AES-256 text encryption and decryption

What happens when you encrypt

A password is not a key: it is too short and too predictable to use directly. So the tool first stretches the password into a 256 bit key with Argon2id, a function built to be slow and memory hungry on purpose, using a random salt so the same password gives a different key every time. The text is then encrypted with AES-256 in GCM mode, the cipher used by HTTPS and messaging apps, which also adds a tag that detects any change to the encrypted text.

The result is one block of text that contains everything needed to decrypt it except the password: a short header naming the method and its settings, the salt, the nonce, the encrypted text and the tag. Paste it into an email, a note or a chat, and send the password through a different channel, such as a phone call.

How strong it is

AES-256 itself is not the weak point; the password is. Anyone who has the encrypted text can try passwords offline, as fast as their hardware allows. That is why the key derivation is deliberately slow:

• Argon2id, the default, uses 64 MiB of memory and 3 passes for every guess. Memory hard functions blunt the advantage of graphics cards and custom chips, which have lots of computing power but little memory per core.
• PBKDF2 with SHA-256, from 310,000 to 2,000,000 iterations, is the older standard, available everywhere. 600,000 iterations follows current OWASP advice.

Even so, a short or common password falls quickly. Use a passphrase of five or six random words, which the password generator makes; how long it takes to crack a password shows the numbers.

Decrypting

Switch to Decrypt, paste the block and type the password. The settings are read from the header, so you do not need to remember which method was used. If a single character of the block was changed, or the password is wrong, decryption fails with a clear message rather than producing garbage, thanks to the GCM tag. There is no way to recover the text without the password, here or anywhere: keep it somewhere safe.

What it is good for

• Sending one secret, such as a Wi-Fi password, an account recovery code or bank details, through a channel you do not fully trust.
• Keeping a note in a cloud document or email draft that the provider cannot read.
• Not a replacement for end to end encrypted messengers, or for a password manager that keeps many secrets.

For files, protect a PDF with protect PDF or a set of files with a password protected archive. To check a file has not changed, without hiding it, use the hash generator.

If something goes wrong

• It says it could not decrypt. The password is wrong, or the text was changed or cut short when copying. Copy the whole block again.
• It says the text is not from this tool. The block was made by another program, or the start of it is missing.
• Encrypting or decrypting takes a moment. That is the key derivation working as intended; every guess costs an attacker the same time.