Re: [Bitcoin-development] CoinShuffle: decentralized CoinJoin without trusted third parties
One issue I do see is the protocol requires participants to check the inputs submitted by others are valid. Lite clients (at least of the p2p variety) cannot perform this check. You could skip the verification part and if the inputs turn out to be invalid then you'll find out when it doesn't confirm. This would problem open the protocol up to dos attacks and prevent part of the blame phase from working properly. Alternatively you can have the participants submit the merkle proof for the input. This would require inputs to have at least one confirmation, however. On Aug 6, 2014 6:42 PM, Tim Ruffing tim.ruff...@mmci.uni-saarland.de wrote: Hey, We (a group of researchers in Germany) propose a decentralized protocol for CoinJoin, a way to mix coins among users to improve anonymity. Our protocol is called CoinShuffle. We believe that CoinShuffle is a way to implement CoinJoin in the original spirit of Bitcoin, i.e., decentralized and without trusted third parties. (If you are not familiar with CoinJoin, the idea is explained here: https://bitcointalk.org/index.php?topic=279249.0 ) The protocol is essentially a clever way to create a CoinJoin transaction. Recall that the idea of CoinJoin is mixing with one SINGLE transaction that has multiple input addresses and multiple fresh output addresses (i.e., one pair of addresses per user). The advantage of CoinJoin over mixing with a server or trusted party is that nobody can steal coins. Each user can check if the single transaction sends enough coins to his fresh output address. If this is not the case, the user can just refuse to sign the transaction and nothing (bad) happens. The difficulty in CoinJoin is to let the participants announce their fresh output addresses without breaking anonymity: Of course, if a participant of the protocol just announces I have 1 BTC at address X now and I would like to have it back at address Y, then everybody can link X and Y and mixing is useless. A naive approach is to send these two messages via a secure channel to a server that organizes the whole mixing. While the server cannot steal coins, the server still has to be trusted for anonymity, because it knows which input addresses belong to which output addresses. We present the list of CoinShuffle's features at the end of this e-mail. An overview over the technical details can be found on the project page: http://crypsys.mmci.uni-saarland.de/projects/CoinShuffle/ Moreover, for the full details, have a look at the research paper on CoinShuffle that can be found here: http://crypsys.mmci.uni-saarland.de/projects/CoinShuffle/coinshuffle.pdf The paper has been accepted at a major European academic conference on security (ESORICS). We will present the idea there. Our Proof-of-concept Implementation --- There is a proof-of-concept implementation (written in Python) available on our project page. It is really only a proof-of-concept and it implements only the announcement of the addresses, not the creation of the transaction. Moreover, the code is CERTAINLY INSECURE and not well-written; our only goal was to demonstrate feasibility and estimate the performance of our approach. Our Future Plans Now we are planning a full, open-source implementation of the protocol. Of course, we would like to build on top of an existing wide-spread client. Since we do not have much experience in the design of existing Bitcoin clients, we would appreciate any help in the process. In particular, we did not decide which of the existing clients we would like to extend. Any hints towards this decisions would very helpful. Help in design and coding would be great but we also would like to hear your comments, criticism, and improvements for the protocol itself. CoinShuffle Features CoinShuffle has the following features: - Decentralization / no third party: There is no (trusted or untrusted) third party in a run of the protocol. (Still, as in all mixing solutions, users need some way to gather together before they can run the protocol. This can be done via a P2P protocol if a decentralized solution is desired also for this step.) - Unlinkability of input and output addresses: Nobody, in particular no server (there is none!), can link input and output addresses of a mixing transaction, as long as there are at least two honest participants in run of the protocol. (This is not a weakness: If there is only one honest participant, meaningful mixing is just impossible, no matter how it is organized. If all the other participants collude, they know all their input and output addresses and can immediately determine the output address of the honest participant.) - Security against thefts: As explained above, nobody can steal coins during the mixing because of the CoinJoin principle. Every participant can verify that his money will not be stolen.
