Data protection

When a session is opened, the server draws at random a token of 8 characters from an alphabet of 26 signs that cannot be confused (no I, L, O, U, 0, 1, nor 2, 5, 6, 8, too close to Z, S, G, B): more than 200 billion possible tokens. A digit typed for the letter it looks like is read as that letter.

The token is the only key to the session: it allows it to be resumed on another device, and it is used to encrypt your data. Resuming the session on another device disconnects the previous one.

The server does not store the token: it keeps only an identifier derived with scrypt, which does not allow it to be recovered. To slow down random attempts, the same address cannot try more than 10 session resumptions in 15 minutes.

s2calc asks for no account, no name and no e-mail address. The data entered concern the undertaking, never a person.

The page loads no script, font or image from another site: no audience measurement, no advertising, no tracker.

Everything you enter and everything the application calculates is encrypted in the database with AES-256-GCM, block by block, each with its own random nonce.

The 256-bit key is derived from your token and from a server-specific secret, kept outside the database: scrypt (about 32 MB of memory per derivation), then HKDF-SHA-256. Neither the token nor the key is stored.

Without the token, a session cannot be read: neither by a person accessing the database, nor by the host in its backups, nor by the site operator. A copy of the database alone is not enough, even for someone trying every token, because the server secret is missing.

A session is erased 30 days after its last use, by a daily task of the server. You can delete your calculations, your inputs or the whole session at any time.

The downloaded session file contains your settings and your inputs, encrypted as in the database, never the results: it can only be read with your token, including to recreate a session after its erasure. A reopened or reloaded session is recalculated.

The other side: a lost token cannot be recovered, and nobody can decrypt the session. Keep it somewhere safe.

A single cookie, needed for operation: it carries the session token and a random witness, so that your browser finds its session again. It cannot be read by any script of the page, is sent only over HTTPS and never to another site, and expires after 30 days at the latest. The server keeps only the fingerprint of the witness.

To limit resumption attempts, the server keeps for 24 hours a fingerprint of the IP address of each attempt, calculated with its secret: it does not allow the address to be recovered. As for any site, the host (o2switch, France) keeps its technical logs; s2calc does not use them.

s2calc processes no identification data. The only technical information that could relate to a person (IP address fingerprint, cookie) is used only for operation and security, and is erased within the periods stated.

For any question about your data: contact@s2calc.com.