Quote from: grondilu on November 27, 2010, 22:21:27
To me it seems that cooperative mining is a tough task, because the honnesty of participants has to be checked. What’s preventing someone to run a modified version of the client, that would just keep generated bitcoin for himself, while receiving bitcoins from others ?

<sigh>

Either I haven’t been very good at explaining why there’s no possibility to cheat, or I’m wrong. But if I’m wrong, no-one has posted a specific objection. So I’ll try to explain it again, by presenting a specific design to show that a dishonest client cannot cheat.

Suppose I operate a pooled mining server, and I recruit some clients who wish to pool their mining.

My server asks each client to do some hashing for it. It asks each client to submit any hashes they find that are above a certain threshold of difficulty. The server chooses a difficulty that is one-fortieth (1/40th) of the current “official” difficulty level.

My server gets a constant trickle of candidate hashes sent back by the remote mining clients. Every now and then, one of those hashes meets the official difficulty level and my server can generate a block, which earns my server 50 bitcoins.

I now distribute bitcoins to the remote mining clients, at the rate of one bitcoin for each hash that was submitted for the current block that was at or above 1/40th of the official difficulty level.

In the long run, I would expect to distribute 40 coins out of every 50 that my server generates, although there will be some fluctuation from block to block. Nothing in this scheme requires the clients to be honest, because there is no way that a dishonest client can cheat!

The client is calculating hashes that will generate 50 BTC for my server. Those same hashes are not of any use to a dishonest client. They cannot be used to generate 50 BTC for the dishonest client, because a different hash code is needed to encode the payment of the generated bitcoins to someone else. And if the dishonest client tries to cheat by generating hashes that will pay the generated bitcoins to themselves, then the hash codes they submit won’t validate at my server and I won’t distribute any share of the payouts to them.

So this scheme requires absolutely no trust of the client.

This scheme also does not require the mining client to have faith that the server is honest. If the server advertises that it is paying out 1BTC for each hash that is at least 1/40th of the official difficulty level, then every client that submits an “easy” hash for a block that was generated can check that they received their bitcoin. Any fraud would show up immediately.

ribuck’s description is spot on.

Pool operators can modify their getwork to take one additional parameter, the address to send your share to.

The easy way for the pool operator would be to wait until the next block is found and divy it up proportionally as:
user’s near-hits/total near-hits from everyone

That would be easier and safer to start up.  It also has the advantage that multiple hits from the same user can be combined into one transaction.  A lot of your hits will usually be from the same people.

The instant gratification way would be to pay a fixed amount for each near-hit immediately, and the operator takes the risk from randomness of having more or less near-hits before a block is found.

Either way, the user who submits the hit that solves the block should get an extra amount off the top, like 10 BTC.

New users wouldn’t really even need the Bitcoin software.  They could download a miner, create an account on mtgox or mybitcoin, enter their deposit address into the miner and point it at anyone’s pool server.  When the miner says it found something, a while later a few coins show up in their account.

Miner writers better make sure they never false-positive near-hits.  Users will depend on that to check if the pool operator is cheating them.  If the miner wrongly says it found something, users will look in their account, not find anything, and get mad at the pool operator.

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