguably not a good use of the Bitcoin distributed database, but nothing stops people from doing this so we should be aware that it may be done.