How anonymous is a suggestion box in an office of four? A note shows up complaining about the coffee budget, and the boss solves it before lunch. One person never drinks coffee and one is on vacation, so that leaves two suspects. Nobody opened the box or broke a lock. The crowd was too small.
Privacy people call that crowd an anonymity set, the group of people you can't be told apart from. Move the same box to a company of 40,000 and the same note tells the boss almost nothing.
Bitcoin privacy has the same weak spot, and I'd rather say so out loud. Every payment sits on a public ledger, readable by anyone, forever. A mixing tool blends your coins with coins from other people, so it can only hide you as well as that group can. If only a few people use it, you are one of a few guesses. Worse, the choice to use it can become the signal, because the only people in the box are people who wanted a box. The math can be perfect and the privacy can still fail.
I can hear the pushback, and I take it seriously. Optional privacy is the version regulators and exchanges will put up with, and coins with forced privacy keep getting delisted. A small real crowd beats a large banned one. Fair. If an optional crowd ever grows big enough that joining says nothing about you, I'm wrong to worry about this.
I've held Bitcoin since 2015, and wallet defaults worry me more than math does now. Each boring payment that goes through a privacy tool makes the crowd bigger for somebody who needs it more than I do.
If this changed how you see a privacy tool, Zap ⚡ https://blossom.primal.net/a011e9489f1ea6d5f98404367b95375dcc0f676701aeeaccce76af452f13ed5b.png
