How to send an encrypted file safely

You need to send someone a copy of your passport, a payslip or medical results, and the usual routes, email, a chat attachment, a cloud link, all leave the file readable by more people and systems than you’d like. Locking the file first fixes that, but only if the passphrase doesn’t follow the same path. Most mistakes with encrypted files aren’t about the encryption at all.

The rule: two routes

An encrypted file is only as private as its passphrase. If both travel together, anyone who gets one gets both, and the encryption achieves nothing. So the file and the passphrase go by different routes:

• The file can go any convenient way: an email attachment, a chat message, a shared folder, a memory stick. It’s unreadable without the passphrase, so it doesn’t matter much who handles it on the way.
• The passphrase goes another way: a phone call, said in person, or a text to a number you already know is theirs. A voice call has the extra benefit that you know who you’re telling.

The point is that someone would have to break into two separate things, for example both your email and your phone, to read the file.

Step by step

1. Make a passphrase. Use a passphrase generator for six random words. They’re easy to read out over the phone and strong enough for any file.
2. Lock the file. Choose it in the file encryption tool, enter the passphrase twice and save the .locked file. The tool checks the locked file unlocks correctly before offering it.
3. Send the locked file by your usual route. You can mention in the message that the passphrase will follow separately, but don’t hint at what it is.
4. Send the passphrase by the second route.
5. Check it arrived. Ask the person to confirm they could open it. If they couldn’t, the cause is almost always a mistyped passphrase, so read it out again slowly.
6. Clean up. Delete copies of the passphrase from your messages once it’s been used, and delete any unlocked copies you made only for sending.

Mistakes that undo the encryption

• Passphrase in the same email or chat. The most common one, and it leaves the file as exposed as if it had never been locked.
• A guessable passphrase. The person’s birthday, the company name with the year, or “password1” can be guessed offline by anyone holding the file, as fast as their hardware allows. Random words avoid this.
• The same passphrase for everything. Once it’s out, every file you’ve sent with it is readable. Use a new one each time.
• Sending the original by mistake. Check you attached the file ending in .locked, not the original next to it.
• A revealing file name. A name like “passport-scan.pdf.locked” tells anyone who sees it what’s inside. Rename the locked file to something neutral before sending. The original name is stored inside, encrypted, and comes back when it’s unlocked.

Who is the file protected from?

It helps to know what this does and doesn’t cover. A locked file is protected from anyone who handles it in between: the email provider, a chat service’s servers, a shared folder’s owner, someone who later gets into the recipient’s inbox. It isn’t protected from the recipient. Once they unlock it, they have the plain file and can keep or forward it. Only send it to people you’d trust with the original.

It also doesn’t hide that you sent something, to whom, or roughly how large it was. Those details stay visible to whoever carries the message.

When a locked file is the right tool

For a one-off document to one person, a locked file is simple and works with any route. For regular private exchanges, an end-to-end encrypted messaging app may be easier, since it handles keys for you. See end-to-end vs zero-knowledge encryption for what those terms mean.

For files you want to keep rather than send, such as photos and documents on your phone, Silo encrypts them on the device behind its own PIN or passphrase. The locking tool uses the same cipher and key-derivation method, applied to a single file.