Hm. I suppose a hacky-ugly-cheapo way to do this would be to run a daemon under root which accesses a local sqlite db or whatnot (and that file is only reachabl

Hm. I suppose a hacky-ugly-cheapo way to do this would be to run a daemon under root which accesses a local sqlite db or whatnot (and that file is only reachable by root). When a web/whatever app needs to check a hash, it asks the daemon whether this user + this hash are valid. This could be done e.g. using unix sockets, and the daemon could do rate limiting (one attempt per two seconds for a specific user, etc). The exchange could also be encrypted using a pre-defined key, so having access to the socket wouldn't even let you sniff the exchange. If a cracker gets access to the web app, they'll be able to monitor the exchange, but they'll need actual privilege escalation to be able to read/dump the whole hash database.