[Bitcoin-development] CoinShuffle: decentralized CoinJoin without trusted third parties
tim.ruffing at mmci.uni-saarland.de
Wed Aug 6 22:22:31 UTC 2014
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
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:
Moreover, for the full details, have a look at the research paper on
CoinShuffle that can be found here:
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
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
Every participant can verify that his money will not be stolen. Otherwise, he
refuses to sign the transaction and nothing will happen.
- Robustness against denial-of-service:
In CoinJoin, a single malicious (or malfunctioning) client suffices to stop
the whole protocol (e.g., by just refusing to sing the transaction without a
proper reason.) This can easily lead to DoS attacks but these can be countered
While in case of disruption, the current run of the protocol has to stop,
CoinShuffle addresses this problem as follows: In case of active disruption,
i.e., some participant sends wrong messages, the protocol provides a proof of
this misbehavior. Then the honest protocol parties can start a new run of the
protocol without the misbehaving participant. Also in case of passive
disruption, i.e., some participant does not respond (for whatever reason), the
remaining participants can agree on starting a new run without this
participant. This ensures that the protocol will finally succeed even in the
presence of malicious participant (although this can take quite a while then).
- Only public-key encryption and signatures:
The protocol requires only well-established cryptographic primitives. Besides
signatures and hash functions (that are already used by Bitcoin), only
standard public-key encryption is necessary.
A run of the protocol with 30 participants takes less than 100 seconds (in a
setting with reasonable bandwidth and delay). This is not much, given that 10
min (on average) are required to confirm the mixing transaction anyway.
The costs are almost completely caused by communication. The computation
overhead is minimal. (This is the main achievement actually. In theory, it is
clear that a protocol with all the properties can be built. However, generic
constructions cannot be used in practice yet, because the computation and
communication costs are huge.)
As CoinShuffle works on top of Bitcoin, it is fully compatible with the
current Bitcoin system. No changes to the Bitcoin protocol are required.
By the way: The NXT cryptocurrency picked up our idea and an implementation of
CoinShuffle for a part of NXT is under way. (
Mixing is the way to improve anonymity in Bitcoin now, as it does not require
changes to the Bitcoin protocol. We propose CoinShuffle, a decentralized
protocol to perform mixing in a secure way without trusted third parties, see
http://crypsys.mmci.uni-saarland.de/projects/CoinShuffle/ for a technical
overview. Our next step is to implement the protocol. Help in design and
coding would be great but we also would like to hear your comments, criticism,
and improvements for the protocol itself.
Tim Ruffing, Pedro Moreno-Sanchez, Aniket Kate
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