How to password protect a zip file, what it hides, and what it does not

Drop the files into STRONGBOX, type a passphrase, and it writes a zip encrypted with AES-256 in the WinZip form that 7-Zip, WinZip, WinRAR, Keka and The Unarchiver all read. It also opens one, given the passphrase. Everything happens in the tab; the files are not uploaded and the page works with the network off. Three things to know before you rely on it: Windows and macOS cannot open an AES zip with the tools they ship with, the older kind of zip password is broken and should not be used, and a locked zip still tells anyone holding it what the files are called.

Zip has two kinds of password

STRONGBOX writes AES-256 only. It will open a ZipCrypto archive, because somebody will send you one, and it says which kind it found.

The part that catches people out

Windows Explorer cannot open an AES-encrypted zip. Neither can the Archive Utility that macOS uses when you double-click one. Both handle zip files perfectly well and both only understand the old ZipCrypto password. Double-clicking an AES zip on a stock Windows machine gives an unhelpful error, or an empty folder, or a prompt that rejects the correct password.

This is not a fault in the file. It is a gap in the built-in tools, and it has been there for twenty years. If you are sending a locked zip to somebody, tell them to open it with 7-Zip (free, Windows), Keka or The Unarchiver (free, macOS), or WinZip or WinRAR if they have them — or send them a link to STRONGBOX, which opens it in their browser with nothing to install.

What a locked zip does not hide

Zip encrypts the contents of each file. It does not encrypt the list of files. Anybody holding the archive can read, without the password:

• Every file name and folder path. Dismissal letter - Hannah Reid.docx tells the story before anyone types a password.
• The size of each file, before and after compression, and how well each one compressed.
• The timestamps on each file.
• The number of files.

For most jobs this does not matter. For some — a leak, a grievance, a medical record — the names are the disclosure. The fix is to put the revealing zip inside a second, plain zip named something dull, or to use a container that encrypts its own index: CAPSULE makes a single encrypted file that opens in a browser and gives nothing away from the outside.

Choosing the passphrase

A zip passphrase is stretched with PBKDF2 at a thousand rounds, which was a reasonable number in 2003 and is a thin margin now: a modern graphics card tries a great many guesses a second. That makes the passphrase itself the whole of the protection. Four or five unrelated words beat any amount of punctuation stuffed into a short one — PHRASE generates them, and SHADOWKEY will tell you what a given one is actually worth.

And send it separately from the file. A password in the same email as the archive is a key taped to the envelope.

What gets compressed and what does not

STRONGBOX compresses what compresses and stores what does not. Text, spreadsheets, documents and logs shrink by a lot. Photographs, video, music and anything already zipped are already compressed, so they are stored as they are rather than churned through an algorithm that would add a fraction of a percent. The listing shows which happened to each file.

Opening one

Drop the archive in, type the passphrase, and the contents are listed: what is in it, how large each file is, and whether the encryption is AES or the old ZipCrypto. Save one file or save all of them. A wrong passphrase is caught immediately — the format carries a two-byte check for exactly that — and said in plain words rather than as a failure halfway through.

Zip against the alternatives

The short version