Open a password-protected Word, Excel or PowerPoint file with your password, nothing uploaded

What it does

Checks the password against the file itself. Every encrypted Office file carries a small encrypted test value (the verifier). The password you type is turned into a key and tried on that value once. If it does not match you get a plain “wrong password” message and nothing else happens: no second try, no guessing.
Decrypts the whole document in the browser with the browser's own Web Crypto (AES), then opens it in place in the Word, Excel or PowerPoint viewer, which shows the text, tables, sheets, slides, comments and tracked changes.
Saves an unencrypted copy, named like the original plus -unlocked (Budget.xlsx becomes Budget-unlocked.xlsx). For a Word, Excel or PowerPoint file it is the exact document Office encrypted, byte for byte. Anyone who gets that copy can open it, so keep it somewhere private.
Checks the file's integrity when it has an integrity code (an HMAC; Office 2010 and later add one). If the code does not match, the page says so in plain words: the password was right but the file was changed or damaged after it was saved.
Says how the file is locked before you type anything, for example “AES-256-CBC, the password hashed with SHA-512 100,000 times”.

What it cannot open

Word, Excel and PowerPoint 97–2003 files (.doc, .xls, .ppt) with a password to open. They use the old RC4 encryption (or, for some .xls files, XOR obfuscation), which this page does not decrypt. Office 2007 and later still use RC4 when they save a password-protected file in the old format. Open it in Office or LibreOffice with the password and save it as .docx, .xlsx or .pptx; if you keep a password on the new file, it opens here.
OpenDocument files encrypted with Blowfish. OpenOffice.org and early LibreOffice versions used Blowfish-CFB, until LibreOffice moved to AES-256 around 2011 (Blowfish is still written when a file is saved in the old ODF 1.0/1.1 format). The page recognises a Blowfish file and says so. Opening it in LibreOffice and saving it again re-encrypts it with AES.
OpenDocument files with the newer AES-GCM and Argon2id encryption, which recent LibreOffice versions (24.2 and later) can write. The page recognises the scheme and names it. LibreOffice can save a copy in the older AES-256-CBC scheme when the ODF format version is set to 1.2 or 1.3 (Tools › Options › Load/Save › General).
Files protected with IRM (Information Rights Management: Microsoft Purview, Azure RMS, Windows RMS; Office shows “Restricted Access”). Their key is handed out by the organisation's rights server after you sign in, not made from a password, so no password can open them. They open only in Office signed in to an account with rights to the file.
Files you do not know the password for. This is not a password cracker. It never guesses, never tries a list of common passwords and never retries on its own. Every attempt is one you typed.
Protection that is not encryption is not removed. An Excel sheet or workbook protected against changes (“Protect Sheet”, “Protect Workbook” structure), a Word document with “Restrict Editing” or “Mark as Final”, and a “password to modify” all leave the file unencrypted: it opens in the normal viewer without any password, and this page leaves that protection in the file as it is.
Files encrypted to a certificate rather than a password (rare, set up by an administrator), and Office's Extensible encryption with a third-party provider. Both are named, not opened.

Useful to know about password-protected Office files

A locked .docx is not a ZIP any more: An ordinary .docx, .xlsx or .pptx is a ZIP of XML files. With a password to open, Office encrypts that whole ZIP and wraps it in an old-style compound file (the same container as a .doc from 1997), holding two streams: EncryptionInfo, which says how it is locked, and EncryptedPackage, the encrypted ZIP. That is why an unzip tool says a locked .docx is not an archive, and why the any-file page names it as a password-protected Office file from its bytes.
How strong the lock is: Office 2013 and later use AES-256 and run the password through SHA-512 100,000 times before it becomes a key; Office 2010 used AES-128 with SHA-1, and Office 2007 used AES-128 with SHA-1 50,000 times. The 100,000 rounds are there to make guessing slow. With the right password that costs well under a second here; for someone guessing, it is 100,000 hashes per guess. A short or common password can still be guessed offline, so a long one matters more than the algorithm.
LibreOffice files: An encrypted .odt, .ods or .odp stays a ZIP. Its mimetype and the list of parts in META-INF/manifest.xml are readable; each part is encrypted separately (AES-256-CBC, key from PBKDF2-HMAC-SHA1 over SHA-256 of the password), and the manifest keeps a checksum of the first 1,024 bytes of each part, which is how a wrong password is noticed.
The password and this page: The password field has autocomplete turned off and is emptied as soon as you press Unlock. The password is not written to browser storage, cookies, the address bar or the console, and the page sends no request containing it; the only request this page makes that is not part of viewhack itself is the page-view count when it loads, before any file is chosen.