Topic: So now that blockchain supports CoinJoin, what pitfalls should we be aware of? (Read 11296 times)

Unfortunately, today I ran into an issue.

I made a SharedCoin transaction with 10 repetitions, the receiving address showing unconfirmed incoming transactions after some time.

Waited for a few blocks, then these incoming transactions vanished and the balance was restored to my blockchain wallet.

Trying to resend those coins via SharedCoin again to the same address failed with some error messages in JavaConsole:

"executeOffer failed Outpoint [...] already Spent in Completed Transaction"
"Outpoint [...] already Spent in Completed Transaction"

The only way I could resend those coins using SharedCoin was to use a new receiving address.

Then the SharedCoin transaction apparently went through, but the coins were quickly back in my blockchain wallet, before the new receiving address even showed an unconfirmed transaction.

Tried resending again to another new address, which failed again, leaving the coins in my wallet:

"executeOffer failed Outpoint [...] already Spent in Completed Transaction"
"Outpoint [...] already Spent in Completed Transaction"

Can anyone advise how to get the coins out of my blockchain wallet using SharedCoin?

EDIT: Trying a 4th new address with "recover intermediate addresses" before (which as expected recovered 0.00 BTC, since I had the full balance restored) didn't help either. Maybe I should try sending them without SharedCoin to another blockchain wallet and try SharedCoin again from there? Or will it not let me do that either since the coins are "already Spent in Completed Transaction"?

Would anyone be so kind to confirm or deny whether the shared coin service is currently operational?

All I get, when trying to access it with the "shared coin" feature of my wallet is "fetching sharedcoin info" in the upper right corner briefly with the repetitions menu just offering "loading" as its only option forever with "review payment" being greyed out.

api.sharedcoin.com is giving me 502, which is shown in the Java console, too. So I assume the service is most likely just down?

Is this feature still working? I haven't been able to click the "review payment" button for quite some time now when I try to submit a shared coin transaction.

I like the recent updates to the Shared Coin website. It looks like you are standardizing the name of the service to Shared Coin from SharedCoin, which will help SEO if used consistently.

Can you expand on this point please: since blockchain.info is now offering two mixing services, if Shared Coin can not guarantee 0% taint but Send Shared can, why would anyone choose to use Shared Coin rather than Send Shared? Thanks.

The website and FAQs are not available yet, so as a quick summary:

Coinjoin is a method of coin mixing which requires less trust in the mixing service. The coinjoin service available on blockchain.info is known as "Shared Coin" and is a separate entity to blockchain.info hosted at https://sharedcoin.com. Shared coin hosts a server which acts as a meeting point for multiple people to join together in a single transaction. Having multiple people in a transaction improves privacy by making transactions more difficult to analyse. The important distinction between traditional mixing services is the server cannot confiscate or steal your coins.

A sharedcoin transaction will look something like this: https://blockchain.info/tx/e4abb15310348edc606e597effc81697bfce4b6de7598347f17c2befd4febf3b (picked at random). As you can see multiple inputs and outputs make the determining the actual sender and receiver more difficult.

The server does not need to keep any logs and transactions are only kept in memory for a short time. However If the server was compromised or under subpoena it could be force to keep logs and deanonymize the users. If this were to happen although you haven't gained any privacy you haven't lost any either. For this reason, especially for those desiring strong anonymity, it will likely be wise to rely on multiple different coinjoin implementations (even if using a p2p system). Infiltrating one system is much easier than infiltrating multiple systems and correlating everything together. If enough people use coinjoin and it is integrated into multiple different clients the privacy of everyone in the bitcoin network improves.


- Tor can prevent leaking of information through your ip address.

- Sending coins to or from tagged addresses reduces privacy.

- When creating a shared coin transaction your browser generates a number or temporary/intermediate addresses. The balance of these addresses is not checked on blockchain.info itself unless the "Recover Intermediate Addresses" button is used. Looking up the balance of these addresses is an information leak as if blockchain.info was keep logs it would show you had an interest in these addresses. So don't use the "Recover Intermediate Addresses" tool unless there has been an error with a transaction.


Coinjoin can never completely sever the link between the input and destination address, there will always be a connection between them, it is just more difficult to analyse.


Shared send can handle larger amounts more effectively, it also guarantees 0% taint which coinjoin can never do. Shared send will most likely be depreciated in time but at the moment there are use cases for both services.

So sharedsend does not give you a link between the destination and input address?

Thanks for such service, now i wonder why we need sharedcoin.com

Can we have an option to specify change addresses for shared coin transactions? And an option to send to multiple addresses (similar to how is possible in custom send)?

Also, will the shared coin feature be coming to the browser extensions and mobile apps in the foreseeable future? Thanks a bunch.