How safe is a password-protected zip, really?

A password-protected zip can be genuinely strong or effectively decorative, and the archive gives no visible sign of which one you made. The difference is the encryption scheme, and the default is usually the bad one.

Two schemes wearing the same name

ZipCrypto (ZIP 2.0) is the original scheme from 1990. It is broken. A known-plaintext attack recovers the internal state given a modest amount of content the attacker can guess — and "content the attacker can guess" is easy when archives contain predictable things like a standard header, a familiar logo, or a file the attacker already has a copy of. Tools that do this are free and run in minutes.

AES-256 (WinZip AES) is the modern scheme. It has no equivalent weakness. Against AES-256 the only avenue is guessing the password, which puts the entire security of the archive on how good that password is.

What your tools actually produce

The practical consequence is awkward: on a stock Windows or Mac, the obvious way to make a zip cannot encrypt at all, and the obvious way to add a password from a terminal makes the broken kind. Getting a strong encrypted zip means installing something.

The leak nobody expects: file names

In a standard encrypted zip, the archive directory is not encrypted. File contents are protected; names, sizes, folder structure and timestamps are readable by anyone who has the file, without the password.

That is often the sensitive part. 2026-Q3-layoffs-final.xlsx, medical-report-surname.pdf and passport-scan.jpg tell the story without a single byte being decrypted. If the names matter, a plain encrypted zip is the wrong container: 7-Zip's .7z format with Encrypt file names enabled hides the directory too, and that checkbox is off by default.

And then there is the password itself

With AES-256 the cipher is not the target — you are. Offline guessing against an archive runs at enormous speed on a GPU, with no rate limit and no lockout, because the attacker holds the file and can try forever. A dictionary word plus a year falls in seconds. A name and a birthday falls in seconds.

What survives is length: four or five unrelated words, or a generated passphrase. And it has to travel separately from the archive — emailing the zip and then emailing the password puts both in the same mailbox and reduces the whole exercise to theatre. Say it on the phone, send it on a different app, or use a channel where the secret is not written down next to the payload.

When a zip is the right tool anyway

• Bundling many files into one, which is what zip is actually for.
• Archiving at rest on a drive you control, where AES-256 with a long passphrase is perfectly reasonable.
• Handing a file to a technical recipient who will definitely have 7-Zip and to whom you can pass a passphrase out of band.

Where it goes wrong is as a transfer mechanism for a non-technical recipient: they get a file they may not be able to open, plus a password you now have to deliver securely anyway. You have not removed the hard problem, you have moved it — and added a support call. If the file is being sent rather than stored, encryption that happens as part of the transfer solves the same problem without asking the recipient to install anything.

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