I checked out LeakCheck 1.002 and the “run everything locally on your PC” claim sounds reassuring at first, but it really depends on what they mean by “locally.”

If it truly runs locally, then the only thing your computer should be doing is downloading a database of known breached password hashes, then comparing your password (or its hash) against that dataset on your own device. In that case, nothing sensitive would need to leave your PC, and the internet would only be used for updates.

But the moment anything is sent to their servers, even just for comparison, it stops being a local check in any meaningful privacy sense. At that point you’re trusting them with either your password or at least a hash of it, plus trusting that nothing is logged, stored, or exposed through a breach or misuse.

And that’s really the core issue here: your password always exists in plaintext for a moment because you typed it in. The question isn’t whether your computer sees it (it has to), but whether anyone else ever gets access to it, directly or indirectly.

A few things that would make this a lot clearer, and honestly should be front and center from the developers:
-Does any data leave the computer at all during a check?
-If yes, is it the full password, a hash, or something anonymized?
-Is anything logged on their side, even temporarily?
-Is the breach database fully local or partially cloud-based?
-Has the system been independently audited?

Without clear answers to that, “local” can become more of a marketing word than a technical guarantee.

One more important point: even if LeakCheck says “no breach found,” that doesn’t really prove a password is safe. It just means it wasn’t in their dataset. A password can still be weak, reused, or just not yet leaked.

So overall, the idea of local checking is good in theory, but the trust model matters a lot in practice. If it’s truly offline with local comparison, that’s solid. If anything is being sent to servers, then it’s basically just another online breach checker with the usual privacy tradeoffs—no matter how it’s phrased.

I'm not suggesting this company is sketchy, but after being told for the past 30+ years to never give out my passwords, it seems to me to be a bit risky to tell anyone all of my e-mail addresses and passwords. What protection is there for my info if I use this service?