Would a fixed ring size improve anonymity?

Question

Currently the ring size of a transaction can be set by the sender. One would think that using a ring size of 10 makes the transaction more anonymous than using a ring size of 5 (current default) as the transaction had more input candidates. But does an increase of the ring size really improve anonymity? In my opinion, transactions not using the default ring size will step out of the mass and can therefore be traced more easily ("Interesting, every time this address XYZ appears as a ring member, a ring size of 9 is used. This can't be a coincidence! Someone must be using a ring size of 9 for all of her transactions!")

Answer 1

There are several people close to the Monero project that would tentatively agree with you that a fixed ring size would improve anonymity.

As you point out, if you use a non-default ring size, your transaction will stand out.

Even if you were to use a random ring size each time, you'll be one of a small number of people that are not using the default ring size each time, so you'll stand out.

If the ring size does become fixed, then the way to increase anonymity beyond the fixed ring size would be to use a 'churn' transaction. This means you'd send the funds to yourself first, and then send the funds to the intended recipient. If the fixed ring size is 10, and you churn twice prior to sending the funds, your anonymity set would increase to 10^3 = 1000.

There are limitations to churning. For example, it is important to ensure that churns are not obvious on the blockchain. If it is rare to see a chain of outputs being spent 3 times in quick succession on the blockchain, then your churn will stand out.

More research therefore needs to be done by the Monero Research Lab to determine the effectiveness of different techniques to increase anonymity before any changes are made to fix the ring size or to recommend any particular anonymity techniques such as 'churning'.