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.

If you are on the receiving end of this right now (Explorer calling the archive invalid, or refusing a password you know is correct), the failure on its own page goes through each of the errors Windows gives, how to tell which of the two schemes your archive uses, and the three ways to open it.

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