Encrypt / Decrypt Text Online

Both modes encrypt data with the same AES block cipher, but they differ in how they turn that cipher into something you can use on real messages, and that difference matters for security.

AES-CBC (Cipher Block Chaining) splits your plaintext into fixed-size blocks and XORs each block with the previous ciphertext block before encrypting, using a random initialisation vector (IV) to seed the chain. It hides patterns in the plaintext well, but CBC on its own provides no way to tell whether the ciphertext has been tampered with: a modified byte will decrypt into a different message rather than fail outright, an issue known as a lack of authentication.

AES-GCM (Galois/Counter Mode) combines encryption with a built-in authentication tag. When you decrypt, the tag is verified first: if the ciphertext, IV, or associated data has been altered in any way, decryption fails outright instead of silently producing corrupted plaintext. This property, called authenticated encryption, is why GCM is the modern default for most new systems: it protects both confidentiality and integrity in one pass, with no separate MAC step required.

In practice: reach for AES-256-GCMunless you're interoperating with a system that specifically requires CBC. This tool generates a fresh, random IV for every operation. Never reuse an IV with the same key, as doing so can catastrophically weaken both modes.

The “128” and “256” in AES-128 and AES-256 refer to key length, not block size: AES always operates on 128-bit blocks regardless of key size. A longer key means more possible key values for an attacker to search through, which is why AES-256 is generally preferred for data that needs to remain confidential for a long time or against well-resourced attackers, while AES-128 is already considered secure against any currently known practical attack and remains faster in software that lacks hardware AES acceleration.

It's worth being explicit about what this tool is and isn't for. It's built for testing an implementation against known inputs, understanding how a given mode behaves, or quickly encrypting a value you're about to hand to a colleague over a channel you don't fully trust. It is not a substitute for a proper key management system in a production application; a real system needs to think about where keys are stored, how they're rotated, and who can access them, none of which a browser-based tool can solve on its own since nothing here persists a key between visits.