FossCrypt

What is FossCrypt?

FossCrypt is a free file encryption tool that runs entirely in your browser. You give it a file and a password, and it produces an encrypted copy that can only be opened with that same password. The file is never uploaded anywhere: all the math happens on your own device, inside this page. There is no account to create, no limit to accept, and no copy of your data sitting on some server.

Typical uses: sending a scanned passport or a contract over email or a messenger, archiving documents with sensitive information, or handing a file to a cloud drive you do not fully trust. Encrypt it first with FossCrypt, and the channel only ever carries an unreadable blob.

How to use it

1. Pick the mode. Encrypt protects a file; Decrypt opens a file that was encrypted with FossCrypt before (it carries the .foss extension).
2. Drop the file into the box — drag it in or click to choose. The file stays on your device.
3. Type a password and, in encrypt mode, repeat it. Long passphrases beat short complicated ones: four random words are easier to remember and harder to brute-force than P@ss1!.
4. Press the button. A second or two later the encrypted (or decrypted) file is downloaded automatically.

Sharing with someone? Send them the encrypted file one way — email, chat, cloud — and the password a different way: a phone call, an SMS, a meeting. If an attacker intercepts only one of the two, they still have nothing.

The crypto inside, in plain words

FossCrypt does not invent its own cryptography. It uses libsodium, a widely audited, industry-standard library, with two building blocks:

• Argon2id turns your password into a 256-bit encryption key. Argon2id is the winner of the Password Hashing Competition and is deliberately slow and memory-hungry, which makes massive password guessing on GPUs and specialized hardware expensive. Each file gets its own random 128-bit salt, so two files with the same password still produce completely different keys and different ciphertexts.
• XChaCha20-Poly1305 then encrypts the file. It is an authenticated scheme (AEAD): besides scrambling the content, it computes a Poly1305 tag, so any modification of the encrypted file — even a single flipped bit — is detected and decryption refuses to produce a result. The nonce is a random 192-bit value, fresh for every file, and the file header is authenticated too, so tampering with the salt or version markers is also caught.

FossCrypt does not rename your file — it simply adds .foss to the end of the name, so report.pdf becomes report.pdf.foss. You are free to rename the result yourself afterwards: nothing changes, because the extension is purely cosmetic and decryption works with any file name. The original file name is not written in the clear either — it is encrypted together with the file contents, so a .foss file on a flash drive or in an email attachment reveals nothing about what is inside until the correct password is supplied.

Nothing leaves your device

The encryption runs in JavaScript and WebAssembly on your machine. The crypto engine is served from this website itself — no third-party CDN is involved — and once the page has loaded, it even keeps working with your connection switched off. No cookies, no analytics on this page, no accounts, no logs: there is simply no server component to receive anything. Curious? Open your browser's developer tools, switch to the Network tab, and encrypt a file — you will see zero requests leave the page.

File format and limits

A FossCrypt file is a small, documented header followed by the encrypted payload:

Practical limits: the file is processed in memory, so on an ordinary computer anything up to a few hundred megabytes is comfortable; on phones be a little more conservative. And the one thing FossCrypt cannot do is recover a lost password — there is no back door, no reset link and no "hint", because there is no server holding anything. If the password is gone, the data is gone. For important files, store the passphrase in a password manager.