Oh. Ok. I get it. This is good then.

But for people to do the CoinJoin, they all have to sign the same transaction, thus the possibility of a denial of service attack (by pretending to be part of the process but not signing the transaction.)

From what I understand, this "network level snooping" can only be done live, or someone or something is watching a good number of nodes, because the blockchain data itself does not store ip addresses. The transaction doesn't have any ip address information either. And where the transaction is first relayed is no indication that that is the one that actually sent it. And in any case, with a CoinJoin with a good number of real people, it doesn't matter who sent it as everyone else signed it, the one who actually broadcasts the transaction can be anyone, or even a completely different entity, or went through Tor or a VPN or from some random internet cafe. (Spammers do it all the time and are rarely or never caught.)

So, the problem is: too big a CoinJoin, (like with a few thousand) is hard to pull off, and some people might not go through with it. Too small, (such as 2 or 3 or 4 inputs and outputs) and its too easy to analyze.

So, if someone comes up with a "trusted" server, or shall I say "trustless" server, even if it is suboptimal, then that's still a good thing as long as that server is not compromised, or it doesn't keep logs after every transaction or something like that.

I mean, I can see at least 2 really big "traditional" mixers on these forums. The biggest one supposedly has a reserve of 3000 BTC. These guys can certainly pull it off, many people already trust them to mix "traditionally", I don't suppose they'd have a problem getting the same users to trust them to "CoinJoin" the whole thing and include their reserve in the process.

I have an idea, and guys, let me know if this is decent or crazy or whatever: or if this is already what is being done by JoinMarket, Shuffle, Whatever.

1. I make a website or a server for this purpose.
2. It asks users to create their transactions, and a given time limit, say 30 minutes or 1 hour.
3. Every 30 minutes, or every hour, it gets back to all the users with the giant CoinJoin transaction formed thus far and asks all the people to sign.
4. The site itself takes a fee, and that's some how inserted into the process.
5. Assuming the site is "trustable", then only outsiders are "attackers" and they're not going to learn much except "I see 100 inputs and 200 outputs, and I own 20 of them. I dunno who the other 80 are."
6. Repeat every hour or every whenever. Or every 100 people. Or once a day.
7. Java script or open source client side signing thingy that can sign for you if you're away from your computer so the site doesn't have to wait an hour when it reaches the minimum number of people.

The other idea I had, which I mentioned a long time ago, is like a traditional mixer, in that the mixer site itself has to be trusted, but it does everything internally, and you just send your coins. Instead of the traditional mixing of unrelated inputs and outputs, it's basically a CoinJoin mixer that gets all deposits and spits out the transactions just like a traditional mixer. The only downside is having to trust the site.