[Bitcoin-development] CoinShuffle: decentralized CoinJoin without trusted third parties

Sergio Lerner sergiolerner at certimix.com
Sat Aug 9 13:10:50 UTC 2014

Hi Tim,
 It's clear from the paper that the second party in the protocol can
de-anonymize the first party. So it's seems that dishonest shufflers
would prefer to be in that position in the queue.
There are two possible solutions to this:

1. Derive the first order of parties in the shuffle from the hash of all
inputs provided (as a seed for a pseudo-random number generator).
2. Repeat the shuffle several times with an different party order (e.g.
an order that is deterministically derived from the hash of all the outputs)

Best regards,

On 09/08/2014 07:04 a.m., Tim Ruffing wrote:
> You are raising valid questions and one goal of our posting here is indeed to 
> discuss exactly these system issues.
> On Thursday 07 August 2014 15:00:11 you wrote:
>> I think the description at your website leaves out the truly interesting
>> part: How do you decentralize this securely?
>> - How do Alice, Bob, Charlie and Dave communicate, i.e. which network is
>> used for communication and how?
> The simplest approach is obviously to use direct connections to a randomly 
> elected leader, who is also responsible for the broadcasts.
> One advantage of CoinShuffle is that the unlinkability between input and 
> output addresses is guaranteed, no matter which underlying network you use. 
> (Still, it is a good idea in general to hide your IP address but we can let 
> the user decide here.)
> Of course, there would be other possibilities, such as overlay networks. 
> Coinmux, a CoinJoin prototype by Michael Pearce (http://coinmux.com/) uses 
> TomP2P, a distributed hash table, for communication. 
> Do you have any hints regarding this point?
>> - How does Alice know that Bob, Charlie and Dave are not the same person?
>> (= how do you prevent a Sybil attack?)
>> Because thats the real problem with mixing it seems - ensuring that your
>> mixing partners are actually 100 people and not just 1 attacker. There are
>> probably many mixing algorithms which work if you solve that problem, but I
>> don't see how you offer a solution for it :(
> It's true that there are a few proposals for mixing protocols which all have 
> their advantages and disadvantages. However, it's not true that the mixing 
> itself becomes simple if you solve the problem of Sybil attacks. Still, mixing 
> is difficult to get right: Even if there are no Sybil attacks, you have to 
> ensure that the participants (or a server) cannot break unlinkability or steal 
> money. Actually that's why there are several proposals for mixing protocols, 
> because there is no obvious perfect solution.
> Regarding your question:
> It is indeed very important to get this right. Fundamentally, there is nothing 
> that prevents the attacker from creating a lot of identities participating in 
> a lot of CoinJoins. However, there are ways that make it hard for the attacker 
> to put an honest user together only with malicious users.
> For a moment, assume that you can reliably establish a pool of users that 
> would like to participate in the protocol. (I will discuss this later.) 
> You have to divide the users to individual groups, i.e., CoinJoins runs. If 
> the assignment cannot be influenced by the attacker, then the probability that 
> there are also honest users in a run is quite high. Of course, the attacker is 
> able to reduce your anonymity set but they cannot just put you together only 
> with their malicious identities.
> Note that the attacker has to pay transaction fees for joining many 
> transaction. One could even increase the required fee depending on the number 
> of users in the pool (enforced by honest CoinShuffle participants that would 
> not accept CoinJoins that pay a lower transaction fee).
> And making sure that the attacker cannot influence the assignment is simple: 
> One can use the hash of all users' public keys in the pool to determine the 
> assignment for example.
> For the initial setup step, i.e., creating the pool of participants, you need 
> some kind of "bulletin board". 
> One possibility is to use an underlying peer-to-peer network. Bitcoin itself 
> is the first that comes to the mind but it does not allow arbitrary messages. 
> So if we do not want to change the Bitcoin protocol, chans in Bitmessage are a 
> very promising possibility. Bitmessage relies basically on the same broadcast 
> mechanism as Bitcoin. If you as a peer use enough outgoing connections to 
> other peers, it's very difficult for an attacker to ensure that your message 
> will not be spread among the network. (Btw, people have used this to do 
> CoinJoin  manually already 
> https://forum.namecoin.info/viewtopic.php?f=2&t=1694 .)
> Solutions like distributed hashtables (TomP2P again) are another possibility. 
> We are not sure which of those approaches provides the best robustness against 
> malicious nodes that try to stop single participants from reaching the 
> network. For the setup step, latency is not an issue, so Bitmessage is indeed 
> a promising candidate here.
> I think that in general, P2P is the way to go here, but there are other 
> approaches as well:
>  - A possibility is to have a lot of servers acting as bulletin 
> boards. The user sends his announcement message to all of the servers and 
> the user waits until at some of the servers send back a guarantee to 
> include the user. After some time, the servers agree on the pool of the users 
> just by taking all the users that have registered with at least one of the 
> servers. There are well-understood protocols to achieve this goal or similar 
> goals, because essentially the same problem arises in e-voting (see 
> http://arxiv.org/pdf/1401.4151 for just one example. this paper provides also 
> a detailed discussion of related protocols in section 9).
> Of course, the disadvantage of this approach is that the protocol is not 
> really decentralized anymore.
>  - If you really want to be on the safe side, you can include your 
> announcement messages in the Bitcoin blockchain, e.g., by adding your 
> announcement message to an unspendable output, at the cost of an additional 
> transaction. I know that putting data to the blockchain is discouraged but let 
> me explain why it is useful here: If you want to do several CoinJoins in a 
> row, you can include your announcement message for the second CoinJoin in the 
> transaction of the first CoinJoin, so your announcement is very robust but you 
> do not need an additional transaction, because you can piggy-back on the frist 
> transaction.
> Additionally, it is possible to combine these approaches by joining several 
> pools. 
> Another interesting point that my co-author Aniket Kate mentioned is that you 
> can look at that problem as a social issue: You could combine this with 
> information from your friends. You can participate in a CoinJoin only if your 
> friends tell you that they also participate in the same run. They do not even 
> have to reveal their input address, they just have to reveal that their 
> address is in a particular run. Of course, this is not yet a technical 
> solution but a very interesting idea.
> Don't get me wrong. We don't think that there is a perfect solution the 
> two issues that you mentioned but we are pretty sure there are several that 
> work well enough in practice if they are implemented correctly. 
> Tim
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