How do I encrypt a single file with a password?

Use a tool that applies AES encryption with a key derived from your passphrase. A browser-based one encrypts the file locally without uploading it, then you download the encrypted result and share it. The critical step people miss is sending the password by a different channel than the file.

What makes a strong passphrase for file encryption?

Length over complexity — four or five random words is stronger and more memorable than a short string of symbols. Don't reuse a password from anywhere else, and store it in a password manager if it's a file you need to keep. The passphrase, not the cipher, is what an attacker actually attacks.

How do I send the password to the recipient safely?

Never in the same channel as the file. Email the encrypted file, then send the password by text, a messaging app, or over the phone. Splitting the file and the key across two channels means an attacker has to compromise both, which is far harder than intercepting one email that contains everything.

Is it safe to encrypt a file on a website?

Only if the tool encrypts it in your browser rather than uploading it. If the site uploads your file to encrypt it, your plaintext sits on their server for those moments — which defeats the point for a sensitive file. Use a client-side tool that never transmits the file.

How to encrypt a file with a password (and share it safely)

What encrypting a file actually does

Encryption turns your file into a block of random-looking bytes that can only be turned back by someone with the key. Modern tools use AES — the same cipher governments use for classified material — and in a good implementation the key is derived from a passphrase you choose. Without the passphrase, the encrypted file is noise: there's no 'forgot password' link, no backdoor, and no amount of computing power that brute-forces a strong passphrase in a human lifetime.

That strength is also the trap. Encryption doesn't care that it's you who lost the passphrase — a file you can't decrypt is as gone as a file an attacker can't. So the two things that matter are choosing a passphrase strong enough to resist guessing but memorable or stored well enough that you don't lose it, and getting that passphrase to your recipient without leaking it.

Choosing a passphrase that actually protects the file

Length beats complexity. Four or five random words — 'correct-horse-battery-staple' style — is both stronger and more memorable than 'P@ssw0rd!'. Each extra word multiplies the guessing effort.
Don't reuse a password you use anywhere else. If it leaks from some unrelated breach, your encrypted file leaks with it.
Don't put the passphrase in the same email, chat or folder as the file. That's the equivalent of taping the key to the lock — covered in the next section.
If it's a file you need to keep, store the passphrase in a password manager. A file you can't decrypt is a file you've lost.

The strength of the whole thing rests on the passphrase, because the encryption itself is not the weak link — the passphrase is. An attacker doesn't attack AES; they guess your password. So the passphrase has to be hard to guess, which is not the same as hard to type.

The part everyone gets wrong: sharing the password

Here is where most people undo the entire exercise. They encrypt a file, attach it to an email, and put the password in the same email — or the next one, to the same inbox. If that inbox is compromised, or the email is intercepted, the attacker now has both the locked box and the key. You've added a step and protected nothing.

The rule is simple: the file and the password must travel by different channels. Email the encrypted file, then send the password by text message, or a signal/whatsapp message, or say it over the phone. Two channels means an attacker has to compromise both to get in, which is dramatically harder than compromising one. For something high-stakes, agree the passphrase in person beforehand and never transmit it at all.

Doing it without uploading the file first

There's a contradiction in most 'encrypt your file online' sites: to encrypt your file, they upload it to their server — so for the moments before it's encrypted, your plaintext file sits on someone else's machine. If the file is sensitive enough to encrypt, it's sensitive enough not to hand over unencrypted first.

Browser-based encryption avoids this entirely: the file is encrypted in the tab, on your machine, and only the encrypted result exists anywhere. SnapToolz's [AES Encryption](/security/aes/) works this way — it uses AES-256-GCM, derives the key from your passphrase locally, and never transmits the file. If you want to understand exactly what GCM mode adds over plain AES, the [AES-GCM explainer](/guides/aes-gcm-encryption-explained/) walks through it. And for a strong passphrase you don't have to invent, the [password generator](/security/password/) makes one.

Tools used in this guide

Run it as a workflow

Privacy-clean a PDF
Unlock the password, strip metadata + author info, then watermark as your own copy.

FAQ

How do I encrypt a single file with a password?: Use a tool that applies AES encryption with a key derived from your passphrase. A browser-based one encrypts the file locally without uploading it, then you download the encrypted result and share it. The critical step people miss is sending the password by a different channel than the file.
What makes a strong passphrase for file encryption?: Length over complexity — four or five random words is stronger and more memorable than a short string of symbols. Don't reuse a password from anywhere else, and store it in a password manager if it's a file you need to keep. The passphrase, not the cipher, is what an attacker actually attacks.
How do I send the password to the recipient safely?: Never in the same channel as the file. Email the encrypted file, then send the password by text, a messaging app, or over the phone. Splitting the file and the key across two channels means an attacker has to compromise both, which is far harder than intercepting one email that contains everything.
Is it safe to encrypt a file on a website?: Only if the tool encrypts it in your browser rather than uploading it. If the site uploads your file to encrypt it, your plaintext sits on their server for those moments — which defeats the point for a sensitive file. Use a client-side tool that never transmits the file.