Geth ethereum mining guide
35 commentsBlockchain technology consumer solutions
Kristov Atlas Version of this Document: The SharedCoin mixing service provided by Blockchain. Bitcoin users should carefully consider their privacy requirements and evaluate other mixing services if they require serious privacy guarantees.
A tool for analyzing SharedCoin and other CoinJoin-based mixing protocols will be released approximately two weeks following this advisory to allow SharedCoin users adequate time to protect their privacy. As of January the site has over 1. SharedCoin is modeled after the CoinJoin privacy protocol designed to mix bitcoins from multiple users. When mixing their coins, customers select the number of times to repeat the SharedCoin process, between two and ten. Shared Coin is based on the CoinJoin concept 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 analyze. The important distinction between traditional mixing services is the server cannot confiscate or steal your coins. SharedCoin is purportedly an open source project, with a source code repository available at: The CEO of Blockchain. In a sample of 20, consecutive transactions across 45 blocks in the Bitcoin blockchain, 2.
This small sample constitutes only 7 hours of Bitcoin transactions from March 27, Three criteria were used to identify a given transaction as a potential SharedCoin transaction: The transaction contained more than four inputs, the transaction contained more than four outputs, and the transaction was relayed internally by Blockchain. Pending further research into this area, the SharedCoin service should be used only as a light protective measure for financial privacy.
SharedCoin will be suitable for protection against unskilled examiners of the Bitcoin blockchain until user-friendly analysis tools are released to the public. Users who require the aid of mixing services to protect themselves from agents with intermediate programming skills should carefully evaluate other services and technologies to meet their financial privacy requirements.
It is currently unclear whether adding additional rounds of SharedCoin will positively or negatively affect privacy. Until changes are made to SharedCoin to address the weaknesses identified in this advisory, users who continue to utilize SharedCoin are most likely better off using the maximum number of rounds permitted 10 in order to increase the amount of computation required by adversaries to analyze SharedCoin activity across multiple transactions.
Users on the network adopt pseudonyms in the form of Bitcoin addresses, typically represented as strings of letters and numbers. Despite the adoption of pseudonyms, the openness of the blockchain permits both targeted and broad analysis of Bitcoin users. The original design of the protocol encourages users to avoid privacy pitfalls by generating new addresses for each new transaction, and by avoiding the pooling of funds from multiple addresses whenever possible.
This is generally difficult for many users for a variety of reasons including lack of education, lack of Bitcoin client features, and poor client interface design. Bitcoin developers and entrepreneurs have proposed a number of improvements at the client level to help mitigate some of these privacy risks. Early on in the Bitcoin ecosystem, entrepreneurs launched third-party mixing services that are capable of breaking down the path of bitcoins on the blockchain through off-chain accounting.
However, users of these third-party mixing services must trust the services not to steal — or fail to protect from theft — customer funds.
To address the weaknesses of third-party mixing services, Bitcoin developers proposed a set of client-side augmentations that would permit peer-to-peer mixing. Developer Gregory Maxwell formalized one of those proposals as a client protocol in a post to the BitcoinTalk forum on August 22, [6] and named it CoinJoin. CoinJoin takes advantage of the fact that a Bitcoin transaction can have many input and output reservoirs of funds.
It combines the funds of multiple Bitcoin users together in a single transaction without the need for any third party service to act as a temporary custodian. CoinJoin users synchronously take turns signing a CoinJoin transaction until all involved parties agree on the final state of the transaction, before finally committing it to the blockchain. The end result is that the users are able to mix their funds together without any opportunity for theft to occur. Some party must act as conductor to orchestrate this process between CoinJoin users.
SX took a simple approach to implementing CoinJoin, allowing users to gather in a chat room, make use of a CoinJoin server to orchestrate the protocol, and mix their coins. User could only use this CoinJoin feature when they were willing to each contribute and receive the exact same amount of funds.
In this transaction, three different users or one user using three instances of the SX client, perhaps provided 0. Three addresses received 0. Because all of the input amounts are identical 0. The chance of co-ownership of each of them is one in three. This approach is significantly restricted in that all participants must be willing to input and receive identical amounts of funds.