Re: [Bitcoin-development] CoinShuffle: decentralized CoinJoin without trusted third parties
Actually getUTXO would probably work here as well. It isn't authenticated but it should be good enough for this purpose. The worst that would happen is the tx doesn't confirm. On Aug 11, 2014 2:25 AM, Chris Pacia ctpa...@gmail.com wrote: One issue I do see is the protocol requires participants to check the inputs submitted by others are valid. Lite clients (at least of the p2p variety) cannot perform this check. You could skip the verification part and if the inputs turn out to be invalid then you'll find out when it doesn't confirm. This would problem open the protocol up to dos attacks and prevent part of the blame phase from working properly. Alternatively you can have the participants submit the merkle proof for the input. This would require inputs to have at least one confirmation, however. On Aug 6, 2014 6:42 PM, Tim Ruffing tim.ruff...@mmci.uni-saarland.de wrote: Hey, We (a group of researchers in Germany) propose a decentralized protocol for CoinJoin, a way to mix coins among users to improve anonymity. Our protocol is called CoinShuffle. We believe that CoinShuffle is a way to implement CoinJoin in the original spirit of Bitcoin, i.e., decentralized and without trusted third parties. (If you are not familiar with CoinJoin, the idea is explained here: https://bitcointalk.org/index.php?topic=279249.0 ) The protocol is essentially a clever way to create a CoinJoin transaction. Recall that the idea of CoinJoin is mixing with one SINGLE transaction that has multiple input addresses and multiple fresh output addresses (i.e., one pair of addresses per user). The advantage of CoinJoin over mixing with a server or trusted party is that nobody can steal coins. Each user can check if the single transaction sends enough coins to his fresh output address. If this is not the case, the user can just refuse to sign the transaction and nothing (bad) happens. The difficulty in CoinJoin is to let the participants announce their fresh output addresses without breaking anonymity: Of course, if a participant of the protocol just announces I have 1 BTC at address X now and I would like to have it back at address Y, then everybody can link X and Y and mixing is useless. A naive approach is to send these two messages via a secure channel to a server that organizes the whole mixing. While the server cannot steal coins, the server still has to be trusted for anonymity, because it knows which input addresses belong to which output addresses. We present the list of CoinShuffle's features at the end of this e-mail. An overview over the technical details can be found on the project page: http://crypsys.mmci.uni-saarland.de/projects/CoinShuffle/ Moreover, for the full details, have a look at the research paper on CoinShuffle that can be found here: http://crypsys.mmci.uni-saarland.de/projects/CoinShuffle/coinshuffle.pdf The paper has been accepted at a major European academic conference on security (ESORICS). We will present the idea there. Our Proof-of-concept Implementation --- There is a proof-of-concept implementation (written in Python) available on our project page. It is really only a proof-of-concept and it implements only the announcement of the addresses, not the creation of the transaction. Moreover, the code is CERTAINLY INSECURE and not well-written; our only goal was to demonstrate feasibility and estimate the performance of our approach. Our Future Plans Now we are planning a full, open-source implementation of the protocol. Of course, we would like to build on top of an existing wide-spread client. Since we do not have much experience in the design of existing Bitcoin clients, we would appreciate any help in the process. In particular, we did not decide which of the existing clients we would like to extend. Any hints towards this decisions would very helpful. Help in design and coding would be great but we also would like to hear your comments, criticism, and improvements for the protocol itself. CoinShuffle Features CoinShuffle has the following features: - Decentralization / no third party: There is no (trusted or untrusted) third party in a run of the protocol. (Still, as in all mixing solutions, users need some way to gather together before they can run the protocol. This can be done via a P2P protocol if a decentralized solution is desired also for this step.) - Unlinkability of input and output addresses: Nobody, in particular no server (there is none!), can link input and output addresses of a mixing transaction, as long as there are at least two honest participants in run of the protocol. (This is not a weakness: If there is only one honest participant, meaningful mixing is just impossible, no matter how it is organized. If all the other participants collude, they know all their input and output addresses and can
Re: [Bitcoin-development] CoinShuffle: decentralized CoinJoin without trusted third parties
Hmm, you are right. Lightweight clients are an interesting point, we have to think about a policy for them. As you said, the worst case is that the tx will not confirm. So the only possible attack is DoS. For clients that rely on servers it's reasonable to trust their servers not to perform DoS. (Anyway, the servers could do worse attacks.) For SPV-clients (without servers), I'm not sure at the moment. Something like getUTXO seems to be a possibility. I think even SPV-clients can verify the validity of the tx that created the input that is designated for mixing. Then the only remaining reason why it could be invalid is that the input could have been spent already otherwise. But in this case, only one honest client with full information would suffice: a signed transaction that spends the money would convince even SPV-clients that the participant with this inputs tries to cheat. This transaction could even be provided by lightweight client that got if from a server; the transaction is signed by the cheating participant anyway. Tim On Monday 11 August 2014 02:30:16 Chris Pacia wrote: Actually getUTXO would probably work here as well. It isn't authenticated but it should be good enough for this purpose. The worst that would happen is the tx doesn't confirm. On Aug 11, 2014 2:25 AM, Chris Pacia ctpa...@gmail.com wrote: One issue I do see is the protocol requires participants to check the inputs submitted by others are valid. Lite clients (at least of the p2p variety) cannot perform this check. You could skip the verification part and if the inputs turn out to be invalid then you'll find out when it doesn't confirm. This would problem open the protocol up to dos attacks and prevent part of the blame phase from working properly. Alternatively you can have the participants submit the merkle proof for the input. This would require inputs to have at least one confirmation, however. signature.asc Description: This is a digitally signed message part. -- ___ Bitcoin-development mailing list Bitcoin-development@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bitcoin-development
