AES Decryption Online

What is AES decryption?

AES (Advanced Encryption Standard) is a symmetric cipher. This tool lets you decrypt AES ciphertext by providing the key, mode, IV/nonce, padding, and optional GCM authentication tag. Everything runs in your browser.

How to use the AES decryption tool

• Enter Input: Add your plain text for encryption or your AES-encrypted text for decryption.
• Select Cipher Mode: Choose the AES mode (CBC, ECB, etc.). Use the same mode for decryption that was used during encryption.
• Choose Padding: Pick the padding method required for your operation (must match during decryption).
• Add IV (If Required): Enter the Initialization Vector if your selected cipher mode uses one. For decryption, use the exact IV from encryption.
• Set Key Size: Select the key length (128/192/256 bits). This must match between encryption and decryption.
• Enter Secret Key: Provide the key used for encryption. Use the same key while decrypting.
• Select Output Format: Choose how you want the result displayed - Base64, Hex, or Plain Text (for decrypted output).
• Run the Action: Click Encrypt to generate encrypted output or Decrypt to retrieve the original text.

Supported modes and padding

• CBC: Use a 16-byte IV and PKCS5/PKCS7 padding. NoPadding isn’t supported in WebCrypto.
• CTR: Stream-like; requires a unique 16-byte IV/nonce for every message.
• GCM: Authenticated encryption; needs a 12-byte IV/nonce and an auth tag. Include the tag when decrypting.

Tips for successful decryption

• Match mode, key length, IV/nonce, padding, and tag (for GCM) exactly to the original encryption.
• Verify encodings: ciphertext/IV/tag can be Base64 or Hex—use the correct format switch if available.
• Don’t reuse IVs with the same key; generate fresh IVs per encryption.
• If decryption fails, double-check tag length (GCM) and that the IV is the right size.

AES best practices

• Avoid ECB: ECB leaks patterns; stick to CBC, CTR, or GCM.
• Use unique IVs: Generate a fresh IV per encryption; for GCM use 12 bytes, for CBC/CTR use 16 bytes.
• Don’t reuse keys across systems: Rotate keys and separate test/prod secrets.
• Prefer authenticated encryption: GCM provides integrity; pair CBC/CTR with an HMAC.

Frequently Asked Questions (FAQs)

Which AES mode should I choose?

GCM is recommended because it encrypts and authenticates data. CBC is common for legacy systems; pair it with HMAC. CTR is stream-like and efficient but needs a unique IV per message.

What IV length do I need?

Use 12 bytes for GCM and 16 bytes for CBC or CTR. Reusing an IV with the same key weakens security, so generate a fresh IV every time.

Can this tool decrypt AES from other libraries?

Yes, match the mode, key size, padding, IV, and encoding (base64/hex). If the ciphertext is IV:ciphertext, paste it directly; otherwise, provide the IV separately.

Do you store my keys or ciphertext?

No. Everything runs locally in your browser; TestMu AI does not transmit or store your input, IV, or keys.