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You shouldn’t use XWallet if you care about your privacy

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2 days ago the source code for Monero XWallet for iOS was released. Justin Smith (rusticbison) said on different occasions that he wants to monetize this app. Let me be clear from the start: I don’t have anything against this model. If you want to monetize your hard work, you should! But there is an issue with the way Justin wants to monetize the app. Monero’s core value is privacy. I have good reasons to assume that the way XWallet will be monetized will be detrimental for the users’ privacy. Let’s explore this in this article. Note that for the larger part of the article I try to simplify the explanation by ignoring the fact that ring signatures are being used.

This is what the source tells us about how the app will be monetized:

The wallet provider fee is a flat 0.0005 XMR per outgoing transaction.

This means that every transaction will usually have 3 outputs: “payment”, “change” and “wallet fee”, while a non-XWallet transaction will usually only have 2 outputs: “payment” and “change”. Let’s look at how a normal transaction looks like compared to an XWallet transaction. Note that the image only shows one input. In general a transaction will have one or two inputs. However, if a user has a lot of small unspent outputs in their wallet and wants to send big amount, it’s possible that a lot more inputs are present in this transaction.

But this is not the whole picture… One could expect that people using wallets will spend their change in the same way as they spend the original input. This is how in general normal user behavior looks like: (note that tx3, tx4 and tx6 have 2 inputs and the other transactions only have 1 input)

afb_2What’s clear from the image is that it’s not at all obvious to see which output is the “payment” and which output is the “change” due to the fact that every transaction has 2 outputs. And due to the xmr stealth addresses, there won’t ever be a reuse of the same (one time) address, making it nearly impossible to “cluster” certain txo’s. This means that the wallet privacy is preserved.

Now let’s look at how an XWallet transactions will look like. Tx1 is the first transaction using XWallet. It generates 3 outputs, as explained at the start. We see that tx3 and tx4 only have 2 outputs, so we can assume that these aren’t XWallet transactions. Tx2 on the other hand does have 3 outputs! So the first obvious conclusion we can make is that tx2 is probably the XWallet user who send out tx1 as well as using his change (o1) stemming from tx1. This means that we can cluster every transaction this user broadcasts using his XWallet.

Let’s analyze this transaction graph a bit more: tx4 is very likely a transaction made by the XWallet company grouping a bunch of the 0.0005 XMR wallet fee txo’s and sending them to a another wallet. We already established that o1 is the change, so this means that it’s very likely that the payment output in tx1 is o2. This transaction analysis is very bad for your privacy. Let’s expand our transaction graph a bit to show why:


In this example I’ve added 4 transactions. tx5 and tx7 are clearly XWallet-transactions while tx6 and tx8 are not. Tx4 is still grouping wallet fees. It’s obvious using the same analysis techniques that tx1, tx2, tx5 and tx7 are performed by the same user of XWallet. These transactions are clustered. If tx1 received funds from an exchange wallet, this cluster of transactions will be identified and the user’s privacy is at risk.

Imagine that a darknetmarket is taken down and they are seizing the wallets. Law enforcement has possession of the logs of outgoing and incoming transactions. A vendor has taken out XMR via tx6 and tx8 and received XMR from tx5 and tx7. If this vendor did perform a few successive transactions after tx6 and tx8, his funds will be anonymous. But… in this case there is a way to trace back the origin of the funds!

Tx6 gets input i1 from tx5o1. Note that this was received using a ring signature, so it’s either tx5o1 or one of the decoys in the ring signature. If law enforcement would visit the user of XWallet based on this information alone, he would still somewhat be able to plausibly deny that he send out tx5. Thanks to the ring signatures, it could have been one of the decoys. But…

Tx7 also shows up in the logs of the darknetmarket as an incoming transaction. i2 from tx8 originated from tx7o2! And this transaction is part of the cluster of the XWallet user! What are the odds that 2 times a transaction accidentally picked a decoy from the same cluster? This doesn’t really sound that likely. So now it becomes almost a certainty that the suspicious user of XWallet did indeed performed tx5 and tx7, regardless of the use of ring signatures.

If the DNM buyer would have used a normal wallet (the GUI, Monerujo, …) the blockchain data wouldn’t contain any identifying transactions with 3 outputs and his privacy would be preserved. I think this example clearly shows why people should avoid XWallet if they want to preserve their privacy using Monero.

XWallet weakens other people’s privacy as well!

We already established that when an XWallet user spends XMR he is clustering his transactions. But this also means that certain txo’s can be marked as “spent”. In the above example all change outputs and wallet fee outputs from the cluster tx1, tx2, tx5 and tx7 are likely spent.

To be exact, this list of txo’s is likely spent: tx1o1, tx1o3, tx2o1, tx2o3, tx5o2, tx5o3 and tx7o3. Note that we can know this just by analyzing the blockchain.

Normally, thanks to ring signatures, we can’t really know if a certain txo is spent or not. But due to the fact that we can cluster the user behaviour, we can, with a very high degree of certainty “guess” that these txo’s are in fact spent.

This means that if your wallet picks a decoy from this set of outputs, chain analyzing software can just ignore this decoy. It was already spent in a previous transaction. So in case you’re using the default ring size of 5, your effective ring size is reduced to 4. So if people are using XWallet, it can actually weaken other people’s privacy as well!

Suggestions on how to monetize XWallet in a privacy-preserving way

As I said, I am not against paying for software. But it’s important to note that the “3rd output model” is bad for privacy and should be replaced by something else. I have a few suggestions:

1. Paying for the app using the Apple Store. By doing this, only the credit card company knows you bought the app, but doesn’t know what you do with the app. But I can see why (for ideological reasons) XWallet wants to support payments in XMR

2. Wallet Deposit Fee System. When you deposit money to the app, the app could “lock” the money and could ask the user to send a certain percentage to the XWallet company in a separate transaction. Once this fee is paid, the wallet will add a “fee paid flag” to the txo. This flag will carry over to the change so your wallet won’t ask for a fee when you receive change. This would introduce a little inconvenience, but you will be able to pay for the app using XMR.
Another disadvantage is that when you receive a small amount of xmr (for example a payment for a beer) you will need to “unlock” this payment by sending an additional transaction, which will cost you not only the wallet fee, but also the miners fee. Maybe it’s a good idea that the wallet sums up incoming payments and only asks for payment of the fee once the total of incoming transactions reaches a certain threshold.
An additional benefit would be that the fee transaction would also function as an additional “churn” making your funds a bit more private if you received them from a KYC/AML exchange. Note that if the user doesn’t want to pay the fee, he can import his seed in another wallet.

3. Transaction credits or pay per x transactions. XWallet could have a built-in system of “transaction credits” where the user pays a certain amount so he’s able to send out a few successive transactions. He could buy “packs” of 10, 100 or 1000 transactions. The wallet can keep track of how many transactions the wallet sent out. Once all the credits are used, the user needs to buy a new package (or he can import his seed in another wallet and get his funds out).
Another option could be that XWallet would lock the wallet every x (10, 20, 50, …) transactions. If the user wants to continue to use his wallet, he’ll need to pay a fee. If he doesn’t want to pay the fee, he can import the seed in another wallet.

I think these 3 solutions are worth taking a look at. The privacy issues that are currently present in XWallet are just too dangerous. Monero is about protecting the users. Currently if you use XWallet, your privacy (and maybe your life) is at risk. I would be very happy if eventually another way of monetizing the app were to be implemented.