Re: [Bitcoin-development] CoinShuffle: decentralized CoinJoin without trusted third parties
Putting the efficacy of coinjoin to one side: On Mon, Aug 11, 2014 at 1:38 PM, Tim Ruffing tim.ruff...@mmci.uni-saarland.de wrote: Then the only remaining reason why it could be invalid is that the input could have been spent already otherwise. But in this case, only one honest client with full information would suffice: a signed transaction that spends the money would convince even SPV-clients that the participant with this inputs tries to cheat. Bear in mind that getutxo does not return the spending transaction - it can't because the UTXO set doesn't record this information (a spent txo is deleted). However, if you have sufficient peers and one is honest, the divergence can be detected and the operation stopped/the user alerted. If all peers are lying i.e. your internet connection is controlled by an attacker, it doesn't really make much difference because they could swallow the transaction you're trying to broadcast anyway. Ultimately if your peers think a TXO is spent and refuse to relay transactions that spend them, you can't do much about it even in the non-SPV context: you *must* be able to reach at least one peer who believes in the same world as you do. -- ___ Bitcoin-development mailing list Bitcoin-development@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bitcoin-development
Re: [Bitcoin-development] CoinShuffle: decentralized CoinJoin without trusted third parties
There should not be a requirement at this level to ensure validity. That would too constrain use cases of implementations of your protocol. It is not difficult to imagine use cases where parties generate chained transactions on top of unconfimed transactions. Although malleability currently makes this difficult to do safely in general, it is not inconceivable that there are circumstances where it would nevertheless be safe or otherwise desireable. It is a good security recommendation that clients validate the inputs to a shuffle they are participating in. What this means depends on the client -- checking the UTXO set for a full node, making some getutxos queries for a SPV client. But this should remain a recommendation, not a requirement. On Mon, Aug 11, 2014 at 4:38 AM, Tim Ruffing tim.ruff...@mmci.uni-saarland.de wrote: Hmm, you are right. Lightweight clients are an interesting point, we have to think about a policy for them. As you said, the worst case is that the tx will not confirm. So the only possible attack is DoS. For clients that rely on servers it's reasonable to trust their servers not to perform DoS. (Anyway, the servers could do worse attacks.) For SPV-clients (without servers), I'm not sure at the moment. Something like getUTXO seems to be a possibility. I think even SPV-clients can verify the validity of the tx that created the input that is designated for mixing. Then the only remaining reason why it could be invalid is that the input could have been spent already otherwise. But in this case, only one honest client with full information would suffice: a signed transaction that spends the money would convince even SPV-clients that the participant with this inputs tries to cheat. This transaction could even be provided by lightweight client that got if from a server; the transaction is signed by the cheating participant anyway. Tim On Monday 11 August 2014 02:30:16 Chris Pacia wrote: Actually getUTXO would probably work here as well. It isn't authenticated but it should be good enough for this purpose. The worst that would happen is the tx doesn't confirm. On Aug 11, 2014 2:25 AM, Chris Pacia ctpa...@gmail.com wrote: One issue I do see is the protocol requires participants to check the inputs submitted by others are valid. Lite clients (at least of the p2p variety) cannot perform this check. You could skip the verification part and if the inputs turn out to be invalid then you'll find out when it doesn't confirm. This would problem open the protocol up to dos attacks and prevent part of the blame phase from working properly. Alternatively you can have the participants submit the merkle proof for the input. This would require inputs to have at least one confirmation, however. -- ___ Bitcoin-development mailing list Bitcoin-development@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bitcoin-development -- ___ Bitcoin-development mailing list Bitcoin-development@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bitcoin-development
Re: [Bitcoin-development] CoinShuffle: decentralized CoinJoin without trusted third parties
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=2t=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
Re: [Bitcoin-development] CoinShuffle: decentralized CoinJoin without trusted third parties
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, Sergio/ 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=2t=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
Re: [Bitcoin-development] CoinShuffle: decentralized CoinJoin without trusted third parties
On Sat, Aug 9, 2014 at 6:10 AM, Sergio Lerner sergioler...@certimix.com wrote: 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. That's not clear to me. The 2nd party doesn't know how the 3rd, 4th, 5th, etc. parties shuffled the outputs, since it doesn't have their decryption keys. -- ___ Bitcoin-development mailing list Bitcoin-development@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bitcoin-development
Re: [Bitcoin-development] CoinShuffle: decentralized CoinJoin without trusted third parties
On Thursday, August 07, 2014 12:22:31 AM Tim Ruffing wrote: - Decentralization / no third party: There is no (trusted or untrusted) third party in a run of the protocol. (Still, as in all mixing solutions, users need some way to gather together before they can run the protocol. This can be done via a P2P protocol if a decentralized solution is desired also for this step.) [...] http://crypsys.mmci.uni-saarland.de/projects/CoinShuffle/ for a technical overview. 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? - 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 :( signature.asc Description: This is a digitally signed message part. -- Infragistics Professional Build stunning WinForms apps today! Reboot your WinForms applications with our WinForms controls. Build a bridge from your legacy apps to the future. http://pubads.g.doubleclick.net/gampad/clk?id=153845071iu=/4140/ostg.clktrk___ Bitcoin-development mailing list Bitcoin-development@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bitcoin-development
