File Encryption Tool

Lock any file (a PDF, a photo, a ZIP archive, a spreadsheet) with a password before you email it or store it in the cloud. The file is encrypted with AES-256-GCM on your own device, and the same page decrypts the .enc file back to the original.

Last updated: October 24, 2026

How to use the File Encryption Tool

1. Choose Encrypt files and drop one or more files onto the box, or press Choose files.
2. Enter a strong password twice, or press Generate and store the generated password in your password manager.
3. Press Encrypt files and download each .enc file (or all of them as one ZIP).
4. To open a file later, drop the .enc file here: the tool switches to Decrypt. Enter the same password and press Decrypt files.
5. Download the decrypted file: it gets its original name and type back, and the tool confirms that it was not modified.

How the file encryption works

1. Key derivation: a random 16-byte salt is generated for every file, and a 256-bit AES key is derived from your password with PBKDF2-HMAC-SHA256. The iteration count (600,000 by default) is stored in the file so it can be decrypted later even if you picked a different setting.
2. Metadata: the original file name, MIME type, size and modification time are encrypted as a small first record, so the .enc file reveals nothing about what is inside except its approximate size.
3. Chunked AES-GCM: the content is split into 1 MiB chunks. Each chunk is encrypted with AES-256-GCM using a unique 12-byte nonce (an 8-byte random prefix plus a 4-byte chunk counter) and gets its own 16-byte authentication tag.
4. Tamper detection: the header, the chunk number and a "last chunk" flag are authenticated as additional data. Changing any byte, swapping two chunks or cutting the file short makes decryption fail instead of returning damaged data.

.enc file format

For chunk number i, the IV is nonce prefix || uint32_be(i) and the additional authenticated data is header (37 bytes) || uint32_be(i) || final, where final is 1 only for the last data chunk. An empty file still produces one (empty) final chunk.

Decrypt an .enc file in Python

import json, struct, sys
from cryptography.hazmat.primitives import hashes
from cryptography.hazmat.primitives.kdf.pbkdf2 import PBKDF2HMAC
from cryptography.hazmat.primitives.ciphers.aead import AESGCM

def decrypt_file(path, password):
    data = open(path, 'rb').read()
    header = data[:37]
    assert header[:4] == b'FTCE' and header[4] == 1, 'not an FTCE v1 file'
    iterations, = struct.unpack('>I', header[5:9])
    salt, prefix = header[9:25], header[25:33]
    chunk, = struct.unpack('>I', header[33:37])
    key = PBKDF2HMAC(hashes.SHA256(), 32, salt, iterations).derive(password.encode())
    gcm = AESGCM(key)
    iv = lambda i: prefix + struct.pack('>I', i)
    aad = lambda i, final: header + struct.pack('>I', i) + bytes([final])
    mlen, = struct.unpack('>I', data[37:41])
    meta = json.loads(gcm.decrypt(iv(0), data[41:41 + mlen], aad(0, 0)))
    body, size = data[41 + mlen:], chunk + 16
    pieces = [body[o:o + size] for o in range(0, len(body), size)]
    out = b''.join(gcm.decrypt(iv(i + 1), c, aad(i + 1, int(i == len(pieces) - 1)))
                   for i, c in enumerate(pieces))
    return meta['n'], out

name, content = decrypt_file(sys.argv[1], sys.argv[2])
open(name, 'wb').write(content)
print('wrote', name, len(content), 'bytes')

Tips for encrypting files safely

• Use a password manager. Generate a 20-character password here, save it in your password manager together with a note about which file it opens, and only then delete the unencrypted original.
• Test before deleting. After encrypting, press Test: decrypt them (or decrypt the downloaded file once) to confirm that the password works.
• Encrypt folders as one file. Put many files in a ZIP first (for example with the ZIP File Creator), then encrypt the archive: one password, one file to share, and folder names are hidden too.
• Share the password separately. If the .enc file goes by email, send the password by phone or text message, never in the same thread.
• Encryption does not hide size. The .enc file is about the same size as the original, so someone could guess that a 4 GB file is probably a video.

Frequently asked questions

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