If users wanted privacy from other CoinJoin users, they could be left waiting a long time for other users to partner up with who were seeking to send identical amounts of money. Since this approach does not permit for change addresses, all of the participants must in fact have equivalent balances in their input addresses before participating in the CoinJoin operation. Requiring aspiring CoinJoin participants to wait a long time for partners would prove both confusing and frustrating for many users in that demographic.
In consideration of these limitations, Blockchain. Source code for SharedCoin was first posted publicly to its GitHub repository on December 22, [8]. Assuming that a web wallet service does not want users to wait a long time for partners, they must make some adjustments from the SX baseline. The transaction would have N outputs of that size and potentially N more change outputs if some of the users provided input in excess of the target. Unless a wallet service had a large volume of users and transactions to match up, this could lead to long wait times.
In order to study the behavior of SharedCoin, I performed a number of SharedCoin transactions using my own bitcoins and observed their path through the series of SharedCoin transactions. Out of legal risk considerations, I have decided not to include the details of those transactions in this advisory, but I can compare the results to other transactions I identified within the blockchain that were consistent in nature with the SharedCoin transactions that I participated in.
Some SharedCoin transactions have drastically different numbers of inputs and outputs, suggesting that either a large number of change addresses may be included as outputs, or that SharedCoin attempts to obscure the path of customer funds by splitting and joining them into a random number of addresses, e. Bitcoin privacy analysis is difficult because there are currently few publicly available tools to analyze the blockchain.
The average user may incorrectly assume that SharedCoin privacy can be measured by the number of inputs and outputs, or in terms of the classic Taint analysis provided by Blockchain.
This tool is named CoinJoin Sudoku. To illustrate the success of the tool to date, let us consider a sample SharedCoin transaction. This transaction was selected from a sample of 20, Bitcoin transactions as a likely SharedCoin transaction based on its profile. CoinJoin Sudoku tries to identify individual participants in CoinJoin transactions by searching for common ownership of inputs and outputs. The tool considers all of the possible ways to group inputs and outputs, and eliminates the possibilities that include groups that do not add up between inputs and outputs, since they do not demonstrate common ownership.
For the sake of speed efficiency, the tool currently processes a transaction by examining one digit at a time in the inputs and outputs, working its way from right to left; this is faster because transactions typically involve inputs and outputs with many zeros, which can be ignored while processing a given digit. In order to complete the processing of this transaction in a reasonable period of time using only one processor, I instructed the tool to skip processing the 3rd digit in each number e.
The tool is currently so inefficient that it took Removing the restriction would allow the tool to much more thoroughly de-anonymize the transaction, but would require substantially longer without further efficiency improvements. In this case, two SharedCoin participants were identified, denoted by red and blue. The two participants identified represent a ceiling for the number of participants who own those addresses, since it could in fact be just one participant responsible for all of those addresses.
We can understand this with a simple analogy: The results of the tool clearly indicate that the anonymity set for participants is much smaller than the number of inputs and outputs. In approximately two weeks, CoinJoin Sudoku will be released, open-source. The delay is intended to allow former users of SharedCoin adequate time to take extra steps to protect their financial privacy.
The code will be made available here:. CoinJoin Sudoku should be considered an early step in researching the efficacy of CoinJoin-based privacy services such as SharedCoin.
Aside from efficiency improvements, this tool can be built upon to analyze across multiple transactions. Vendor confirmed that summary of findings has been forwarded to security personnel. Anticipated Release of proof-of-concept tool, CoinJoin Sudoku. The vendor claimed to have fixed this issue not yet confirmed in this GitHub comment [ 11 ]. At the time of releasing this advisory, the vendor has not released any details about planned improvements to the SharedCoin service.
In January , the company contacted me to claim that they have fixed the issues identified in this advisory, though this has not yet been confirmed. Because the blockchain is a permanent and public record of all transactions, weak privacy obtained from previous uses of the SharedCoin service or any mixing service cannot be undone.
It is possible for Blockchain. The SharedCoin transactions I participated in had the following common characteristics: They were relayed internally by a Blockchain.
They contained 9 or more transaction inputs. They included 9 or more transaction outputs. The number of inputs and outputs was often different.
They always include a mining fee, and this fee is always a multiple of some constant currently 0. Relationship analysis of inputs and outputs in the transaction, before and after CoinJoin Sudoku is run. Likelihood of relationship between output and two inputs: Taint Analysis Actual Input 1: The code will be made available here: