Ashigaru - Stonewall

Stonewall is a specific form of Bitcoin transaction designed to increase users' confidentiality when spending by imitating a coinjoin between two people, without actually being one. In fact, this transaction is not collaborative. A user can build it on his own, using only the UTXOs he owns as input. So you can create a Stonewall transaction for any occasion, without having to synchronize with another user.

The Stonewall transaction works as follows: as input to the transaction, the issuer uses 2 UTXO which belong to it. On the output side, the transaction produces 4 outputs, 2 of which are of exactly the same amount. The other 2 will be foreign exchange. Of the 2 outputs of the same amount, only one will actually go to the payee.

So there are only 2 roles in a Stonewall transaction:

Let's take an example to understand this transaction structure. Alice is at the baker's to buy her baguette, which costs 4,000 sats. She wants to pay in bitcoins, while maintaining some form of confidentiality about her payment. So she decides to build a Stonewall transaction for the payment.

By analyzing this transaction, we can see that the baker has indeed received 4,000 sats in payment for the baguette. Alice used 2 UTXO as inputs: one of 10,000 sats and one of 15,000 sats. On the output side, it has recovered 3 UTXO: one of 4,000 sats, one of 6,000 sats and one of 11,000 sats. Alice therefore has a net balance of - 4,000 sats on this transaction, which corresponds well to the price of the baguette.

In this example, I have intentionally neglected the mining fees to make it easier to understand. In reality, transaction costs are borne entirely by the issuer.

The Stonewall transaction works identically to the StonewallX2 transaction, except that the latter requires collaboration, unlike the classic Stonewall transaction, hence the name "x2". This is because the Stonewall transaction is executed without the need for external cooperation: the sender can carry it out without the help of another person. In contrast, for a Stonewall x2 transaction, an additional participant, known as the "collaborator", joins the process. He or she contributes his or her own bitcoins to the transaction, alongside those of the sender, and takes over the entire amount at the end (modulo the mining costs).

Let's go back to our example with Alice at the bakery. If she had wanted to make a Stonewall x2 transaction, Alice would have had to collaborate with Bob (a third party) when setting up the transaction. They would each have brought in a UTXO. Bob would then have received its entire contribution back. The baker would have received payment for his baguette in the same way as in the Stonewall transaction, while Alice would have recovered its initial balance, less the cost of the baguette.

From an outsider's point of view, the transaction would have remained exactly the same.

To sum up, Stonewall and Stonewall x2 transactions share an identical structure. The distinction between the two lies in their collaborative or non-collaborative nature. The Stonewall transaction is developed individually, without the need for collaboration. The Stonewall x2 transaction, on the other hand, relies on cooperation between two individuals to set it up.

The Stonewall structure adds an enormous amount of entropy to the transaction, blurring the lines of chain analysis. Seen from the outside, such a transaction can be interpreted as a small coinjoin between two people. But in reality, like the Stonewall x2 transaction, it's a payment. This method therefore generates uncertainties in chain analysis, or even leads to false leads.

Let's take the example of Alice at the baker's. The transaction on the blockchain would look like this:

An outside observer relying on common chain analysis heuristics might wrongly conclude that "two people have made a small coinjoin, with one UTXO each as input and two UTXOs each as output".

This interpretation is inaccurate, because, as you know, one UTXO was sent to the baker, the 2 incoming UTXOs came from Alices, and she recovered 3 exchange rate outputs.

Even if the outside observer manages to identify the paterne of the Stonewall transaction, he won't have all the information. He won't be able to determine which of the two UTXOs of the same amounts corresponds to the payment. In addition, he won't be able to determine whether the two UTXOs entered are from two different people, or whether they belong to a single person who has merged them. This last point is due to the fact that Stonewall x2 transactions, mentioned above, follow exactly the same pattern as Stonewall transactions. Seen from the outside, and without additional contextual information, it's impossible to tell the difference between a Stonewall transaction and a Stonewall x2 transaction. The former are not collaborative transactions, whereas the latter are. This adds even more doubt to the expense.

Originally developed by the Samourai Wallet team, Stonewall transactions have been taken over by the Ashigaru application, a fork of the original wallet created following the arrest of the Samourai developers. You'll need to install Ashigaru and create a wallet:

Unlike Stowaway or Stonewall x2 (cahoots) transactions, Stonewall transactions do not require the use of Paynyms. They can be executed directly, without prior preparation or collaboration with another user.

In fact, you don't really need a tutorial to make Stonewall transactions, as Ashigaru generates them automatically every time you spend, as soon as your wallet contains enough UTXOs.

Click on the + button at the bottom right of the screen, then select Send.

Select the account from which you wish to make the expenditure.

Then enter the transaction details: the recipient's address and the amount to be sent, and press the arrow to confirm.

Here, you can, of course, adjust the default transaction fees according to market conditions. However, the most interesting element on this page is the transaction type. You'll notice that Ashigaru has automatically selected STONEWALL. Click on the PREVIEW button to find out more.

You can see that the transaction is indeed of the Stonewall type: it comprises 2 inputs of the same amount, 2 outputs of the same amount, as well as the exchange outputs and, in my case, an additional input to satisfy the payment sum.

If you don't wish to make a Stonewall transaction, but prefer a conventional payment, click on the pencil icon at the top right of the screen, then select Simple instead of STONEWALL.

Once you've checked all the details, drag the green arrow at the bottom of the screen to sign and release the transaction.

You now know how to carry out a Stonewall transaction, and more importantly, how it works. If you'd like to find out more, take a look at my tutorial on Ashigaru Terminal, which explains how to make coinjoins via Whirlpool.