Bitcoin Mixer Services Implementation Analysis
In this section, we use the data gathered before regarding current public mixers and our experiments to identify the adoption o facademically proposed solutions in ChipMixer, MixTum, Bitcoin Mixer, CryptoMixer,and Sudoku Wallet.
The proposed solutions were discussed in before and include CoinShuffle, CoinParty, Xim,Obscuro, Mixcoin, Blindcoin, and TumbleBit. Table outlines which mixing services include key characteristics of proposed solutions in their implementation. The characteristics selected include CoinJoin, shuf-fling of output addresses in one transaction, multisignature escrows, TEXT field use to share data, signed warranties, blinding, and off-blockchain transactions. Each of these characteristics are used in at least one of the proposed solutions.
Through tracing our input transactions and outputs received by ChipMixer, we identified that funds sent to the service are routinely involved in the creation of chips ranging from 0.001 BTC to 8.192 BTC. For example, our Trial 1 input of 0.001BTC was involved in the creation of five chips of 8.192 BTC. The creation involves a CoinJoin transaction with UTXOs sent to ChipMixer by users as its input set.The output is a set of chips of a uniform size. Unlike CoinShuffle, this CoinJoin issolely created with funds available in ChipMixer’s wallet. Thus, the need for multiple signatures and shuffling of output addresses is eliminated. ChipMixer incorporates off-blockchain transactions by giving users the option to split, merge, bet, commonize, and donate their given chips. These options have an impact on the amount and distribution of the mix without executing multiple on-blockchain transactions.
The withdrawal of funds via importing private keys is also done off-blockchain. Thus, a complete ChipMixer mixing interaction can be done with only one on-blockchain input transaction. This is comparable to TumbleBit and its incorporation of off-blockchain puzzles to send Bitcoin between two users. ChipMixer claims to provide a signed receipt on withdrawal of chips. Although the service was unable to provide this receipt in all three trials, we do not believe it is comparable to the signed warranties produced in Mixcoin and Blindcoin. WhileChipMixer’s signed receipt aims to prove the origin of output funds, Mixcoin and Blindcoin’s signed warranty outlines the terms of the mix before any input or output.Overall, our analysis did not provide any evidence that ChipMixer implementssigned warranties, blinding, remote attestation, output address shuffling, or multisig nature escrow addresses.
MixTum offers a PGP signed letter of guarantee before any inputs to the service.The letters for all three trials included the generated input address, the output ad-dress(es), the maximum mixing time, the deadline for users to send their input by,and the maximum service fee. This guarantee can be compared to the signed warranty provided in Mixcoin which includes the value to be mixed, the deadline for the input to be sent, the deadline for the service to return funds, the output address,the mixing fee rate, a nonce, and the number of confirmations required on the input. Mixcoin’s protocol requires that users create the terms of the mix and provide themto the service. In the case of MixTum, the service creates the majority of the terms including the fee and deadline to return funds.
The user is only able to set the output addresses. Overall, the PGP signed letter of guarantee from MixTum provides enough information to identify a breach in protocol and holds the service accountable.We did not identify any evidence that MixTum incorporates CoinJoin, output address shuffling, multisignature escrow addresses, TEXT field use, remote attestation,blinding, or off-blockchain transactions.
Through our analysis and experiments with Bitcoin Mixer, we identified that the service does not implement any of the proposed mixing solutions found in CoinShuffle, CoinParty, Xim,Obscuro, Mixcoin, Blindcoin, or TumbleBit. The service doesnot implement CoinJoin transactions or shuffle output addresses of multiple usersin one transaction. In addition, Bitcoin Mixer does not implement multisignature escrow addresses, TEXT fields in transactions, remote attestation, a signed warranty,blinding, or off-blockchain transactions.
CryptoMixer provides a signed letter of guarantee along with each input address.Unlike MixTum, CryptoMixer’s letter of guarantee is signed using its Bitcoin private key. This letter provides confirmation of the origin of the input address, distribution of funds to each output address, delay for each output address, deadline for inputs,minimum and maximum input allowed, and mixing fee. This guarantee can be com-pared to the signed warranty provided in Mixcoin. In this case, the user specifies output addresses, delays, distributions, and the fees. Thus, CryptoMixer’s letter of guarantee ensures accountability and can be used against the service in case of a breach of protocol.Overall, the signed warranty was the only academically proposed solution adopted by CryptoMixer. We did not identify any evidence of CoinJoin, output address shuffling, multisignature escrow addresses, TEXT field use, remote attestation, blinding,or off-blockchain transactions.
Sudoku Wallet claims to provide funds from premixed CoinJoin transactions. Blockchain analysis in all three trials revealed that inputs were not involved in uniform output CoinJoin transactions after being sent to the service. Additionally, output shad not been involved in uniform output CoinJoin interactions in recent history. Thus,we do not believe the service uses CoinJoin transactions like CoinShuffle. However,Sudoku Wallet does make use of off-blockchain transactions on withdrawal. Like ChipMixer, the use of private keys as outputs ensures that outputs are not detectable on the blockchain. Overall, we did not identify any evidence of CoinJoin transactions, output addresss huffling, multisignature escrow addresses, TEXT field use, remote attestation, signedwarranties, or blinding.