Symmetric Encryption for Files, Messages, and Data at Rest
If you need to hide a file's contents so only someone with the right key can read them, you need encryption — and AES is the standard the whole industry trusts. AES-256 is the symmetric cipher used by TLS, disk encryption, and password managers. The problem it solves is confidentiality with a shared secret: one password encrypts, the same password decrypts. You need it when shipping encrypted config files, protecting exported backups, or exchanging sensitive documents out of band. Unlike hashing, AES is fully reversible, which makes it the right tool whenever the recipient must recover the original data.
How to Encrypt a Message
- Enter the plaintext you want to protect in the input box.
- Choose a strong passphrase — at least 16 characters, ideally generated randomly.
- Select the output format; the tool returns ciphertext plus a random salt and nonce.
- Copy the complete output, including the salt and nonce; every part is required to decrypt.
- Share the passphrase through a separate channel from the ciphertext.
- Decrypt by pasting the full ciphertext and entering the same passphrase.
Real Example: Input and Output
Encrypting a one-line secret with a passphrase produces a self-contained blob. The ciphertext itself is binary-safe, and the tool encodes it as text for easy copying.
| Input | Output (abbreviated) |
|---|---|
api_key=sk_live_42 | U2FsdGVkX1+qZx...kR4nT0sA== |
Same plaintext, same passphrase, different run — the random salt guarantees a different ciphertext every time. That is a feature: identical ciphertext would leak that the messages are the same.
Authenticated Encryption: Why the Tag Matters
Modern AES implementations use GCM mode, which appends an authentication tag. The tag proves the ciphertext was not tampered with, so a modified blob fails decryption instead of producing garbage silently. This protects against attackers who flip bits in encrypted data.
- Confidentiality — the cipher hides the plaintext from anyone without the key.
- Integrity — the GCM tag detects any modification of the ciphertext.
- Key derivation — the passphrase is stretched with a salt before AES runs, slowing brute force.
Tips for Best Results
- Use a password manager — humans cannot remember the random passphrases AES deserves.
- Never reuse passphrases — a leaked key compromises every file encrypted with it.
- Back up the passphrase — AES offers no recovery; losing the key means losing the data.
- Prefer GCM over ECB — ECB mode leaks patterns and is unsafe for anything longer than one block.
Everything runs locally in your browser through the Web Crypto API. The plaintext, passphrase, and derived key never leave your device, and nothing is uploaded to a server. This makes the tool practical for quick, sensitive work, though for regulated data you should still use audited, offline software.
Frequently Asked Questions
Is AES the same as hashing?
No. AES is reversible: with the key you get the original data back. Hashing is one-way and used for verification, not recovery.
What does AES-256 mean?
The 256 refers to the key size in bits. AES-256 uses a 256-bit key and is the strongest commonly deployed variant.
What is a nonce in AES-GCM?
A nonce is a unique number used once per encryption. It must never repeat for the same key, or the security of GCM degrades.
Why is the ciphertext different every time?
A random salt and nonce are generated per run and fed into the algorithm, so identical plaintexts encrypt to different outputs.
Can AES be broken by brute force?
Not practically. A 256-bit key space is far beyond the reach of all existing hardware combined.
Why do I need the salt to decrypt?
The salt is part of key derivation: it prevents the same passphrase from always producing the same key. Without it, decryption fails.
What is the authentication tag for?
The tag lets the decryptor verify the ciphertext was not altered. Any tampering makes the tag mismatch and decryption fail.
Should I encrypt passwords with AES?
No. Passwords should be hashed with bcrypt or Argon2id. AES is for data you need to recover, not for credentials.
Does this tool send my data to a server?
No. Encryption and decryption run entirely in your browser with the Web Crypto API, so your plaintext and passphrase never leave your device.