Could a blackballs list be used maliciously?

Question

I do not see any way to trustlessly generate a blackballs database without running full nodes for malicious fork coins, which I do NOT want to do (and don't even have hardware resources to do). However, I need a working blackballs database to protect my rings against using outputs which are publicly known to be compromised across chains. Rock, meet hard place, with privacy-conscious users in between.

In this discussion on another question, a helpful user is distributing a blackballs list. As I observed it, it lists 16,427,618 txids 64-digit hex strings (Edit: see my comment to user36303's helpful answer). I appreciate the apparent helpfulness; however, hmmm... Attack: Distribute a malicious blackballs list containing 16,427,618 txids 64-digit hex strings which should not be blackballed. If I grok, this would be much worse than useless: It would significantly skew the probability toward randomly selecting outputs which should be blackballed.

Additionally, if the malicious blackballs database is large enough, I hypothesize that distributing different versions to different users may fingerprint them by differently skewing the distribution of output selection. I am less worried about this, due to the magnitude of numbers involved. I doubt it is a useful attack.

Note: I am not 100% sure of any of this. I have expertise in other coins, but I'm a n00b with XMR. Trying to do my homework, avoid common pitfalls.

Answer 1

A blackball list is a set of outputs which are known to be spent - or so the claim is. As you surmise, if that list lists outputs which are not known to be spent, but does not list outputs which are known to be spent, this will indeed cause the wallet to use a weaker ring.

If you do not want to scan the blockchains yourself, a way to mitigate this is to ge lists from two different sources and compare them. At the moment, sgp is planning on making such a list available in the near future.

Last, txids aren't blackballed. The blackballing process is done at the output level, since an transaction might create an output which is later known to be spent, and another which is not. For reference, there are currently about 29 million outputs, and 4.9 million transactions on the Monero blockchain. This makes the 16 million txid number highly suspect.