AES Encryptor and Decryptor

Sometimes you need to protect a note, a password list or a small file without setting up a key pair. Passphrase encryption is the simplest way: both sides know a secret phrase, and the phrase is turned into a key. This page implements that with standard building blocks from your browser’s Web Crypto API, so no library code is involved and nothing is uploaded or saved.

How the key and the cipher work

A passphrase is a poor key because it is short and guessable. PBKDF2-HMAC-SHA-256 fixes that by repeating the hash 600,000 times with a random salt, which turns the phrase into a 256-bit key and makes each guess by an attacker costly. The data is then encrypted with AES in Galois/Counter Mode. GCM does two jobs: it scrambles the data, and it computes an authentication tag over everything, so a changed byte is detected instead of producing garbage.

The container

The result starts with a small header: the marker OWE1, a version byte, the iteration count, the 16-byte salt and the 12-byte IV. Those parts are not secret, but they are included in the authentication, so tampering with the iteration count to weaken the key is caught too. When shown as Base64, the marker appears as T1dFMQ at the start of the text. The ciphertext and tag follow. This is the format of this site only; other programs cannot open it, so keep a copy of the tool or use an interoperable standard such as age or GnuPG when you need to exchange files.

A worked round trip

Choose Text, press Sample (which loads “Meeting moved to Friday 10:00. Room code 4471.”), enter a passphrase twice and press Encrypt. The Base64 output starts with T1dFMQEA; for a 32-character sentence it came to 116 characters, because the header, salt, IV and authentication tag add about 50 bytes. Switch to Decrypt, paste that text, and enter the right passphrase to get the sentence back. With a wrong passphrase the page reports “Decryption failed: the passphrase is wrong or the data was changed.” and shows nothing else. It never prints partial plaintext.

Choosing a passphrase

The whole scheme is only as strong as the phrase. The key stretching slows an attacker down by a large factor, but it cannot rescue a guessable phrase. Use four or more random words, or a long phrase from the passphrase generator, and do not reuse it. The page asks for the phrase twice when encrypting to avoid a typing slip that would lock you out permanently.

Files

Switch the content type to File to encrypt any file up to the memory your device can spare. The encrypted result downloads with .owenc added to the name, and decrypting a .owenc file removes that ending to restore the original name. Files are processed on your device.

Limits

If you lose the passphrase, the data is gone; there is no recovery. The tool does not hide file size or the fact that data is encrypted. It does not sign messages or manage keys, and its format is not interoperable.

Related tools

• Diceware Passphrase Generator makes memorable passphrases of known strength.
• PBKDF2 and HKDF Calculator lets you explore the key derivation step.
• Shamir Secret Sharing splits a passphrase between several people.
• Image Steganography can hide the Base64 output inside a picture.