# Local File Encryption Studio

> Encrypt and decrypt files with AES-256-GCM and a password, locally in your browser. Encrypts any file up to 100 MB with AES-256-GCM, using a password (PBKDF2-SHA-256, at least 600,000 iterations) or a raw 256-bit key, and decrypts it again. The output is a small documented container. With JavaScript on, everything runs in your browser via WebCrypto and the file never leaves your device.

Part of usefulweb.site (https://usefulweb.site/). Results are deterministic calculations from the inputs and the listed assumptions; the API returns exactly what the web page shows.

## Use it
- Human interface: https://fileencryptionstudio.usefulweb.site/ (interactive tool at https://fileencryptionstudio.usefulweb.site/tool)
- JSON API: POST https://fileencryptionstudio.usefulweb.site/api/v1/calculate with a JSON object of inputs (schema: https://fileencryptionstudio.usefulweb.site/api/v1/tool)
- OpenAPI: https://fileencryptionstudio.usefulweb.site/openapi.json
- MCP (Streamable HTTP): https://fileencryptionstudio.usefulweb.site/mcp - tool `file_encryption_studio`

## Inputs
- action (select): Action
- file (file, required): File - Up to 100 MB. With JavaScript on, the file is processed in your browser and never uploaded.
- keyMode (select): Key
- password (text): Password - Use a long passphrase. It cannot be recovered: without it the file is lost.
- rawKey (text): Raw key (64 hex or 44 Base64 characters) - Raw-key mode. Leave empty when encrypting to generate a random key (shown once).
- iterations (integer): PBKDF2 iterations (encrypt) - At least 600,000 (OWASP 2023 recommendation for PBKDF2-HMAC-SHA-256). Decryption reads the value from the file.
- seed (text): Demo seed (optional; fixes salt and IV — never for real files) - Only for reproducible demos. Reusing a salt/IV with the same password breaks AES-GCM security.

## Method
- Encryption: AES-256 in GCM mode (authenticated encryption) with a random 96-bit IV. The 16-byte authentication tag detects a wrong key and any modification of the file or its header.
- Password mode: the key is PBKDF2-HMAC-SHA-256(password in Unicode NFC, UTF-8; random 16-byte salt; ≥ 600,000 iterations; 32 bytes). Raw-key mode uses your 32-byte key directly (or a random one generated for you).
- Container "UWFE" v1, all integers big-endian: bytes 0–3 "UWFE"; 4 version (1); 5 KDF (1 = PBKDF2-SHA-256, 0 = raw key); 6–9 iterations (0 for raw key); 10 salt length S; then S salt bytes; then IV length (12) and the IV; then ciphertext followed by the 16-byte tag. The whole header is authenticated as GCM additional data.
- With JavaScript on, the browser does everything with WebCrypto (crypto.subtle: PBKDF2, AES-GCM) and offers the result as a local download; the file and password never leave your device.
- Without JavaScript or via the API/MCP, the server performs the same steps with Node.js crypto. It then receives the file and the password; the output is kept as a download for about 2 hours and then deleted.

## Limits
- If you forget the password or lose the key, the file cannot be recovered. Keep an unencrypted backup until you have tested decryption.