Tells you whether a wallet.aes.json is structurally intact or truncated, and therefore whether "decrypt failed" is about your file or about your password. It reads the format version, PBKDF2 iteration count, base64 validity, and AES block alignment, none of which need your password. This file runs entirely on your machine; the whole point of downloading it is to disconnect from the internet before you use it on a real wallet file.
Your backup file never leaves this browser tab. There is no upload, no server, and no network request of any kind. The file is read locally, in your own browser, purely to measure its structure. Nothing about it is transmitted, stored, or logged.
You do not need a password to use this. Everything the checker reports about your file, including whether it is intact or has lost bytes, is readable from the file's structure alone. That answer comes back before a password is mentioned.
If the file is intact, you can then test one password you have in mind. It runs in this tab like everything else here: your password is not transmitted, not stored, and not written into the page, and only the two blocks needed to answer yes or no are decrypted. It tests the one password you type. It does not guess, and it cannot find a password you have forgotten.
The rule that matters has not changed. Never send your wallet.aes.json and your password to somebody else's server. Together they are the entire wallet. A site that uploads both is asking for enough to take your coins, and that is a different thing from a page that reads a file you chose, in your own browser, and sends nothing anywhere.
Given that this is live money, the safest way to run this is exactly how you're using it now: offline, disconnected from the internet. It's the recommended path for this tool, not just an option.
wallet.aes.json, or whatever the file was renamed to. Nothing leaves this browser tab.Nothing here is uploaded. Verify it yourself: open your browser's network tab before dropping the file, and confirm nothing fires.