AES Decryption Tool - Decrypt Text Online

Decrypt your encrypted messages with our free Online AES Decryption Tool. Paste the ciphertext, supply the secret key that was used to encrypt it, and recover the original plaintext. All processing happens in your browser — no data is sent to a server.

AES is a symmetric cipher, so the same key encrypts and decrypts. This tool is the reverse of the AES Encryption Tool: feed it the ciphertext and the matching key, and it reconstructs the original message. Without the correct key, AES cannot be reversed in any practical timeframe.

Live Example

With the correct key and ciphertext, the original plaintext is restored exactly:

Key: my-secret-passphrase
Ciphertext: U2FsdGVkX1+3xY9pQr2...

Plain text: Attack at dawn

How to Use the AES Decryption Tool

1. Enter Secret Key: Type the same secret key that was used for encryption.
2. Enter Encrypted Text: Paste the encrypted text you want to decrypt.
3. Click Decrypt: Click the Decrypt button to retrieve the original message.
4. Copy Result: Copy the decrypted text for your use.

Key Features

• AES (Advanced Encryption Standard) for industry-grade security
• Decrypt text with a custom secret key
• All processing runs in your browser with zero server communication
• Copy decrypted output to clipboard
• Free to use with no registration required

Common Use Cases

• Secure Communication: Decrypt messages received from others via email, chat, or other channels.
• Password Retrieval: Retrieve sensitive text stored in encrypted form.
• Data Privacy: Access confidential information in documents and files.
• Developer Testing: Test AES decryption logic for applications.

Benefits

• Complete client-side processing so your data never leaves your device
• No registration or account needed
• Works on desktop and mobile devices
• Clean, intuitive interface

About AES Decryption

Decryption reverses encryption: it takes the ciphertext and the secret key and reconstructs the original plaintext. AES is symmetric, so the key you use here must be the exact same one used during encryption — including the same passphrase-based key derivation. The tool also needs the salt/IV that was generated at encryption time; when those are embedded in the ciphertext payload (as is standard), decryption reproduces the bytes faithfully.

If the key is wrong, you do not get a "close" plaintext — you get either an explicit error (a padding or authentication failure) or meaningless bytes. That is by design: AES has no notion of "almost right." This is what makes it secure, and also why losing the key means the data is gone for good. Authenticated modes like GCM add a verification tag so tampered ciphertext is rejected outright rather than decrypted to garbage.

Because decryption runs entirely in your browser, your key and ciphertext never leave your device — important when handling sensitive material. Always confirm the ciphertext came from a trusted source, and never paste a real production secret into an untrusted online tool. To generate fresh strong keys, use the Password Generator.

How the Decryption Process Works Internally

When you click Decrypt, the tool performs the same key-derivation step that encryption used, then reverses the cipher. Because AES itself works on a raw key (256 bits by default) rather than a human passphrase, the two steps below reconstruct that key from your secret key plus a salt that is embedded in the ciphertext:

1. Parse the payload: the base64 ciphertext is decoded back into bytes. CryptoJS output begins with the marker Salted__ followed by an 8-byte random salt, so the tool splits off the salt and keeps only the real ciphertext that follows.
2. Re-derive the key: your secret key and that same salt are fed through OpenSSL-compatible key derivation (the same routine used at encryption time) to reproduce the exact 256-bit AES key and IV. This is why decryption fails when even one character of the passphrase differs.
3. Run AES-CBC in reverse: the ciphertext is decrypted block by block (AES processes data in 128-bit blocks), and PKCS#7 padding is stripped from the final block to recover the original byte length.
4. Decode to text: the recovered bytes are interpreted as UTF-8 and displayed as plaintext. If the key was wrong, this step typically throws a "Malformed UTF-8" error because random bytes are rarely valid UTF-8.

Every step runs locally inside your browser through the Web Crypto-compatible crypto-js library. Your secret key and ciphertext are never transmitted over the network, logged, or persisted after you leave the page.

Understanding the Ciphertext Format

Look closely at any ciphertext this tool accepts and you will notice it almost always starts with U2FsdGVkX1. That is not a coincidence — it is base64 for the literal string Salted__, which is how the OpenSSL format signals that a random salt follows:

base64("Salted__" + 8-byte-salt + real-ciphertext)
         ^^^^^^^^^   ^^^^^^^^^^^^^^   ^^^^^^^^^^^^^^^^
         marker       random salt      AES-CBC output

The salt is not secret — its job is to ensure that encrypting the same message twice with the same key produces two different ciphertexts. The salt must be sent alongside the ciphertext, otherwise the key cannot be re-derived and decryption is impossible. This is why you should copy the entire base64 string, not a truncated middle portion.

Because the salt is baked into the payload, you only need to remember your passphrase — there is no separate "salt" or "IV" field to manage. The Encrypt tool generates a fresh random salt on every run and bundles it into the output for you.

AES Decryption Modes: CBC vs GCM

This tool uses AES-CBC (Cipher Block Chaining) with PKCS#7 padding, the OpenSSL default and the most widely interoperable mode for passphrase-based encryption. It is fast and well-supported, but it is not authenticated — it has no way to prove the ciphertext was not tampered with, so a modified payload decrypts to garbage (or throws a padding error) rather than being rejected outright.

AES-GCM is an authenticated mode that appends a verification tag. If a single bit of the ciphertext is altered, GCM detects it and refuses to decrypt. GCM is preferred for modern systems that need integrity guarantees, but it is a different format and is not interchangeable with the CBC output this tool produces — an OpenSSL/CryptoJS CBC ciphertext will not decrypt in a GCM tool, and vice versa.

If you are unsure which mode produced your ciphertext, the Salted__ prefix is a reliable tell: it is emitted by CBC passphrase-based encryption (the format this tool expects). Raw hex or binary payloads without that marker were likely produced by a different library or mode and may need their native tool to decrypt.

Common Errors and How to Fix Them

• "Malformed UTF-8" or garbage output: The key does not match, the ciphertext is corrupted, or the salt/IV is missing. Use the exact key from encryption.
• Wrong key yields no error, just junk: Some modes decrypt silently to random bytes. Always verify the result reads sensibly.
• Ciphertext truncated: If the base64 string was cut during copy-paste, decryption fails. Re-copy the full value.
• Lost the key: AES is irreversible without it. There is no backdoor; keep keys in a password manager.