Received: from sog-mx-1.v43.ch3.sourceforge.com ([172.29.43.191] helo=mx.sourceforge.net) by sfs-ml-2.v29.ch3.sourceforge.com with esmtp (Exim 4.76) (envelope-from ) id 1XG3WM-0003go-MQ for bitcoin-development@lists.sourceforge.net; Sat, 09 Aug 2014 10:05:06 +0000 Received-SPF: pass (sog-mx-1.v43.ch3.sourceforge.com: domain of mmci.uni-saarland.de designates 139.19.1.49 as permitted sender) client-ip=139.19.1.49; envelope-from=tim.ruffing@mmci.uni-saarland.de; helo=hera.mpi-klsb.mpg.de; Received: from infao0809.mpi-klsb.mpg.de ([139.19.1.49] helo=hera.mpi-klsb.mpg.de) by sog-mx-1.v43.ch3.sourceforge.com with esmtps (TLSv1:AES128-SHA:128) (Exim 4.76) id 1XG3WK-0007pQ-Ar for bitcoin-development@lists.sourceforge.net; Sat, 09 Aug 2014 10:05:06 +0000 Received: from zak.mpi-klsb.mpg.de ([139.19.1.29]:55288) by hera.mpi-klsb.mpg.de (envelope-from ) with esmtp (Exim 4.80) id 1XG3WB-0007Ng-94 for bitcoin-development@lists.sourceforge.net; Sat, 09 Aug 2014 12:04:57 +0200 Received: from [141.70.80.5] (port=29311 helo=calzone.localnet) by zak.mpi-klsb.mpg.de (envelope-from ) with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) id 1XG3WA-0007u2-VZ for bitcoin-development@lists.sourceforge.net; Sat, 09 Aug 2014 12:04:55 +0200 From: Tim Ruffing To: bitcoin-development@lists.sourceforge.net Date: Sat, 09 Aug 2014 12:04:51 +0200 Message-ID: <5456835.U3gAI91RM4@calzone> User-Agent: KMail/4.13.3 (Linux/3.15.8-1-ARCH; KDE/4.13.3; x86_64; ; ) In-Reply-To: <1530801.palqu9XdN4@1337h4x0r> References: <8137823.B0x87S28xY@calzone> <1530801.palqu9XdN4@1337h4x0r> MIME-Version: 1.0 Content-Type: multipart/signed; boundary="nextPart2588564.XKcU0yCBmf"; micalg="pgp-sha1"; protocol="application/pgp-signature" X-MPI-Local-Sender: true X-Spam-Score: 0.7 (/) X-Spam-Report: Spam Filtering performed by mx.sourceforge.net. See http://spamassassin.org/tag/ for more details. -1.5 SPF_CHECK_PASS SPF reports sender host as permitted sender for sender-domain -0.0 SPF_PASS SPF: sender matches SPF record 2.3 URI_NO_WWW_INFO_CGI URI: CGI in .info TLD other than third-level "www" -0.1 DKIM_VALID_AU Message has a valid DKIM or DK signature from author's domain 0.1 DKIM_SIGNED Message has a DKIM or DK signature, not necessarily valid -0.1 DKIM_VALID Message has at least one valid DKIM or DK signature X-Headers-End: 1XG3WK-0007pQ-Ar Subject: Re: [Bitcoin-development] CoinShuffle: decentralized CoinJoin without trusted third parties X-BeenThere: bitcoin-development@lists.sourceforge.net X-Mailman-Version: 2.1.9 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 09 Aug 2014 10:05:06 -0000 --nextPart2588564.XKcU0yCBmf Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" 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 --nextPart2588564.XKcU0yCBmf Content-Type: application/pgp-signature; name="signature.asc" Content-Description: This is a digitally signed message part. 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