RLN (Rate Limit Nullifier) is is not an application but a novel building block for anonymous, permissionless environments. As it’s name suggest, RLN has the ability to rate limit anonymous activity per user w/o necessarily requiring an economic value at stake (such as PoW, PoS or fees).
Imagine a completely anonymous chat forum where members must ZK prove they own an NFT to join and start sending messages. Chat is anonymous. You know they own an NFT (thanks to zkp) but don’t know which NFT holder is sending a msg. Users can use ZKPs to build other id abstractions (for example prove they are a genesis ETH holder, a Punk owner etc.) That’s pretty much a solved problem.
The challenge is spamming. With RLN, we can rate limit activity in this forum (for example to 5 msgs per minute for each user) w/o asking fees for messages. Everytime a user sends a message, a share of their private key gets leaked. If they don’t break the rule (send <5 msg per min) shares don’t mean much. Their private key is safe. If they break the rule (send>5 msgs), anyone can reconstruct their private key using those 5 shares. With their private key exposed anyone can revoke their participation from the group.
This is new solution in that it opens the door for non-plutocratic spam prevention. Even if users have an empty address, they lose access to their private key, which potentially controls valuable on-chain history. Their reputation gets slashed!