How friend-lock works

The core idea: public-key cryptography

The tempting first design is: my friend hashes their password, gives me the hash, I encrypt with it. That can't work. With any symmetric scheme, whatever locks also unlocks — if you can encrypt with a secret, you can decrypt with it too. So you'd be able to reopen the file, which defeats the whole purpose.

The primitive that separates "can lock" from "can unlock" is public-key cryptography. Your friend holds a key pair:

• a public key — can only lock. Safe to share. This is the "lock code".
• a private key — can unlock. Never leaves your friend; in fact it's never stored anywhere — it's re-derived from their password only for the moment they unlock.

Step by step

1. Your friend creates a lock codeThey pick a password. friend-lock stretches it with Argon2id into a 32-byte seed, and deterministically derives an X25519 key pair from that seed. You receive only the public key (plus a random, non-secret salt), packed into a short lock code.
2. You lock the filefriend-lock zips your file(s), then encrypts the zip to your friend's public key using a libsodium sealed box. A sealed box generates a throwaway ephemeral key pair, encrypts with it, and discards the ephemeral private key — so nobody, not even you, can decrypt afterwards. Only the recipient's private key can.
3. Your friend unlocksThey enter their password, friend-lock re-derives the exact same private key (same password + same salt → same key), opens the sealed box, and unzips the original file(s).

The building blocks

All cryptography is provided by libsodium (the audited -sumo build), running as WebAssembly in your browser.

The "it's just a photo" carrier

The locked output is a real, viewable .jpg: a cover image with a short notice and a QR code. A JPEG officially ends at its end-of-image marker, and viewers ignore anything after it — so the encrypted data is appended there. Open the file in any photo viewer and you see the picture; drop it into friend-lock and it reads past the image and decrypts. A plain .flock download is offered too, for channels that re-compress images.

File formats

lock code    = base64url( u8 version | u32 ops | u32 mem | 16B salt | 32B public key )
.flock blob  = "FLK1" | u8 version | u32 ops | u32 mem | 16B salt | sealed ciphertext
.jpg carrier = <valid JPEG cover> | .flock blob | "FLCKPLD1" | u64 payload length

Privacy — where everything runs

friend-lock is a single static web page. All processing happens in your browser. Your files and passwords are never uploaded — there is no server to upload them to, no database, no account, and no tracking. You can even save the page and use it fully offline. See our privacy note for details.

Security notes & disclaimer

• Everything rests on the password. Because the private key is derived from it, a stolen locked file is open to offline password guessing. Argon2id makes each guess slow, but a weak password is still weak — choose a strong passphrase.
• There is no password reset. Forget the password and the file is unrecoverable. That is by design.
• The lock code is safe to share — it holds only a public key and a salt.
• The sealed box provides confidentiality and tamper-detection, but the sender is anonymous: your friend can't cryptographically prove you were the one who locked it.